Crash Course Astronomy Marathon • Full Series: transcript

Hello and welcome to Crash Course Astronomy. I'm your host, Phil Plate, and I'll be taking you on a guided tour of the entire universe. You might want to pack a lunch. [music] course of this series, we'll explore planets, stars, black holes, galaxies, subatomic particles, and even the eventual fate of the universe itself.

But before we step into space, let's take a step back, I want to talk to you about science. There are lots of definitions of science, but I'll say that it's a body of knowledge and a method of how we learned that knowledge. Science tells us that stuff we think we know may not be perfectly known. It may be partly or entirely wrong. We need to watch the universe, see how it behaves, make guesses about why it's doing what it's doing, and then try to think of ways to support or disprove those ideas. That last part is important. Science must be above all else honest if we really want to get to the bottom of things. Understanding that our understanding might be wrong is essential, and trying to figure out the ways we may be mistaken is the only way that science can help us find our way to the truth, or at least the nearest approximation to it. Science learns. We meander a bit as we use it, but in the long run, we get closer and closer to understanding reality. And that is the strength of science, and it's all around us. Whether you know it or not, you're

soaking in science. You're a primate. You have mass. Mitochondria in your cells are generating energy. Presumably, you're breathing oxygen. But astronomy is different. It's still science, of course, but astronomy puts you in your place. Because of astronomy, I know we're standing on a sphere of mostly molten rock and metal 13,000 km across with a fuzzy atmosphere about 100 km high, surrounded by a magnetic field that protects us from the onslaught of subatomic particles from the sun 150 million km away, which is also flooding space with light that reaches across space to illuminate the planets, asteroids, dust, and comets, racing out past the Kyper belt, through the Orort cloud into interstellar space, past the nearest stars which orbit along with gas clouds and dust lanes in a gigantic spiral galaxy we call the Milky Way that has a super massive black hole in its center and is surrounded by 150 globular clusters and a halo of dark matter and dwarf galaxies some of which it's eating all of which can be seen by other galaxies in our local group like

Andromeda and Triangulum and our group is on the outskirts of the Virgo galaxy cluster which is part of the Virgo supercluster which is just one of many other gigantic structures that stretch most of the way across the visible universe which is 90 billion lightyears across and expanding every day even faster today than yesterday due to mysterious dark energy. And even all that might be part of an infinitely larger multiverse that extends forever both in time and space. See, astronomy puts you in your place. But what exactly is astronomy? This isn't necessarily an obvious thing to ask. When I was a kid, it was easy. Astronomy is the study of things in the sky, the sun, moon, stars, galaxies, and stuff like that. But it's not so easy to pigeon hole these days.

Take for example Mars. When I haul my scope out to the end of my driveway and look at Mars, that's astronomy, right? Of course. But what about the rovers there? Those machines aren't doing astronomy. Surely they're doing chemistry, geology, hydrarology, prology, everything but astronomy. So nowadays, what's astronomy? I'd say it's still studying stuff in the sky, but it's branched out quite a bit from there. Borders between it and other fields of science are fuzzy, a theme I'll be hitting on several times over this series. Humans might like firm delineated boundaries between things, but nature isn't so picky. And that brings us to our first edition of Focus on this week's topic. Astronomers. Who are we? What do we do? I used to look through telescopes for a living, or at least study the data that came from detectors strapped onto them. But now I talk and write and make videos about astronomy and relegate my viewing to my personal backyard telescope. But I still consider myself an astronomer. So that should give you an idea that there's a lot of wiggle room in the profession. In fact, when I worked on Hubble Space

Telescope, I was actually hired as a programmer. I coded in the language used by the folks helping to build and calibrate a camera that was due to launch into space and be installed on Hubble by an astronaut. Once the data from that camera are taken and analyzed, you have to know what to do with them. Do the observations fit the physical model of how stars blow up or how galaxies form or the way gas flows through space? Well, you better know your math and physics because that's how we test our hypothesis. And someone who does that is generally called an astrophysicist. Of course, those telescopes and detectors don't create themselves. We need engineers to design and build them and technicians to use them. Most astronomers don't actually use the telescopes themselves anymore. Someone who's trained in their specific use does that for them. Some of those instruments go into space and some go to other worlds like the moon and Mars. We need astronomers and engineers and software programmers who can build those too. And then at the end of all this, we need people to tell you all about it. Teachers, professors, writers, video

makers, even artists. So I'll tell you what. If you have an interest in the universe, if you love to look up at the stars, if you crave to understand what's going on literally over your head, then who am I to say you're not an astronomer? However you define astronomy, humans have been looking up at the sky for as long as we've been human. Certainly, ancient people noticed the big glowy ball in the sky and how it lit everything up while it was up and how it got dark when it was gone. The other fainter glowy thing tried, but wasn't quite as good at lighting up the night. They probably took that sort of thing pretty seriously. They probably also noticed that when certain stars appeared in the sky, the weather started getting warmer and the days longer. And when other stars were seen, the weather would get colder and daytime shortened.

And when humans settled down, discovered agriculture, and started farming, noticing those patterns in the sky would have had an even greater impact. It told them when to plant seeds and when to harvest. The cycles in the sky became pretty important. So important that it wasn't hard to imagine gods up there looking down on us weak and ridiculous humans interfering with our lives. Surely if the stars tell us when to plant and control the weather, seasons, and the length of the day, they control our lives, too. And astrology was born.

Astrology literally means study of the stars. As a word, it's been used before science became a formal method of studying nature. It irks me a bit since it got the good name and now we're stuck with astronomy which means law or culture of the stars and that's not really what we do. But what the heck? Words change meaning over time and now it's pretty well understood that astronomy is science and astrology isn't. Millennia ago, astrology was as close to science as you got. It had some of the flavors of science. Astrologers observed the skies, made predictions about how it would affect people, and then those people would provide evidence for it by swearing up and down it worked. The thing is, it really didn't. The fault of astrology lies in ourselves and not our stars. People tend to remember the hits and forget the misses when predictions are made, which is why they sometimes sit in casinos pumping nickels into machines that are proven to be nothing more than a method for reducing the number of nickels you have. But astrology led people to really study

the sky and find the patterns there, which led to a more rigorous understanding of how things really worked in the heavenly vault. It wasn't overnight. Of course, this took centuries. Before the invention of the telescope, keen observers built all sorts of odd and wonderful devices to measure the heavens. And in fact, it was before the telescope was first turned to the sky that a huge revolution in astronomy took place. It is patently obvious that the ground you stand on is fixed, rooted if you will, and the skies turn above us. The sun rises, the sun sets, the moon rises and sets. The stars themselves wheel around the sky at night. Clearly, the earth is motionless and the sky is what is actually moving. In fact, if you think about it, geocentrism makes perfect sense that all the objects in the sky revolve about the Earth and are fixed to a series of nested spheres, some of which are transparent, maybe made of crystal, which spin once per day. The stars may just be holes in otherwise opaque spheres letting sunlight through. Sounds

silly to you, doesn't it? Well, here's the thing. If you don't have today's modern understanding of how the cosmos works, this whole multiple shells of things in the sky thing actually does make sense. It explains a lot of what's going on over your head. And if it was good enough for Plato, Aristotle, and Tommy, then by God, it was good enough for you. And speaking of which, it was endorsed by the major religions of the time. So maybe it's better if you just nod and agree and don't think about it too hard. But a few centuries ago, things changed. Although he wasn't the first, the Polish mathematician and astronomer Capernicus came up with the idea that the sun was the center of the solar system, not the Earth. His idea had problems, which we'll get to in a later episode, but it did an incrementally better job than geocentrism. And then along came Tiko Brahhee and Johannes Kepler, who modified that system, making it even better. Then Isaac Newton. Oh, Newton.

He invented calculus partly to help him understand the way objects moved in space. Over time, our math got better, our physics got better, and our understanding grew. Applied math was a revolution in astronomy, and then the use of telescopes was another. Galileo didn't invent the telescope, by the way, but made them better. Newton invented a new kind that was even better than that, and we've run with the idea from there. Then, about a century ago came another revolution, photography. We could capture much fainter objects on glass plates sprayed with light sensitive chemicals which revealed stars otherwise invisible to us. Details in galaxies, beautiful clouds of gas and dust in space. And then in the latter half of the last century, digital detectors were invented which were even more sensitive than film. We could use computers to directly analyze observations and our knowledge leaped again. When these were coupled with telescopes sent into orbit around the Earth where our roing and boiling atmosphere doesn't blur out observations, we began yet another

revolution. And where are we now? We've come such a long way. What questions can we routinely ask that our ancestors would not have dared? What statements made with a pretty good degree of certainty? Think on this. The lights in the sky are stars. There are other worlds. We take the idea of looking for life on alien planets seriously and spend billions of dollars doing it. Our galaxy is one of a h 100red billion others. We can only directly see 4% of the universe. Stars explode and when they do they create the stuff of life, the iron in our blood, the calcium in our bones, the phosphorus that is the backbone of our DNA. The most common kind of star in the universe is so faint you can't even see it without a telescope. Our solar system is filled to overflowing with worlds more bizarre than we could have dreamed. Nature has more imagination than we do. It comes up with some nutty stuff. We're clever, too, we big brain apes. We've learned a lot, but there's still a long way to go.

So, with that, I think we're ready. Let's explore the universe today. You learned what astronomy is, and that astronomers aren't just people who operate telescopes, but include mathematicians, engineers, technicians, programmers, and even artists. We [music] also wrapped up with a quick history of the origins and development of astronomy from ancient observers [music] to the Hubble Space Telescope. Hey, Phil Plate here. Welcome to episode two of Crash Course Astronomy, Naked Eye Observations. Despite the salacious title, nudity is not required. In fact, given that a lot of astronomical observations are done at night, you may want to bundle up.

As it relates to astronomy, naked eye means no binoculars, no telescope, just you, your eyeballs, and a nice dark sight from which to view the heavens. After all, that's how we did astronomy for thousands of years. And it's actually pretty amazing what you can figure out about the universe just by looking at it. Imagine you're somewhere far away from city lights where you have an unobstructed view of the cloudless sky. The sun sets and for a few minutes you just watch as the sky darkens. Then you notice a star appear in the east just over a tree. Then another and another and within an hour or so you are standing beneath an incredible display. The sky spangled with stars. What do you notice right away? First, there are a lot of stars. People with normal vision can see a few thousand stars at any given time. And if you want a round number, they're very roughly 6 to 10,000 stars in total that are bright enough to detect by eye alone, depending on how good your sight is. The next thing you'll notice is that they're not all the same brightness. A handful are very bright. A few more are a bit fainter, but still pretty bright, and so on. The

faintest stars you can see are the most abundant, vastly outnumbering the bright ones. This is due to a combination of two effects. One is that stars aren't all the same intrinsic physical brightness. Some are dim bulbs, while others are monsters, blasting out as much light in 1 second as the sun does in a day. The second factor is that not all stars are the same distance from us. The farther away a star is, the fainter it is. Interestingly, of the two dozen or so brightest stars in the sky, half are bright because they're close to Earth, and half are much farther away, but incredibly luminous, so they still appear bright to us. This is a running theme in astronomy and science in general. Some effects, you see, have more than one cause. Things aren't always as simple as they seem. The ancient Greek astronomer Hipparus is generally credited for creating the first catalog of stars, ranking them by brightness. He came up with a system called magnitudes, where the brightest stars were first magnitude, the next brightest were second magnitude down to sixth magnitude. We still use a variation of this system today,

thousands of years later. The faintest stars ever seen using Hubble Space Telescope are about magnitude 31. The faintest star you can see with your eye is about 10 billion times brighter. The brightest star in the night sky called Sirius, the dog star, is about 1,000 times brighter than the faintest star you can see. Let's take a closer look at some of those bright stars like say Vega. Notice anything about it? Yeah, it looks blue and Beetlejuice looks red. Arcturus is orange, Capella yellow. Those stars really are those colors. By eye, only the brightest stars seem to have color, while the fainter ones all just look white. That's because the color receptors in your eye aren't very light sensitive, and only the brightest stars can trigger them. Another thing you'll notice is that stars aren't scattered evenly across the sky. They form patterns, shapes. This is mostly coincidence, but humans are pattern recognizing animals. So, it's totally understandable that ancient astronomers divided the skies up into constellations, literally sets or groups

of stars and named them after familiar objects. Orion is probably the most famous constellation. It really does look like a person arms raised up, and most civilizations saw it that way. There's also tiny delinus. It's only five stars, but it's easy to see it as a dolphin jumping out of the water. And Scorpius, which isn't hard to imagine as a venomous arthropod. Others, well, not so much. Pisces is a fish. Yeah, okay. Cancer is a crab if you say so. Although they were rather arbitrarily defined in ancient times, today we recognize 88 official constellations and their boundaries are carefully delineated on the sky. When we say a star is in the constellation of Ocus, it's because the location of the star puts it inside that constellation's boundaries. Think of them like states in the US. The state lines are decided upon by mutual agreement, and a city can be in one state or the other. Mind you, not every group of stars makes a constellation. The Big Dipper, for example, is only one part of the constellation of Ursa Major, the Big Bear. The bowl of the dipper is

the bear's honches, and the handle is its tail. But bears don't have tails, so astronomers might be great at pattern recognition, but they're terrible at zoology. Most of the brightest stars have proper names, usually Arabic. During the dark ages, when Europe wasn't so scientifically minded, it was the Persian astronomer Abed al- Raman al-Sufi who translated ancient Greek astronomy texts into Arabic and those names have stuck with us ever since. However, there are a lot more stars than there are proper names. So, astronomers use other designations for them. The stars in any constellation are given Greek letters in order of their brightness. So, we have Alpha Orionis, the brightest star in Orion, then Beta, and so on. Of course, you run out of letters quickly, too. So most modern cataloges just use numbers. It's a lot harder to run out of those. Of course, just seeing all those faint stars can be tough, which brings us to this week's focus on light. Pollution is a serious problem for astronomers. This is light from street lamps, shopping centers, or wherever, where the light gets blasted up into the sky instead of toward the

ground. This lights up the sky, making fainter objects much more difficult to see. That's why observatories tend to be built in remote areas as far from cities as possible. Trying to observe faint galaxies under bright sky conditions is like trying to listen to someone 50 ft away whispering at you at a rock concert. This affects the sky you see as well. From within a big city, it's impossible to see the Milky Way, the faint glowing streak across the sky. That's actually the combined light of billions of stars. It gets washed out with even mild light pollution. Your view of Orion probably looks like this.

When from a dark site, it looks like this. It's not just people who are affected by this either. Light pollution affects the way nocturnal animals hunt, how insects breed, and more by disrupting their normal daily cycles. Cutting back light pollution is mostly just a matter of using the right kind of light fixtures outside, directing the light down to the ground. A lot of towns have worked to use better lighting and have met with success. This is due in large part to groups like the International Dark Sky Association, Globe at Night, The World at Night, and many more who advocate using more intelligent lighting and to help preserve our night sky. The sky belongs to everyone, and we should do what we can to make sure it's the best possible sky we can see. Even if you don't have dark skies, there's another thing you can notice when you look up. If you look carefully, you might see that a couple of the brightest stars look different than the others. They don't twinkle.

That's because they aren't stars. They're planets. Twinkling happens because the air over our heads is turbulent, and as it blows past, it distorts the incoming light from stars, making them appear to slightly shift position and brightness several times per second. But planets are much closer to us and appear bigger, so the distortion doesn't affect them as much. There are five naked eye planets, not counting Earth. Mercury, Venus, Mars, Jupiter, and Saturn. Uranus is right on the edge of visibility, and people with keen eyesight might be able to spot. Venus is actually the third brightest natural object in the sky after the sun and moon. Jupiter and Mars are frequently brighter than the brightest stars, too. If you stay outside for an hour or two, you'll notice something else that's pretty obvious. The stars move like the sky is a gigantic sphere wheeling around you over the course of the night. In fact, that's how the ancients thought of it. If you could measure it, you'd find this celestial sphere spins once every day. Stars toward the east are rising over the horizon and stars in the west are setting, making a big circle over the course of the night and presumably day.

This is really just a reflection of the Earth spinning. Of course, the Earth rotates once a day and we're stuck to it. So, it looks like the sky is spinning around us in the opposite direction. There's an interesting thing that happens because of this. Look at a spinning globe. It rotates on an axis that goes through the poles and halfway between them is the equator. If you stand on the equator, you make a big circle around the center of the Earth over a day. But if you move north or south toward one pole or the other, that circle gets smaller. When you stand on the pole, you don't make a circle at all. You just spin around in the same spot. It's the same thing with the sky.

As the sky spins over us, just like with the Earth, it has two poles and an equator. A star on the celestial equator makes a big circle around the sky, and stars to the north or south make smaller ones. A star right on the celestial pole wouldn't appear to move at all and would just hang there like it was nailed to that spot all night long. And this is just what we see. Photographic time exposures show it best. The motions of the stars show up as streaks. The longer the exposure, the longer the streaks as the stars rise and set, making their circular arcs in the sky. You can see stars near the celestial equator making their big circles. And by coincidence, there's also a middling bright star that sits very close to the north celestial pole. That's called Polaris, the north or pole star. Because of that, it doesn't appear to rise or set, and it's always to the north, motionless. It really is coincidence. There's no southern pole star unless you count sigma octans, a dim ball barely visible by eye. It's not all that close to the south pole of the sky anyway. But even Polaris isn't exactly on the pole. It's offset a teeny bit. So it does make a

circle in the sky, but one so small you'd never notice. By eye, night after night, Polaris is the constant in the sky, always there, never moving. Remember, the sky's motion is a reflection of the Earth's motion. If you were standing on the north pole of the Earth, you'd see Polaris at the sky's zenith, that is straight overhead, fixed and unmoving. Stars on the celestial equator would appear to circle the horizon once per day. But this also means that stars south of the celestial equator can't be seen from the Earth's north pole. They're always below the horizon. So, this in turn means that which stars you see depends on where you are on Earth. At the North Pole, you only see stars north of the celestial equator. At the Earth's south pole, you only see stars south of the celestial equator. From Antarctica, Polaris is forever hidden from view. Standing on the Earth's equator, you'd see Polaris on the horizon to the north and Sigma Octton on the horizon to the south. And over the course of the day, the entire celestial sphere would spin around you.

Every star in the sky is eventually visible. While Polaris may be constant, not everything is. Sometimes you just have to wait a while to notice. And to that point, you'll have to wait a while to find out what I mean by this because we'll be covering that in next week's episode. Today, we talked about what you can see on a clear, dark night with just [music] your eyes. Thousands of stars, some brighter than others, arranged into patterns called constellations. Stars have colors, even if we can't see them with our eyes alone, and they rise and set [music] as the Earth spins. You can see different stars depending on where you are on Earth. And if you're in the northern hemisphere, Polaris will always point you toward north. Under a dark sky, you can see thousands of stars. If you watch for a few hours, you can see them rise and set as the Earth rotates once a day. And if you go outside the next night at the same time, you'll see that things look pretty much the same as they did the night before. The stars rise and set, Polaris hangs to the north, and so on. One day hardly makes any difference to the sky's appearance. But what if you wait for another night

or a week? If you're that patient and observant, you can spot subtle changes in the sky. [music] A couple of weeks have passed. Remember that star that was just over a tree in the east when the sun set? The one that made you first notice the stars are rising and setting. Go look at it again. If you happen to be out at the same time, you'd expect that star to be in the same place, but it's not. It's actually a bit higher above the tree. And if you look west, stars that were well off the horizon just after sunset last week are now lower. If you wait a month, this effect is even more pronounced. All new constellations will be visible in the sky after sunset. This is because the earth is going around the sun, literally changing our viewpoint on the sky. The earth takes a year to orbit the sun once. Every day it moves a little bit along its orbit. And as it

does from Earth's perspective, distant stars appear to move their positions relative to the sun. So one day we might see a star very near the sun, but the next day the angle is a bit bigger. At some point about 6 months after we first saw it, the star is directly opposite the sun in the sky. Then the angle starts to shrink again as the star approaches the sun from the opposite side until after a full year, the cycle repeats. What this means to you, the naked eye observer, is that the stars appear to rise and set at different times over the course of the year. Stars in the east rise about 4 minutes earlier every night, and stars in the west set 4 minutes earlier. A constellation that was entirely below the eastern horizon at sunset one month might be completely visible after sunset the next month. Another way to think about it is that the stars appear to be fixed. And as the Earth circles the sun, the sun moves through the stars over the course of the year, making a complete circle around the sky once per year. The path it takes is a reflection of the Earth's path

around the sun, a line in the sky. We call that line the ecliptic. That means the sun passes through the same constellations in the sky every year. We give those constellations a special name, the zodiac. Every year during a given month, the sun will appear to be in a certain zodiacal constellation from Sagittarius through Scorpius, Libra, Virgo, Leo, Cancer, and the rest. Eventually over a year, the sun returns to Sagittarius and the cycle starts again. But even though we talk about this process in terms of the sun's movement, it's really the path traveled by the Earth that creates this effect.

as our perspective moves with it. And of course, the planets move in the sky as well. Mercury, Venus, Mars, they orbit the sun, too. And they do so in approximately the same plane the Earth does. If you could see the solar system from the side, it would look flat. So to us on Earth, the planets go around the sky over the course of a year, and they also appear to change their positions relative to the sun and the stars. The inner planets, Mercury and Venus, move so rapidly, you can see their motion after a single night. The outer planets are more leisurely, but wait long enough and they too will be seen to move, sliding through the constellations. By the way, the word planet is Greek for wanderer. There's another aspect of all this you might notice over time. You've probably seen a globe and notice that the axis of it is tilted. That is, it's not straight up and down perpendicular to how it sits. That's because a globe is modeling the Earth and the Earth is tilted. The Earth spins on its axis once per day and orbits the Sun once per year. But the Earth's axis is tilted

with respect to its orbital plane by 23.5°. And this has a profound effect on our planet. Imagine for a moment that the Earth's axis were exactly perpendicular to its orbit, straight up and down. If that were the case, every day the sun would take the same path across the sky. If you were on the equator, the sun would rise, go exactly overhead, and then set. If you're on the pole, the sun would appear to go around the horizon every day, neither rising nor setting. It would always be twilight. But that's not the case. The Earth is tilted. In the months of June and July, the Earth's north pole is tipped toward the Sun. 6 months later, it's pointed away. This affects the path the Sun takes across the sky. Instead of it taking the same path every day in the northern summer, when we're tipped toward the sun, the sun takes a higher path in the sky. Because that path is longer, the days are longer, too. 6 months later, in December and January, the Earth's pole is tipped away. The sun takes a lower path in the sky. And because the path is shorter, days are shorter, too. That's why we have seasons. When the sun is up

high in the sky, it shines straight down on the ground, heating it better. And days are longer, so it has more time to heat us up. It gets hot. In the winter, it's the reverse. The sun is lower, so it can't warm us up as efficiently, and it has less time to do so. It gets cold. There you go. Seasons. The Earth's axis is tipped. If it weren't, the seasons wouldn't occur, and the temperature of the Earth wouldn't change monthtomonth. There's a common misconception that the Earth has seasons because it orbits the sun on an ellipse and so it's closer to the Sun in summer and farther in winter. While it's true the orbit is elliptical, Earth is closer to the Sun in January on the order of 5 million km or so than it is in July. It's the angle of the Sun's rays that makes winter cold and summer hot, not our distance from the sun. Also, you may know that when it's summer in the northern hemisphere, it's winter in the southern. When the north pole is tipped toward the sun, the south pole is tipped away. So, northern and southern hemisphere seasons are opposite each other. But nothing in astronomy is

permanent. The north pole is not always going to point toward the sun in June. And Polaris is not always going to be the north star. That's because our planet's axis is actually moving. Have you ever seen a spinning top start to wobble? Its axis moving in a slow circle even as the top itself spins. This is called procession. And the Earth does it too. Our planet spins on its axis once per day, but the axis wobbles, making a very slow circle that takes 26,000 years to complete. This affects a lot of what we see in the sky. For example, Polaris won't always be the pole star. Every year, the pole points a little farther from that star, making a big circle 47° across. For ancient Egyptians, the star Thubin was the pole star. And in about 11,000 years, that position will be held by the bright star Vega. Also, the date the sun is in a particular zodiac constellation changes slowly due to procession as well. When the ancients first thought up this idea, the sun was in Aries on March 22nd, the vernal equinox, what some people call the first day of spring. But due to procession,

it's now in Pisces. That's why your astrological sign doesn't match where the sun actually is in the sky. 2,000 years of procession has changed them. One of the many reasons astrology is silly. It's incredible to think about. the Earth, the Sun, the stars. They allow us to tell the time and time of year just by looking up and paying attention. This is why the stars were so important to ancient humans. The stars were like a clock and a calendar in the sky long before we had invented either. We've actually learned a lot about the sky just by looking at it. Of course, some of the stuff I've explained we've learned through other means. The Earth is spinning. Stars have different intrinsic brightnesses and so on. But all of that knowledge and far more got its start by people who went outside and looked up. Later, as we applied math and physics to what we observed, we learned even more and could then go back and explain what we saw. So, don't discount naked eye astronomy. It's all we had for thousands of years. In fact, I think we lost something when we started using clocks and calendars and moving to cities with bright lights that washed

away the stars from the sky. Those folks long ago were tied to the sky. They knew it like, you know, the streets in your neighborhood. They could see the stars rise and set. They knew the glory of the Milky Way sprawled across the heavens, even if they didn't know exactly what it was. We do know now with our knowledge gained over the centuries. But it comes at the cost of losing touch with the sky, not living under it as much as we once did. I've spent thousands of hours over my life at night just simply looking up, watching the stars, appreciating the universe as I can see it. The things I have witnessed have shaped my life and instilled in me a permanent and endless sense of wonder and joy. The universe belongs to everyone. [music] Go outside and if you can soak up your share. Today we talked about cycles. As the earth goes around the sun, we see stars rising and setting at different [music] times. The sun moves along a line in the sky called the ecliptic through a set of constellations called the zodiac. Really a reflection of the earth's motion around the sun. that the planets move more or less along the

ecliptic as well and that seasons are caused by the tilt of the Earth's axis together [music] with its annual orbit around the sun. Besides the sun, the moon is the most obvious object in the sky. Bright, silvery with tantalizing features on its face. It's been the target of imagination, poetry, science, and even the occasional rocket. If you pay even the most cursory attention to it, you'll see that it changes every day. Sometimes it's up in the day, sometimes at night, and its shape is always changing. What causes this behavior? The moon is basically a giant ball of rock 3,500 km across hanging in space. Its surface is actually pretty dark with about the same reflectivity as a chalkboard or asphalt.

However, it looks bright to us because it's sitting in full sunlight. The sun illuminates it and it reflects that light down to us here on Earth. And because it's a sphere and orbiting the Earth, the way we see it lit by the sun changes with time. That's what causes its phases, geometry. The important thing to remember through all of this is because the moon is a ball and in space, half of it is always illuminated by the sun. This is true for the Earth, too, and every spherical object in space. Half faces the sun, half faces away. We call the part facing the sun the daylight or bright side and the half facing away the night or dark side. The phase of the moon refers to what shape the moon appears to us, how much of it we see illuminated from the earth. The key to all this is this line dividing the lit day side from the unlit night side. We call that line the terminator. If you're facing the moon with the sun behind you, you're seeing the half of the moon that is fully illuminated by sunlight and it looks full. If you're off to the side, you see half of the lit

side and half of the dark side and we say the moon is half full. If the sun is on the other side of the moon, you're looking at the unlit half and it looks dark. Now, mind you, I haven't moved anything except our point of view here. So, at all times, the moon is always half lit and half dark. Remember that the phase of the moon we see depends on from what direction the sunlight's hitting it and the angle we see that from Earth. The moon orbits the Earth roughly once per month. In fact, that's where the word month comes from. Month and moon are cognates, words that have similar etmological histories. And in most languages, including English, the two words are very similar. The length of time we call the month is derived from the length of time it takes the moon to go through all of its phases, 29 1/2 days. So to describe the phases, let's start at the beginning. New moon. New moon happens when the sun, moon, and earth are all more or less in a line. The moon's orbit is actually tipped a bit to the earth's. So sometimes the new moon happens when the moon is below the sun or above it. But at some point in its orbit, at some point in the month, it appears to be as close to the sun as it can. What does this look like from

the Earth? The moon is between the Earth and the Sun. So from our perspective, we only see the dark half, the unilluminated half of the moon. The other side, the far side of the moon, is lit, but we can't see it. It makes sense then to call this the beginning of the moon cycle, hence the term new moon. Now, think about this for a sec. Because the moon is near the sun in the sky, it travels across the sky with the sun. It's up during the day. You can only see it from the part of the Earth that's lit, which is when it's daytime. It's a very common misconception that the moon is only up at night, but it's up during the day literally just as often. At new moon, the moon stays near the sun, so it rises at sunrise and sets at sunset. This makes it extremely difficult to see. It is, after all, sitting next to the brightest object in the sky, and only a little bit of it is lit from our perspective, but not for long. Because the moon is orbiting the Earth, after a couple of days, it's moved a bit to the east. Now we're seeing it along a slight angle and we can see a little bit of the illuminated half of the moon on its side toward the sun. The terminator, the

dayight line, appears curved around the moon. So what we see is a thin illuminated crescent moon. At this point, the crescent is still very thin with the horns of the crescent pointing away from the sun. Note that the moon is still pretty close to the sun in the sky, just a bit to the east, rising maybe an hour or two after sunrise. But this means it's up all day and then sets after the sun does. This is the best time to see the crescent moon. When the sun is already set and the sky starts to get dark, the moon will be low over the western horizon and it'll set soon after the sun does. Let's wait a couple of days. Okay, now the moon has moved a bit more in its orbit around the Earth and is farther from the sun in the sky. We see a little more of the illuminated part and the crescent is wider. Since it's getting thicker, we say this is a waxing crescent moon. Waxing means growing or getting bigger. It's also well away from the sun now, so it's easier to spot even during the day before sunset. 7 or so days after new moon, we get to our first milestone. The moon is now 1/4 of the way around its orbit. It's 90° away from the sun in the sky, which means we're looking straight down on the terminator, the moon's

dayight line. It cuts right down the middle of the visible face of the moon. So, it's half lit with the sunward side of the moon visible and the other side dark. Confusingly, this phase is properly named first quarter because the moon is one quarter of the way through its cycle, one quarter of the way through its orbit around the Earth. even though it looks half full. So, it's not really the half full moon. Astronomers prefer first quarter. So, if you want to sound all astronomer, then you should call it that. But time marches on. The moon continues on its gravitational dance with Earth, swinging around its orbit. Now, more than half full, we say its shape is gibbous, which means swollen or convex. Since it's getting wider, this is actually the waxing gibbus phase of the moon. It rises in the late afternoon and is up most of the night. Our next big step comes 2 weeks after new moon when it's moved halfway through its orbit. It's now opposite the sun in the sky 180° around. The Earth is between the moon and sun. So, we're looking at the fully illuminated half of

the moon. This is the full moon. Because it's opposite the sun, it rises at sunset and sets at sunrise. It's up all night shining down on Earth. But again, wait a couple of days and things change. When the moon is full, it's 180° around the sky from the sun. So as it continues to move around the earth in a circle, the distance between it and the sun is now starting to decrease even as it continues on in the same direction. As before it keeps rising and setting later, but now it rises after sunset and sets after sunrise. If you get up early in the morning as the sun is just rising in the east, you'll see the nearly but not quite full moon setting in the west. Not only that, but we're about to go through all the phases again, but in reverse order. A few days after full moon, the lit side is shrinking. It's in the waning or shrinking gibbus phase.

Then 3 weeks or so after new moon and a week after full, the moon is once again halflit. The terminator splitting the moon's face in two even halves. This is the third quarter moon because the moon is 3/4 of the way through its cycle. It's a lot like first quarter, but the side that was lit is now dark and vice versa. It's 270° around the sky from the sun. It rises at midnight and sets at noon. A few days later and the moon is a crescent again, getting thinner. It's now a waning crescent. It rises just a couple of hours before sunrise and sets a couple of hours before sunset. Then finally, we're back where we started. 1 month after new moon, the moon has traveled 360° around the sky and is once again as close to the sun as it can get. It's new moon and the cycle starts up again as it has for time immemorial. An interesting thing happens if you move your perspective from the earth to the moon. The phases of the moon we see from earth depend on the angle of the moon and the sun in the sky. But on the moon

the angles are exactly 180° reversed. At new moon when the moon is between the earth and sun the earth is opposite the sun as seen from the moon. It's full earth. All the other phases are opposite too. So when we see a full moon a moon dweller would see a new earth and so on. Have you ever looked at the thin crescent moon and seen the ghostly face of the rest of the unlit side? That's because it's not really unlit. The nearly full Earth is reflecting sunlight on the moon, lighting up the otherwise dark part. The Earth is bigger and more reflective than the moon. So, it's actually 50 times brighter than a full moon. This glow is called Earth Shine, a term I quite like. Even more poetically, it's been called the old moon in the new moon's arms, referring to the unlit part surrounded by the crescent new moon's horns. That's lovely, isn't it? The moon is one of the most beautiful and most gratifying objects in the sky to observe. It's different every day. Yet, it's also the same because we see more or less the same half of it, the same face all the time. It's big and bright

and the features on its surface discernable by eye and even better with binoculars or a small telescope. As the phases change inexurably, day after day, the angle of sunlight hitting the surface changes, bringing new things into our view. The motions become comforting, even familiar. It's a reminder that the universe may seem strange and complicated and forbidding at first, but over time, as you get outside and experience it, it becomes your neighborhood. Welcome home. Today, you learned why the moon has phases.

It's a sphere and it orbits the Earth. So, the angle at which we see its lit side changes. It goes [music] from new to waxing crescent to half full, waxing gibbus, full waning gibbus, half full, waning [music] crescent, and then the cycle starts all over again. This also affects when it rises and sets and what we see on the surface. We humans of planet Earth benefit from a great coincidence. It's a coincidence of time and of space and of math. The coincidence is this. The sun is about 400 times wider than the moon. And it's also on average about 400 times farther away than the moon. The apparent size of an object in the sky depends on how big it is and how far away it is. So these numbers being equal means the sun and the moon appear to be about the same size in the sky. And that's where another interesting thing comes in.

Sometimes the moon passes directly between the Earth and the Sun. Doesn't happen all that often, but when it does, you get magic. Or even better, you get science. You get [music] an eclipse for mankind. >> An eclipse is a generic term in astronomy for when one object passes into the shadow of another object. darkening or blocking it. A solar eclipse is when the moon blocks the sun, casting a shadow on the Earth. And a lunar eclipse is when the Earth blocks the sun, casting a shadow on the moon.

But how do they work? Well, the moon orbits the Earth once per month, and the Earth orbits the Sun once per year. If the Moon's orbit were perfectly aligned with the Earth's, essentially sharing the same plane, we'd get a solar eclipse every new moon and a lunar eclipse every full moon. But we don't. That's because the moon's orbit is tilted with respect to the Earth's by about 5°. What that means is that at new moon, the moon can be as much as 5° away from the sun, passing above or below the sun and the sky, thereby missing it from our perspective. But sometimes the moon is in the right place at the right time.

And at new moon, it lies perfectly in line between the sun and the earth. And when that happens, we get a solar eclipse. This geometry happens at least twice per year and sometimes as much as five times per year. What's happening physically in space is that the moon is casting a long shadow. Usually that shadow misses the Earth. But during an eclipse, the moon's shadow falls on the Earth's surface. In fact, there are two shadows from the moon, one inside the other. One is a narrow cone tapering to a point away from the moon. If you're anywhere physically inside this cone, the moon appears big enough to completely block the sun. That means the shadow is very dark and we call it the umbra which [music] is Latin for, you guessed it, shadow. Outside of this deep umbal shadow is a wider conical region where if you're in it, the sun is only partially blocked. You can still see some of the sun past the moon. You're getting less light and so you're technically shadowed, but it's not quite as dark as the umbra. This region is called the pen umbra. Pen in this case for Latin meaning almost or nearly. When the umbra touches the earth, we get a

total solar eclipse. But what does that look like from the ground? You don't get a total solar eclipse right away. First, the edge of the moon slips in front of the sun, and we see a little dip in the sun's limb, its edge, as seen from the Earth. That's the start of the penumbumber sweeping over you. As the moon slowly moves, that dip grows, becoming a bite. The sun becomes a thick crescent, then a thin one. As the sun becomes an ever thinner crescent, the sky begins to darken. Then, finally, the moon's black disc completely covers the sun. The umbra sweeps over your location and at that moment totality begins. You might think that this just means the sky gets dark and it's like night outside for a while. But a total eclipse is far more than that. And that's because of the sun's corona. As I'll cover in more detail in a future episode, the corona is the sun's atmosphere, an ethereally thin envelope of gas that stretches from the sun's surface into space for millions of kilometers. It's really faint and therefore usually completely overwhelmed by the intensely bright

light from the sun. But when the moon blocks the sun's face, the corona becomes visible. It surrounds the sun, filaments and tendrils extending into the sky, an incredibly beautiful sight. I know many people who have said it's the most spectacular thing they have ever seen. And there's more. The moon's edge isn't smooth. There are craters and other depressions. Craters right at the moon's edge allow sunlight to stream past. We see these as bright patches around the eclipse sun, which are called Bailey's beads because they were first described by English astronomer Francis Bailey in 1836. Because the moon and sun are very nearly the same apparent size, totality is brief. The longest it can last is only about 7 or 8 minutes, that's how long it takes the umbra to move over one spot on the Earth. When totality ends and the moon starts to move off of the sun's face, for a moment, just a single spot of the sun is unblocked, glowing fiercely on one side of the moon. Sometimes you can get a circle of light around the moon's surface and together with the bright spot, it looks like a celestial wedding ring. In fact, this is called the

diamond ring effect. Then inexerably, the moon pulls away from the sun and the order of events is reversed. The umbra is gone, but you're still in the penumbal shadow. The sun shows a thin crescent, then a thick one, then a dip in its side, and then it's all over. The umbil shadow of the moon is pretty small where it hits the earth. So, a total eclipse is a local event. If you're too far north and south, you don't get a total eclipse. You only get a partial one, which is still cool, but lacks the mystique of a total eclipse. Remember, too, that the moon's orbit around the Earth is an ellipse. That means sometimes it's closer to the Earth and sometimes farther. If a solar eclipse happens when the moon is at the far end of its orbit, it can actually be smaller than the sun in the sky. It doesn't block the entire face of the sun and it leaves a ring of light around the black circle of the moon. The technical name for this shape is annulus. So, this event is called an annular eclipse. A lot of people think if you look at a total solar eclipse, you can go permanently and completely blind. That's really not true. But some parts of eclipse watching are more dangerous than others. I mean, obviously, it's not a

good idea to stand there and stare at the sun. Looking at even the uneclipsed sun for more than a moment is painful, and that pain is the result of the damage that solar radiation is doing to your retinas. So, I don't recommend it. Duh. But when viewing an eclipse, the real concern is right after totality ends. During totality, it's dark, so your pupils have dilated to let more light in. But then there's the flash of sunlight when the moon moves off, and that's intense enough to damage your retinus. That's why astronomers recommend extreme caution when viewing an eclipse, because that flash can catch you by surprise. When viewing the sun, don't just stand there and stare at it.

You should always have eye protection. And make sure you have safety approved filters. Don't try the homemade tricks you might have heard of, like looking through an old CD or DVD or using old style camera film as a filter. These can let through too much infrared and ultraviolet light and again can dilate your pupils, actually making things worse. Lots of companies make inexpensive filters that are great for sunspotting. We have links in the doobly-doo for more information on eye safety. Now, you don't have to worry about hurting your eyes at all when viewing a lunar eclipse because in that case, it's the Earth that blocks the sun and the Earth's shadow falls on the moon. So, go nuts. But one big difference between the two kinds of eclipses is who can see them. A solar eclipse is localized to one spot on the Earth, or really a swath along the ground as the moon's umbrell shadow sweeps across the Earth's surface. But a lunar eclipse is when the moon moves into Earth's shadow. So, anyone on Earth facing the moon can see a lunar eclipse.

This is why I've seen dozens of lunar eclipses, but never a total solar one. I've never been at the right place at the right time. Not that I'm bitter. The Earth has umbil and penumbal shadows, too. When the moon first enters the Earth's penumbra, the dimming is so slight, you hardly notice it. But as the moon moves deeper into the penumbra, it starts to darken. Sometimes it changes color, turning a deep orange or blood red. That's because the Earth is starting to block the sunlight heading toward the moon, and the only light that gets through is coming through the thickest part of our atmosphere. This blocks blue and green light, leaving only red to come through. That's why the moon and sun look red to us when they're on the horizon, rising and setting, too.

When you look upon the red eclipsed moon, you're seeing the light from all the sunrises and sunsets in the world, hitting the moon and reflecting back to us. Finally, the moon starts to enter the Earth's umbra, and the real eclipse begins. At first, it looks like a bite is taken out of it. That curving arc is the shadow of the edge of the Earth. The moon moves deeper and deeper into the shadow until it's completely darkened. The Earth is bigger than the moon, so the Earth's umbra is much wider. While a solar eclipse is over in minutes, a total lunar eclipse can last nearly 2 hours. I once saw a lunar eclipse so deep that it took me a minute to find the moon in the sky. There's not a lot of new science you can do with a lunar eclipse. But if you know a little geometry, you can use the size and shape of the Earth's shadow on the moon to get the relative sizes of the Earth and Moon. Ancient Greeks did just this and got a number that wasn't too far off. They also knew how big the Earth was using other methods, and so they had a decent estimate for the size of the moon nearly 2,000 years before the invention of the telescope. They also knew the

shape of the Earth's shadow was always a circle, which only makes sense if the Earth were a sphere. If the Earth were flat, it would sometimes cast a thin shadow, but it never does. Pretty clever, those ancient Greeks. One final note, because of tides from the Earth, which we'll learn more about in detail in a later episode, the moon is slowly moving away from the Earth by about 4 cm a year. As it recedes, it's slowly getting smaller in the sky. This means that eventually it will be too far away to completely cover the sun and we won't get any more total eclipses. Doing the rough math, that will be in about a billion years. Better watch eclipses while you can. Today you learned that a solar eclipse is when the moon blocks the sun so it shadow falls on the Earth and a lunar eclipse is when the [music] Earth's shadow falls on the moon. We don't get them every 2 weeks because the moon's orbit is tilted. And if you're clever, you can use lunar eclipses to figure [music] out how big the Earth and Moon are. I've talked a lot about observing the night sky with your eyes, just simply going out and seeing what you can see. It's pretty amazing what

you can learn just by doing that. And of course, that's all we humans could do for thousands of years. But now we can do better. We can use telescopes. The first person to invent the telescope is lost to history. Despite common knowledge, Galileo did not invent them. He wasn't even the first person to point one at the sky or the first person to publish results. But he was a loud and persistent voice over the years. and his amazing string of discoveries using his crude instrument landed him firmly in the history books. Aggressive self-marketing sometimes pays off for mankind.

[music] >> You might think the purpose of a telescope is to magnify small objects so we can see them better. That's how a lot of telescopes are marketed, but to be honest, that's not exactly the case. If you want to be really general, the purpose of a telescope is to make things easier to see, to make the invisible visible, and to make the things already visible visible more clearly. A telescope works by gathering light. Think of it like a bucket in the rain. The bigger the bucket, the more rain you collect. If your bucket is big enough, you'll get plenty of water even when it's only sprinkling out. In the case of a telescope, the bucket is an optical device like a lens or a mirror that collects light. We call this device the objective. And the bigger the objective, the more light it collects. Look at your eyes. Well, that's tough. So, let's think about our eyes for a moment. They also work as light buckets, but they only collect light through our pupils, which even under the best of circumstances are less than a centimeter across. A very tiny bucket indeed. But we can do better. To extend the analogy, a telescope is like a bucket with a

funnel at the bottom. All that light that it collects is then concentrated, focused, and sent into your eye. It turns a trickle of light into a torrent. The amount of light it collects depends on the area of the objective. That means if you double the diameter of the collector, you'd collect four times as much light because the area of the collector goes up as the square of the radius. Make a bucket 10 times wider and you collect 100 times as much light. Clearly, as telescopes get bigger, their ability to show us faint objects increases enormously. In fact, that was one of Galileo's first and most important discoveries. Stars that were invisible to the naked eye were easily seen through his telescope, even though it only had a lens a few centimeters across. Those faint stars didn't emit enough light for his eyes to see them.

But when he increased his collecting area with a telescope, they popped into visibility. The primary way telescopes work is to change the direction light from an object is traveling. I can see a star with my eye because light from that star is sent in my direction into my eye. But most of that light misses my eye, falling to the ground all around me. The telescope collects that light, bounces it around, and then channels it into my eye. When the very first telescopes were built, this changing of the direction of light was done using lenses. When light goes from one medium to another, say from going through air to going through water or glass, it changes direction slightly. You see this all the time. A spoon sitting in a glass of water looks bent or broken. The spoon is doing just fine, but the light you see from it is getting bent, distorting the image. This bending is called refraction. The way light bends depends on what's bending it, like water or glass, and the shape of the object doing the bending. It so happens that if you grind a piece of glass into a lens shape, it bends or refracts the incoming light in a cone, focusing it into a

single spot. It's a light funnel. This refraction has a couple of interesting results. For one thing, the light from the top of a distant object is bent down and the light from the bottom is bent up. When this light comes to a focus, it means you see the object upside down. It also flips left and right, which can be a little disconcerting and takes getting used to when you're using a refracting telescope. For another thing, the lens can magnify the image. That's again because the light is bent and the image created of an observed object can appear larger than the object does by eye. It depends on a lot of factors, including the shape of the lens, the distance of the object, and how far away the lens is. But in the end, what you get is an image that looks bigger. That has obvious advantages. A planet like Jupiter is too far away to see as anything other than a dot to the eye. But a telescope makes it appear bigger and details can then be seen. When Galileo and other early astronomers pointed their telescopes at the sky, multitudes were revealed. Craters on the moon, the phases of Venus, Jupiter's moons, the rings of Saturn, and so much

more. The universe itself came into focus. When astronomers talk about using a telescope to make details more clear, they use a term called resolution. This is the ability to separate two objects that are very close together. You're familiar with this. When you're driving on a road at night, a distant car coming toward you appears as a single light. When it gets closer, the light separates out, resolves into two headlights. A telescope increases resolution, making it easier to say split two stars that are close together or to see details on the moon's surface. The resolution depends in part on the size of the objective. In general, the bigger the telescope objective, the better your resolution is. Resolution is more useful than magnification when talking telescopes. Fundamentally, there's a limit to how well your telescope resolves two objects, but there's no limit to how much you can magnify the image. If you magnify the image beyond what the telescope can actually resolve, you just get mush. Refracting telescopes are great, but they suffer from a big problem. Big lenses are hard to make. They get thin near the edge and break

easily. Also, different colors of light bend by different amounts as they pass through the lens. So you might focus a red star, say, and a blue one will still look fuzzy. No less a mind than Isaac Newton figured a way around this. Use mirrors. Mirrors also change the direction light travels. And if you used a curved mirror, you can also bring light rays to a focus. Telescopes that use mirrors are called reflectors. The advantages of reflectors are huge. You only have to polish one side of a mirror, where a lens has two sides.

Also, a mirror can be supported along its back, so they can be manufactured much larger, more easily, and for less money. Although, there have been many improvements made over the centuries. Most big modern telescopes at their heart are based on the Newtonian design, and in fact, no large professional-grade telescopes made today have a lens as their objective. Nowadays, it's all done with mirrors. And that brings us to this week's aptly named Focus On. The most common question I'm asked besides, "Hey, who does your hair?" is, "Hey, Phil, what kind of telescope should I buy?" It's a legitimate question, but it's very difficult to answer. Imagine someone walked up to you and asked, "What kind of car should I buy?" That's impossible to answer without a lot more information. Same for telescopes. Do you want to look at the moon and planets or fainter, more difficult to spot galaxies? Are you really devoted to this or is it more of a pastime? Is this for a child or an adult? These questions are critical. Most small scopes are refractors, which are good for looking at detail on the moon and planets. They tend to magnify the image more than reflectors do, but they're tricky to use because they flip the image left and

right and up and down. Bigger scopes are good for fainter objects, but are more expensive and can be difficult to set up and use. I hate hearing about a scope that just collects dust because it was bought in haste. So, here's what I recommend. Find an observatory, planetarium, or local astronomy club. They're likely to have star parties, public observing events where you can look at and through different kinds of telescopes. Their owners are almost universally thrilled to talk about them. As an astronomer, I can assure you that the problem with astronomers isn't getting them to talk, it's shutting them up. So, you'll get lots of great firsthand advice and experience. Also, I usually recommend getting binoculars before a telescope. They're easy to use, fun to use, easy to carry around, and you can get good ones for less money and still see some nice things. Even if you decide not to get more into astronomy as a hobby, they can also be used during the day on hikes and for bird watching.

I have a couple of pair of binoculars and I use them all the time. There's a third aspect to telescopes that's very important. Beyond resolution and making faint things easier to see, they can literally show us objects outside of the range of colors our eyes can see. In the year 1800, William Herschel discovered infrared light, a kind of light invisible to our eyes. In the time since, we've learned of other forms of invisible light, radio, microwave, ultraviolet, x-rays, and gamma rays. Astronomical objects can be observed in all these flavors of light if we have telescopes that are designed to detect these flavors of light. Radio waves pass right around normal telescopes, ones that we use to observe visible light. X-rays and gamma rays pass right through them as if they aren't even there. But we're smart, we humans. We learned that giant metal dishes can and will bend radio waves and can be formed just like gigantic Newtonian mirrored telescopes. In fact, different forms of light need different kinds of telescopes. And once we figured out how, we've built them. We can now detect cosmic phenomena across the entire spectrum of light from radio waves to gamma rays. and have even built unconventional telescopes that detect

subatomic particles from space as well, such as nutrinos and cosmic rays. Because of this, we've learned far more about the universe than Galileo could have imagined. And we're in the midst of another revolution, too. The actual bioysics is complicated, but in a sense, our eyes act like movie cameras, taking pictures at a frame rate of about 14 images per second. That's a short amount of time. Photographs, though, can take far longer exposures, allowing the light to build up, allowing us to see much fainter objects. The first photographs taken through a telescope were done in the 1800s. This has led to innumerable discoveries. For example, in the 20th century, giant telescopes with giant cameras revealed details in distant galaxies that led to our understanding that the universe is expanding, a critically important concept that we'll dive into later in the series. And now we have digital detectors, similar to the ones in your phone camera, but far larger and far more sensitive. They can be dozens of times more light sensitive than film. able to detect in minutes objects that would have taken hours or more to see using film. These digital

cameras can also be designed to detect ultraviolet light, infrared, and more. We can store vast amounts of that data easily on computers and use those computers to analyze that huge ocean of information, performing tasks too tedious for humans. Most asteroids and comets are discovered using autonomous software, for example, looking for moving objects among the tens or hundreds of thousands of fixed stars in digital images. This is also ushered in the era of remote astronomy. A telescope can be on a distant mountain and programmed to scan the sky automatically. It also means we can loft telescopes into space above the sea of air in our atmosphere that blurs and distorts distant faint objects. We can visit other worlds and send the pictures and data back home or put observatories like the Hubble Space Telescope into orbit around the Earth and have it peer into the vast depths of the universe. I would argue that the past century has seen a revolution in astronomy every bit as important as the invention of the telescope in the first place. In the early 17th century, the entire sky was new, and everywhere you pointed a telescope, there was some treasure to behold. But with our huge telescopes and

incredibly sensitive digital eyes, now that's still true. We learn more about the universe every day, just as we learn that there's more to learn every day, too. That's one of the best parts of being an astronomer. The universe is like a jigsaw puzzle with an infinite number of pieces. The fun never ends. And remember, even with all the wonders revealed by telescopes, your eyes are still pretty good instruments, too. You don't need big fancy equipment to see the sky. The important thing is to go outside. Look up. That's fun, too.

Today, you learn that telescopes do two things. Increase our ability to resolve details and collect light so we can see fainter objects. [music] There are two main flavors of telescope. refrators, which use a lens, and reflectors, which use a mirror. There are also telescopes that are used to look at light, our eyes can't see. And with the invention of film and later electronic detectors, [music] we've been able to probe the universe to amazing depths. We live, and stop me if I'm going too fast, on a planet. I mean, sure, duh. But this isn't the natural state of the universe, or at least it's not the way things usually are. Most of the universe is pretty empty. That's why we call it space. And if I were to magically transport you someplace randomly in the cosmos, the chances are you'd be a million lighty years from the nearest substantial object. Evolving on a planet has warped our sense of physics. If I throw an object away from me, it comes back. That's bizarre. It should just keep going, moving away from me at a constant speed. Instead, though, it goes up, slows, stops, then falls back down toward me. The difference between living

on a planet and being in deep space is gravity. Gravity from an object goes on forever, but it gets weaker rapidly with distance. A zillion lightyears away, the Earth's gravity is fantastically weak. But here on Earth, it's literally a force to be reckoned with. And in some places, it can be a lot stronger than what we experience right here. For mankind, [music] >> for most of history, gravity was just a fact of life. neither understood nor examined terribly closely. In the mid600s, scientists like Robert Hook and Isaac Newton started investigating it using math. In fact, the two men got into a bitter feud over who thought of what first. But whoever it was who first got it right, now we have a much better understanding of how gravity works. One thing before we get to gravity, an important concept that comes up a lot is mass. It's a bit tricky to define, but you can think of it as how much stuff makes up an object. I know that's not very scientific sounding, but it's not a

bad way to think about it. Something with more mass has more stuff in it. Size doesn't really play into this. Two objects can have the same mass, but one can be much larger than the other. In that case, the bigger object's mass is more spread out. So, we say it has lower density, where density is how much mass is inside a given volume. In science terms, mass tells us how much an object resists having its motion changed. An object with more mass is harder to get moving than an object with less mass, which is pretty obvious if you've ever tried pushing on a toy truck versus a real truck. But mass is also defined using gravity. Everything that has mass also has gravity and can inflict this force on another object. The amount of force you feel from the gravity of an object like a planet depends on three things. How much mass it has, how much mass you have, and how far away you are from it. In fact, distance dominates here. The force of gravity weakens with the square of the distance. Double your distance from an object and the force of gravity drops by 2 * 2 equals four

times. Go 10 times farther away and the force drops by 10 * 10 equals 100 times. Gravity is also attractive. It can only draw things in, not repel them. But how it attracts things is where it gets fun. If I hold up a rock and let go of it, it falls to the ground. What might be hard to see is that it gets faster the longer it drops. Forces accelerate objects. So the longer the force acts, the more the object's velocity changes, in this case getting faster. If I drop a rock from higher up, it'll move faster when it hits the ground. Other forces act on moving objects as well, like friction and air resistance, counteracting gravity, making this acceleration hard to see. But in space, the force of gravity becomes very clear. Two objects that have mass will attract each other.

If there are no other forces acting on them, they'll accelerate toward each other until they meet. Remember though that the force of gravity depends on those masses. If one is really massive and the other not so much, then in more practical terms, the massive one will pull in the less massive one. The more massive one does move, but much less than the other one. When objects are free to move under the effects of gravity, we say they are in orbit. The simplest kind of orbit may not be what you think. It's actually just a line. When you drop a rock, it's very briefly in orbit. ignoring things like the Earth's rotation, which adds a bit of sideways motion, it's close enough to say the rock just falls straight down and is stopped because the Earth itself gets in the way. That's not a terribly interesting orbit. So, what if instead of dropping the rock, we throw it? That gives it a little bit of sideways motion. So, instead of hitting the ground at my feet, it hits a bit farther away. If I throw it harder, it moves horizontally even more before it hits.

What if I throw it really hard? This is where Newton's genius comes in. He realized that if you throw the ball hard enough sideways, it will fall at the exact same rate the Earth would curve away underneath it. As Douglas Adams said in Hitchhiker's Guide to the Galaxy, "Flying is just falling and missing the ground." It turns out that's exactly what orbiting is, too. A rock thrown hard enough sideways will fall toward the Earth, but always miss it, going instead into a circular path around it, guided only by gravity. It'll orbit the Earth in a circle, taking about 90 minutes to go around the planet once. Circles are simple orbits. The speed at which the orbiting satellite travels depends on the mass of the object it's orbiting and its distance from it. The farther it is, the weaker gravity is, so it doesn't have to travel as quickly to maintain the orbit. Roughly 400 years ago, the astronomer Johannes Kepler realized that there can be other shapes of orbits as well. He discovered that planets orbit the sun on ellipses when previously it was thought they orbited in perfect circles. An elliptical orbit happens when you throw

the rock sideways, even harder than it takes for a circular orbit. It goes up higher on one end of the orbit than on the other. In fact, the harder you throw the rock, the more elongated the orbit gets. An orbit like this is still closed. That is, the orbit repeats itself and the rock is still bound to the Earth by gravity. At some point, though, if you throw the rock hard enough, an amazing thing happens. It can escape. Remember, gravity gets weaker with distance. If you throw a rock hard enough, while gravity can slow it down. The gravity gets weaker the farther away the rock is. If the rock has enough velocity, gravity weakens too quickly to stop it. The rock can escape, moving away forever. So, we call this the escape velocity. The escape velocity of an object like a planet or star depends on how much mass it has and how big it is. For the Earth, that turns out to be about 11 km/s. For Jupiter, it's about 58 km/s. And for the sun, it's a whopping 600 km/s.

Whatever the particular escape velocity for your cosmic location is, if you fling a rock away from it faster than that, I hope you kissed it goodbye first, cuz it ain't coming back. One way to think of it is that the rock is always slowing, getting ever closer to stopping, but it never actually stops. If it could travel infinitely far away, it would stop. But that's kind of a long trip. This works in reverse, too. If I go way far away from the Earth and drop a rock, it'll accelerate. When it hits the planet, it'll be moving at escape velocity, that same 11 km/s. And if I give it a little sideways kick, it'll miss the Earth, but still pass us at escape velocity. An escape orbit is open. It doesn't come back and is shaped like a parabola. What if you throw the rock even harder than that? The rock doesn't come back and moves away even faster. The orbit is now a hyperola, which is similar to a parabola, but is even more open. The rock never stops, even at infinity. It just keeps moving on. Like all forces, gravity gets weaker with distance. But its force never quite

drops to zero. It just gets smaller and smaller as you get farther and farther away. So why then are astronauts on the space station weightless. Gravity is still pulling on the astronauts. In fact, at the height of the station, Earth's gravity has only decreased by a little bit. It's still about 90% as strong as it is on the Earth's surface. If they were in a tower 320 km high, they'd weigh 90% of what they do on the Earth's surface. But the big difference is that the astronauts are in orbit falling around the Earth. Weight is actually not just the force of gravity on a mass, but how hard a surface pushes back on that mass. For example, when you stand on the ground, the ground pushes back. Otherwise, you'd fall through. The force of the ground back on you is what causes you to have weight. In freef fall, there's nothing pushing back.

you're falling freely and so you have no weight. NASA likes to call this condition microgravity since there are subtle forces acting on you. This actually highlights the difference between mass and weight. In space, you have the same mass as you do on Earth, but no weight. If another astronaut pushed on you, they'd have to exert a force, but if you stood on a scale in space, it wouldn't register anything. Space is weird. Well, compared to Earth. One more thing, and this is truly weird. Photons, particles of light, have no mass. Yet, they can be affected by gravity, too, bending their direction of flight as they pass a massive object. It turns out gravity can actually warp space. Light travels along the fabric of space like a truck on the road. And if the road curves, so does the truck. I know this is an odd concept, and [music] we'll be dealing with it later in more detail when we push escape velocity to its limits with black holes. Today, you learned that gravity is a force, and [music] everything with mass has gravity. Gravity accelerates objects with mass, changing their speed and/or direction. An object moving along a

[music] path controlled by gravity is said to be in orbit, and there are many different kinds. Straight lines, circles, [music] ellipses, parabola, and hyperbole. You can't ever escape gravity. But if you travel faster than escape velocity for an object, [music] you'll get away from it without falling back. And if you're in orbit in freef fall, you have no weight, but you still have mass. You know, if Shakespeare had been an astronomer, he'd have said that there is a tide in the affairs of the universe, and on such a full sea are we now afloat. He would have been right. You might just think of tides as the ocean going in and out every day. But in fact, what astronomers call tides are a subtle but inexurable force that have literally shaped most objects in the universe. And to understand tides, we start with gravity.

>> [music] >> Gravity is a force and it weakens with distance. An important thing to note is that we measure gravity from the center of mass of an object, not its surface. One way to think of the center of mass of an object is the average position in an object of all its mass. For an evenly distributed sphere, that's its center. Right now, unless you're an astronaut, you're about 6,400 km from the center of the Earth. If you stand up, your head is a couple of meters farther away from the Earth's center than your feet. Since gravity weakens with distance, the force of Earth's gravity on your head is an inensy bit less than it is on your feet.

How much less? A mere 0.00005%. And that's way too small for you ever to notice. But what if you were taller? Well, the taller you are, the farther your head is from the Earth's center and the weaker force it will feel. If you were say about 300 kilometers tall, the force of gravity would drop by about 10% at your head. That probably would be enough to notice if you weren't dying from misfixiation and you know being 300 km tall. This change in the force of gravity over distance is what astronomers call the tidal force. When you have a massive object affecting another object with its gravity, its tidal force depends on several factors.

For one thing, it depends on how strong the gravity is from the first object. The stronger the force of gravity, the stronger the title force will be on the affected object. It also depends on how wide the affected object is. The wider it is, the more the force of gravity from the first object changes across it and the bigger the title force. Finally, it depends on how far the affected object is from the first object. The farther away the affected object is, the lower the title force will be. Tides depend on gravity, and if gravity is weaker, so is the title force. The overall effect of the tidal force is to stretch an object. You're applying a stronger force on one end than you are on the other. So, you're pulling harder on one end. That'll stretch it. And this is where tidal forces become very important. Look at the moon. It has gravity, but much less than the Earth because it's less massive. It's 380,000 km away. So, the gravitational force it has on you is pretty small. And you're pretty small compared to that distance, just a couple of meters long from head to feet. But the Earth is big. It's nearly 13,000 km across. That means the

side of the Earth facing the moon is about 13,000 km closer to the moon than the other side of the Earth. This is a pretty big distance, enough for tides to become important. The side of the Earth facing the moon is pulled harder by the moon than the other side of the Earth. So, the Earth stretches. It becomes ever so slightly football-shaped, like a sphere with two bulges, one pointing toward the moon and one pointing away. This is probably the weirdest thing about tidal forces. You might expect only one bulge on the side of the Earth facing the moon. But remember, we measure gravity from the centers of objects. The side of the Earth facing the moon feels a stronger pull toward the moon than the Earth's center. So, it's pulled away from the center. But the side facing away from the moon feels a weaker force toward the moon than the Earth's center. This means the center of the Earth is being pulled away from the far side. This is exactly the same as if the far side is being pulled away from the center. And that's why you get two bulges on opposite sides of the Earth.

The tidal force is therefore strongest on the sides of the earth facing toward and away from the moon and weakest halfway in between them on each side. A lot of the earth is covered in water and water responds to this changing force, this stretching. The water bulges up where the tidal force is strongest on opposite sides of the earth. If there's a beach on one of those spots, the water will cover it and we say it's high tide. If a beach is where the tidal force is low, the water's been pulled away from it and it's low tide. But wait a second, the Earth is spinning. If you're on a part of the Earth facing the moon, you're at high tide. 6 hours later, a quarter of a day, the Earth's rotation has swept you around to the spot where it's low tide. Six hours after that, you're at high tide again. And then another 6 hours later, you're at low tide for the second time that day. Finally, a day after you started, you're back at high tide once more. And that's why we have two high tides and two low tides every day. Very generally speaking, the ocean tide causes the sea level to rise and fall by a meter or two every day. Incidentally, the solid earth can bulge as well. It's not as fluid as water, but it can move. The tidal force

stretches the solid earth by about 30 cm. If you just sit in your house all day, you move up and down by about that much twice. Like the saying goes, a rising tide lifts all surfaces. The Earth's spin has another effect. Lag in the water flow means the water can't respond instantly to the tidal force from the moon. The Earth's spin actually sweeps the bulges forward a bit along the Earth. So, picture this. The bulge nearest the moon is actually a bit ahead of the Earth moon line. That bulge has mass. Not a lot, but some. Since it has mass, it has gravity, and that pulls on the moon. It pulls the moon forward in its orbit a bit, like pulling on a dog's leash, accelerating it. The moon responds to this tug by going into a higher orbit. The moon is actually moving away from the Earth. The rate of recession of the moon has been measured, and it's something like a few centimeters per year, roughly the same speed your fingernails grow. Now get this. The moon has gravity. Just as the bulge is pulling the moon ahead, the moon is pulling the bulge back, slowing

it down. Because of friction with the rest of the Earth, this slowing of the bulge is actually slowing the rotation of the Earth itself, making the day longer. The effect is small, but again, it's measurable. Okay, let's get a little change of perspective. Everything I've said about the moon's tidal effect on the Earth works the other way, too. The moon feels tides from the earth and they're pretty strong because the earth is more massive and has more gravity than the moon. Just like Earth, there are two tidal bulges on the moon, one facing the Earth and one facing away. Long ago, the moon was closer to the Earth and spinning rapidly. The moon's tidal bulges didn't align with the Earth, and the Earth's gravity tugged on them, slowing the moon spin and moving it farther away. As it moved farther away, the time it took to orbit once around the Earth increased. Its orbital period got longer. Eventually, the lengthening rotation of the moon matched how long it took to go around the Earth.

When that happened, the axis of the bulges pointed right at the Earth. That's why the moon only shows one face to us. It spins once per month and goes around us once per month. If it didn't spin at all over that month, we'd see the entire lunar surface. But since it does spin once per orbit, we only ever see one face. This is called tidal locking, and it's worked on nearly every big moon in the solar system. tides from their home planet have matched their spin in orbital period. These moons all show the same face toward their planet.

Now wait a second. If the moon has gravity, which causes tides and is the root cause behind all these shenanigans, what about the sun? It's even bigger than the moon. Tides depend on the gravity from [music] an object and your distance from it. The sun is far more massive than the moon, but much farther away. These two effects largely cancel each other out. And when you do the math, you find the sun's tidal force on the earth is just about half that of the moon's. The way the sun's tidal force and the moon's tidal force interact on Earth depends on their geometry, which changes as the moon orbits us. At new moon, the Earth, moon, and sun are in a line. The moon's tidal force aligns with the sun's reinforcing it. This means we get an extra high high tide and an extra low low tide on Earth. We call this the spring tide. When the moon is at first quarter, the tidal bulge from the moon is 90° around from the suns. High tide from the moon overlaps low tide from the sun. We get a slightly lower high tide and a slightly higher low tide. We call those neep tides. [music] The pattern

repeats when the moon is full. The moon, earth, and sun fall along a line again and we get spring tides. A week later, the moon is moved around and we get neept tides again. Not only that, the moon orbits the earth on an ellipse. When it's closest to us, we feel a stronger effect. If that also happens at new or full moon, we get an added kick to the spring tides. This is called the proxyian tide and can lead to flooding in low-lying areas. Unless you live on the coast, I bet you had no idea tides were so complex. Tides are universal.

They work wherever there's gravity. If two stars orbit each other, each raises a tide in the other. Just like the Earth and Moon, that can slow their spin and increase their separation. Many planets orbiting other stars may be tidily locked to those stars. Near a black hole where the gravity is incredibly intense, the tides are so strong they would pull you like taffy into a long thin string. Astronomers call this effect spaghettification. No, seriously, that's what we call it today. You learn that tides are due [music] to the changing force of gravity over distance. The strength of the tidal force from an object depends on the gravity of the object and the size of and distance to the second object. Tides raise two bulges in an object, creating two high tides and two low tides per day on Earth. [music] Tides have slowed the Earth's rotation, move the moon away from the Earth, and lock the moon's rotation in orbit so that the moon always has one side [music] facing us.

So, tide goes in, tide goes out. It turns out I can explain that now you can, too. The solar system is the name we give to our local cosmic backyard. A better way to think of it is all the stuff held sway by the sun's gravity. The sun itself, planets, moons, asteroids, comets, dust, and very thin gas. If you took a step back, well, a few trillion steps back and looked at it from the outside, you might define the solar system as the sun. And that's because the sun comprises more than 98% of the mass of the entire solar system.

The next most massive object, Jupiter, is only onetenth the diameter and less than 1% the mass of the sun. But that's a little unfair. Our solar system is a pretty amazing place and you can figure out a lot of what's going on in it just by looking at it. Thousand of years we had to explore the solar system stuck on this spinning revolving ball the earth. The problem was for a long time we didn't know it was a spinning revolving ball. Well the ancient Greeks knew it was a ball. They had even measured its size to a fair degree of accuracy but most thought it was motionless. When a few folks pointed out that this might not be the case like the ancient Greek astronomer Aristarus of Samos, they got ignored. The idea that the sky spins around the earth seems obvious when you look up. And when great minds like those of the astronomer Tomy and the philosopher Aristotle supported that idea, well, people like Aristoarkus got left behind. The basic

thinking was that the moon, sun, and stars were affixed to crystal spheres that spun around the Earth at different rates. While it kind of sort of worked to predict the motions of objects in the sky, in detail, it was really unwieldy and failed to accurately predict how the planet should move. Still, Tomy's idea of a geocentric universe stuck around for well over a thousand years. It was the year 1543 when Nicholas Capernicus finally published his work proposing a sun- centered model, much like the one Aristarus had dreamed up 2,000 years previously. Unfortunately, Capernicus' [clears throat] model was also pretty topheavy and had a hard time predicting planetary motions. The last nail in geocentrism's coffin came a few years later when [snorts] astronomer Johannes Kepler made a brilliant mental leap based on observations by his mentor Tico Brahhe Kepler realized the planets moved around the sun in ellipses not circles as capernicus had assumed. This fixed everything including those aggravating planetary motion. It still took a while but helioentrism won the day and the

night too. This paved the way for Newton to apply physics and his newly created math of calculus to determine how gravity worked, which in turn led to our modern understanding of how the solar system truly operates. The sun being the most massive object in the solar system by far has the strongest gravity and it basically runs the solar system. In fact, the term solar comes from the word Saul for sun. We named the whole shebang after the sun. So there you go. The planets are smaller but still pretty huge compared to us tiny humans. At the big end, we have Jupiter, 11 times wider than the Earth and a thousand times its volume. At the smaller end, we have well, there is no actual smaller end. We just kind of draw a line and say planets are bigger than this. That's a bit unsatisfactory, I'll admit. But it does bring up an interesting point. I've been using the term planet, but I haven't defined it. And that's no accident. I don't think you can. A lot of people have tried, but definitions have always come up short. You might say something is a planet if it's big enough to be

round, but a lot of moons are round and so are some asteroids. Maybe a planet has to have moons. Nope. Mercury and Venus don't, and many asteroids do. Planets are big, right? Well, yeah, but Jupiter's moon Ganymede is bigger than Mercury. Should Mercury be stripped of its planetary status? I could go on, but no matter what definition you come up with, you find there are lots of exceptions. That's a pretty strong indication that trying to make a rigid definition is a mistake. It'll get you into more trouble than it'll help.

Planet can't be defined. It's a concept like continent. We don't have a definition for continent, and people don't seem to mind. Australia is a continent, but Greenland isn't. Okay, by me. So, that's what I tell people if they ask me if Pluto is a planet. I say, "Tell me what a planet is first, and then we can discuss Pluto." Pluto is what it is, a fascinating and intriguing world, one of thousands, perhaps millions more, orbiting the sun out past Neptune. I think that makes it cool enough. All the orbits of the planets lie in a relatively flat disc. That is, they aren't buzzing around the sun in all directions like bees around a hive. The orbit of Mercury, for example, lies in pretty much the same plane as that of Jupiter. That's actually pretty interesting. Whenever you see a trend in a bunch of objects, nature is trying to tell you something. In fact, there are other trends that are pretty obvious when you take a step back and look at the whole solar system. For example, the inner planets, Mercury, Venus, Earth, and Mars, are all relatively small and rocky. The next four, Jupiter, Saturn, Uranus, and Neptune, are much larger and

have tremendously thick atmospheres. In between Mars and Jupiter, is the asteroid belt, comprised of billions of rocks. There are lots more asteroids scattered around the solar system, but most are in the main belt. Then, out beyond the orbit of Neptune is a collection of rocky ice balls called kiper belt objects. The biggest are over a thousand miles across, but most are far smaller. They tend to follow the plane of the planets too. But if you go even farther out, starting tens of billions of kilometers from the sun, that disc of Kyper belt objects merges into a vast spherical cloud of these ice balls called the Orort cloud. They don't follow the plane of the inner solar system, but orbit every which way. So what do all these facts tell us about the solar system? We think they're showing us hints of how the solar system formed. 4.6 billion years or so ago, a cloud floated in space. It was in balance. its gravity trying to collapse it was counteracted by the meager internal heat that boyed it up. But then something happened. Perhaps the shock wave from a nearby exploding star slammed into it. Or maybe another cloud lumbered by and rammed it. Either way, the cloud got compressed, upsetting the

balance and gravity took over. It started to collapse. As it did, angular momentum became important. That's a lot like regular momentum when an object in motion tends to stay in motion. But in this case, it's a momentum of spin, which depends on the object's size and how rapidly it's rotating. Decrease the size and the rotation rate goes up. The usual analogy is an ice skater starting a spin, then drawing their arms in. Their spin is amplified hugely. The same thing happened in the cloud. Any small amount of spin it had got ramped up as it collapsed. It flattened into a disc, much like spinning raw pizza dough in the air will flatten it out. As it collapsed, material fell to the center, getting very dense and hot. Farther out in the disc where it was cooler, material started to clump together as little grains of dust and other matter randomly bumped into other little bits. As these clumps grew, their gravity increased and eventually started drawing more material in. These little blobs are called planetessimals. Wee baby planets. As they grew, so did the center of the disc. The object forming there was a protoar or spoiler alert, the proto sun.

Eventually, its center got so hot that hydrogen fused into helium, which makes a lot of energy. a lot of energy. A star was born. The new sun blasted out fierce light and heat that over millions of years blew away the leftover disc material that hadn't yet been assimilated into planets. The solar system was born. Closer to the sun, it was warmer. Hydrogen and helium are very light gases, and the warm baby planets there couldn't hold on to them. Farther out, there was more material in the disc, and the planets were bigger. Since it was cooler, too, they could hold on to those lighter gases, and their atmospheres grew tremendously, eventually outmassing the solid material in their cores. They became gas giants. There was also a lot of water out there far from the sun in the form of ice. Smaller icy objects formed past Neptune, but space was too big and random encounters too rare. They didn't get very big, maybe a few hundred km across. A lot of them, billions, perhaps trillions of them, got too close to the big planets and were flung hither and

young. Closer in, material between Mars and Jupiter couldn't get its act together to form a planet either. Jupiter's gravity kept agitating it, and impacts between two bodies tended to break them up, not aggregate them together. And there you have it. Our solar system formed from a disc sculpted by gravity. Echoes of that disc live on today, seen in the flatness of the solar system. This isn't guesswork. The math and physics bear this out. And not only that, we see it happening now today. When we look at gas clouds in space, we see stars forming. We see protolanetary discs around them. We see the planets themselves getting their start. We may think of ourselves as the solar system, but we're really just a solar system. The scenario that happened here so long ago plays itself out daily in the galaxy. We're one of billions of such systems. And remember, every atom in your body and everything you see around you, every tree, every cloud, every human, every computer, everything on Earth, even the Earth itself, was once part of that dense cloud. We are quite

literally star [music] stuff. Today you learn that the solar system is one star, many planets, a lot more asteroids, and even more icy comet-like objects. It formed from a collapsing cloud, which flattened into a disc, and [music] that's why the solar system is flat. Rocky planets formed closer to the sun and larger gas giants farther out. Icy objects formed beyond Neptune in a disc as [music] well, and a lot of them were flung out to form a spherical shell around the sun. We see the same thing happening out in the galaxy, too. The motions of the objects in this system [music] caused a lot of confusion to ancient astronomers, but we eventually figured out what's what. The sun is a star. That's a profound statement and one that's not really all that obvious.

Those little sparks in the night sky are pretty, but don't look anything at all like the hot blazing orb that lights up our days. It was a pretty remarkable intellectual leap to understand the sun and the stars are just different flavors of the same kind of object. The only difference is that the sun is close, but the stars are terribly far away, so they're fainter. Right away, let's clear up a misconception. A lot of people say the sun is a middlesized average star. But that's not fair. Sure, it's somewhere in the middle of the size range of stars, but the vast majority of stars are dim red dwarfs, far smaller than the sun. By size and number, the sun ranks in the top 10% of stars in the galaxy. In our solar system, it's clearly the dominant object. brighter, more massive, and more influential than anything else. But what is it? The sun is essentially a big hot ball of mostly hydrogen gas. It's 1.4 million kilometers across, more than a 100 times the Earth's diameter, and big enough that well over a million Earths could fit inside of it, and it's massive,

300,000 times more massive than the Earth, a staggering two octillion tons of gas. But if we want to truly understand the sun, we have to look into its [music] heart. by for mankind. [music] >> At the very core of the sun, conditions are hellish. The pressure is a crushing 260 billion times the Earth's atmospheric pressure and it's a searing 15 million degrees C. Under those conditions, hydrogen is completely ionized, which means the electrons in the atoms are stripped from their protons. This makes the core a thick soup of ultra hot subatomic particles.

In fact, the protons are squeezed together so hard by the octillions of tons of mass lying on top of them that an amazing thing happens. They fuse together through a complicated series of steps. The hydrogen atoms fuse together to form the heavier element helium. Along the way, some of the nuclear energy stored in those atoms is released. That amount of energy is described by Einstein's famous equation E= MC^² which states that mass can be converted into energy and vice versa. Atoms are pretty small though, so each helium atom made in the sun's core generates only a tiny bit of energy. But a lot of helium atoms are made. A lot. Get this. Every second of every day, the sun converts 700 million tons of hydrogen into 695 million tons of helium. The missing 5 million tons, the equivalent weight of 15 Empire State buildings, is converted into energy. And that's a lot of energy. Enough in fact to power a star. It's equivalent to detonating 400 billion 1 megaton nuclear

bombs every single second. That's millions of times the entire nuclear arsenal of our planet. Every second. And that's why even from a distance of 150 million km, the sun is so bright you can't even look at it. Even from that distance, its heat can be felt on your skin when you stand outside. Hydrogen fusion occurs in the core of the sun. The energy released heats the gas above the core, but not quite enough to fuse hydrogen into helium. Further from the sun's center, the gas becomes less dense, and at some point, the heat pouring up from below makes the gas buoyant. It rises in the same way a hot air balloon on Earth rises. This process is called convection, and it's an efficient way of transferring heat. Huge columns of rising hot gas stretch hundreds of thousands of kilometers high, bringing the sun's internal heat to the surface. The gas then cools and sinks back down into the interior. We can actually see the tops of these columns packed together across the sun's face. Above the convecting layer is a much thinner, cooler layer very near the

sun's surface called the photosphere or literally the sphere of light. This is where the density of the material inside the sun gets thin enough that it becomes transparent. Light can shine right through it. At this point, the energy from inside the sun is free to travel into space. It's this light that we see when we look at the sun. The sun is a gas and doesn't have a solid surface. But the gas in the photosphere thins so rapidly compared to the sun's huge size that you can think of it as the sun's surface. And there's one final layer above that, the ethereally thin corona. Sort of like the sun's atmosphere. It's less than 1% as dense as the photosphere, but actually much hotter.

Temperatures there can reach over a million degrees. However, it's so thinly dispersed that it's incredibly faint and can only be seen during a total eclipse or using special telescopes that block the intense light from the sun itself. The corona extends for millions of kilometers. And in a sense, it doesn't actually end. The corona merges into what's called the solar wind, a stream of subatomic particles moving away from the sun. It blows out in all directions, though mostly along the sun's equator. The speed of the wind is usually well over a million kilometers per hour. Yes, seriously. And can reach speeds even much higher than that. When hydrogen fuses into helium in the sun's core, the [music] energy is released in the form of light. This light immediately smacks into a subatomic particle which absorbs it, converts a little bit of the energy into motion and remits the light with a little bit less energy. The light works its way out of the sun this way, losing energy every time it encounters a particle until eventually it gets to the surface and is free to fly away into the

universe as a much lower energy photon of visible light. So, how long does this process take? I've seen different numbers for it, some as much as a million years. But a lot of those calculations don't model conditions inside the sun accurately. For example, they don't take into account the gas convecting for hundreds of thousands of kilometers. More modern calculations show that it takes closer to 1 or 200,000 years for the energy to work its way out. That's still a pretty long time. The light you see from the sun now got its start in the sun's core around the time Homo sapiens first appeared in Africa. The sun's surface is, to put it kindly, a mess. And the key to that mess is magnetism. I've been saying the sun is made of gas. But that's not entirely accurate. It's so hot inside the sun that electrons are stripped from their parent atoms in the gas, creating what's called a plasma, a gaseous soup of charged particles. We'll learn more about that in a later episode. But what's important now is the fact that a moving electric charge generates a

magnetic field. The interior of the sun is essentially all charged particles in motion. Convection coupled with the sun's rotation sets up rivers or streams of plasma inside the sun, each generating and carrying its own magnetic field. When this plasma reaches the sun's surface, their magnetic fields do, too. Maybe you've seen those looping arcs of magnetism around a bar magnet when it affects iron filings on a piece of paper. The solar magnetic fields are like that except there can be zillions of them all over the sun's surface where they can interact and even get tangled up. When the plasma reaches the surface, it cools. But if the magnetic loops tangle up, they prevent the plasma from sinking back down into the sun. Like a knot in a shoelace prevents it from going through the eyelid on your shoe. Plasma shines because it's hot. But as it cools, it dims. It sits on the surface dimming, producing a dark spot on the surface of the sun, which we call a sunspot. Sunspots can be huge. They commonly dwarf the entire Earth, and some are so big they can be seen without

using a telescope, as long as you're wearing adequate eye protection, of course. Around the edges of sunspots, the magnetic field lines are concentrated. This can energize the plasma even further, heating it up. This creates a bright rim around sunspots called facullet, Latin [music] for little torch. The dark parts of sunspots dim the overall light from the sun, but fullet can be so intense they compensate for that and even add more light. Ironically, sunspots actually increase the energy output of the sun. Plasma on the sun's surface can flow along these magnetic loops too. This can create huge arcs of material called prominences or filaments stretching for hundreds of thousands of kilometers across the sun looking like fiery arches. We think these magnetic field lines are feeding energy from the sun's surface into the corona, which is why it's so much hotter. It's not exactly clear how this happens, but scientists are following several leads right now. This long-standing mystery may soon be solved. Magnetic fields on the sun also have a huge amount of energy stored in them. You can think of them like very

tightly wound and very stiff springs. But remember, these magnetic field lines get tangled up. If conditions are right, they can actually snap, in essence, creating a gigantic short circuit. When this happens, all that vast energy stored in the lines explodes outwards all at once in an event we call a solar flare. Even an average solar flare is mindcrossingly powerful. A big one can release as much as 10% of the entire sun energy output. This explosion blasts out high energy light and launches material off the surface of the sun at high speeds, sending it into interplanetary space. Another type of solar eruption is called a coronal mass ejection or CME. It's similar to a flare, but if a flare is like a tornado, intense and localized. A CME is like a hurricane, huge and strong. Like flares, they form when tangled magnetic field lines erupt, blasting out energy, but they occur higher off the sun's surface. Both flares and CMEs eject material into space, billions of tons of it. In fact,

this blast of debris can hit the Earth, and when it does, there can be profound effects. Our atmosphere absorbs the high energy light, protecting us. Also, the subatomic particles are generally deflected by the Earth's magnetic field, so we're okay. But if conditions are right, the Earth's magnetic field can interact with the particles. Massive numbers are funneled down into Earth's atmosphere near the poles, causing the air to glow. This is what we call the aurora, or the northern and southern lights. Depending on the shape of the magnetic field, the auroras can form spectacular multicolored ribbons and sheets. Not all the effects are benign, though. As the magnetic fields interact, they can induce very strong currents of electricity in the Earth's crust. This can overload power grids, causing blackouts. In 1989, Quebec suffered a massive power outage from a solar storm. The very first such storm ever detected was in 1859, and it was also the most powerful ever seen. If an event like that were to happen today, it could cause worldwide blackouts and potentially be very damaging. Satellite electronics would be fried, too. And we

depend on those satellites for our modern civilization. In fact, in 2012, a huge storm, probably the equal of the 1859 event, blasted away from the sun in another direction, missing the Earth. Had it hit us, well, you probably wouldn't be watching this video now. We'd still be recovering. This is why studying the sun is so important. We depend on it for light and heat and the very basis of life itself, but it's entirely capable of knocking our society to its knees. Understanding it is critical to our future. The sun is the two octillion ton gorilla in the room. We need to respect that. Today you learn that the sun is a star powered by nuclear fusion in its core. Hot plasma moves inside the sun creating [music] magnetic fields which in turn can create sunspots, solar flares, and coronal mass ejections. These events can generate aurori on Earth, cause power blackouts, [music] and damage satellites. The Earth is a planet. That's a profound statement and one that's not really all that obvious. For thousands of years, planets were just bright lights in the sky,

one-dimensional points that wandered among the fixed stars. How could the Earth be one of them? With the invention of the telescope, those dots became worlds. And with spacecraft, they became places. The Earth went from being our unique home in the universe to one of many such well planets. The Earth is the largest of the terrestrial planets, the four smaller, denser, rocky worlds orbiting close into the sun. It's about 13,000 km across and has a single large moon, which we'll learn more about next week. Unlike the other three terrestrial planets, Earth has something very important. Water, or more specifically, liquid water on its surface where it can flow around, evaporate, become clouds, rain down, and then mix up chemicals so they can do interesting complex things like support life. Earth's ability to sustain life

depends on that water. It also depends on Earth's atmosphere. Of course, breathing has its advantages. And both, weirdly enough, depend on Earth's magnetic field to exist. And that in turn depends on what's going on deep inside our planet. So let's take a look. Like the sun, the earth is a many layered thing. At its very center is the core which actually has two layers. The inner core and the outer core. The inner core is solid and made mostly of iron and nickel. These are heavy elements and sank to the center of the planet when it was forming leaving lighter elements like oxygen, silicon, and nitrogen to rise to the surface. The solid inner core is about 1,200 km in radius or about 10% the radius of the Earth. The outer core is also mostly iron and nickel, but it's liquid. The material in it can flow. It's about 2200 km thick.

The temperature in the Earth's core is tremendously high, reaching 5,500° C. The pressure is huge as well, as you might expect with the weight of an entire planet sitting on top of it. You might think at such a high temperature, iron would be a liquid. But iron can stay solid if the pressure is high enough. In the inner core, the pressure is extremely high. And even though it's hot, iron is solid. in the outer core where it's still hot, but the pressure is a little bit lower. Iron is a liquid. Above the core is the mantle. It's about 2900 kilometers thick. The consistency of the mantle is weird. Most people think it's like lava, but really it's like very thick hot plastic. It behaves more or less like a solid, but over long periods of time, geologic periods of time, it can flow. We'll get back to that in a sec. On top of the mantle is the crust, a solid layer of rock. The overall density of the rock in the crust is less than in the mantle. So in a sense it floats on the mantle. There are two types of crust on earth. Oceanic crust which is about 5 km thick and continental crust which is a much beefier 30 to 50 km thick. Still the

crust is very thin compared to the other layers. The crust isn't a solid piece though. It's broken up into huge plates and these can move. What drives the movement of these plates is the flow of the rock in the mantle and that in turn is powered by heat. The core of the earth heats the bottom of the mantle. This causes convection. The warmer material rises. It's not exactly a speed demon, though. The rate of flow is only a couple of centimeters per year, so it takes about 50 or 60,000 years for a blob to move a single kilometer. The hot material rises toward the surface, but it's blocked by the crust. The magmatic rock pushes on the plates, causing them to slide around very slowly. Your fingernails grow at about the same rate the continents move. Over millions of years though, this adds up, [clears throat] changing the surface geography of the Earth. Where you see the continents now is not at all where they were millions of years ago. In some places, generally where the plates come together, the crust is weaker. Magma can push its way through, erupting onto the

surface, forming volcanoes. Other volcanoes, like Hawaii or the Canary Islands, are thought to be from a plume of hotter material, punching its way right through the middle of a continental plate. As the plate moves, the hot spot forms a linear chain of volcanoes over millions of years. Volcanoes create new land as material wells out, but they also pump gas out of the Earth, too. A large part of Earth's atmosphere was supplied from volcanoes. The interior of the Earth is hot. In the core, it's about as hot as the surface of the sun. Where did that heat come from? Most of it is left over from the Earth's formation more than 4.5 billion years ago. As rock and other junk accumulated to form the proto earth, their collisions heated them up. As the earth grew, that heat built up and it's still toasty inside even today. Also, as the earth formed and gained mass, it began to contract under its own gravity and this squeezing added heat to the material. Another source is elements like uranium deep inside the earth which add heat as the atoms radioactively decay. And a fourth source of heat is from dense material like iron and nickel

sinking to the center of the earth which warms things up due to friction. All of these things add up to a lot of heat which is why after all these billions of years the earth still has a fiery heart. The outer core of the earth is liquid metal which conducts electricity. The liquid convects and this motion generates magnetic fields similar to the way plasma in the sun generates magnetic fields. The Earth's rotation helps organize this motion into huge cylindrical rolls that align with the Earth's axis. The overall effect generates a magnetic field similar to a bar magnet with a magnetic north pole and south pole which lie close to the physical spin axis poles of the Earth.

The loops of magnetism surround the Earth and play a very important role. They deflect most of the charged particles from the solar wind and they trap some too. Without the geomagnetic field, that solar wind would hit the Earth's atmosphere directly. over billions of years that would erode the Earth's air away like a sand blaster stripping paint off a wall. Mars, for example, doesn't have a strong magnetic field, and we think that's why its atmosphere is mostly gone today. But we do have an atmosphere, and it's more than just air blowing around. Earth's atmosphere is the layer of gas above the crust. Because it's not solid, it doesn't just stop. It just sort of fades away with height. By accepted definition, and by that I mean it's not really science, it's more of a let's just do it this way kind of a thing. The line between Earth's atmosphere and space is set at 100 km up. This is what's called the Carmon line. And if you get above it, congratulations, you're an astronaut. The atmosphere is by volume about 78% nitrogen, 21% oxygen, 1% argon of all things, and then an assortment of trace gases. There's

water vapor, too. Almost all of it below a height of about 8 to 15 km. This part of the atmosphere is warmest at the bottom which means we get convection in the air creating currents of rising air which carry water with them forming clouds which in turn is why we have weather. At a height of about 25 km on average is a layer of ozone a molecule of oxygen that's good at absorbing solar ultraviolet light. That kind of light can break apart biological molecules. So the ozone layer is critical for our protection. Incidentally the Earth's magnetic field does more than trap solar wind particles. It also channels some of them down into the atmosphere where they slam into air molecules about 150 km up.

This energizes the molecules which respond by emitting light in different colors. Nitrogen glows red and blue, oxygen red and green. We call this glow the aurora and it happens near the geomagnetic poles far north and south. The lights can form amazing ribbons and sheets depending on the shape of the magnetic field. I've never seen an aurora someday. You may not be aware of the atmosphere unless the wind is blowing, but it's there. It exerts a pressure on the surface of the Earth of about a kilogram per square centimeter or nearly 10 tons per cubic meter. There's roughly a ton of air pushing down on you right now. You don't feel it because it's actually pushing in all directions down to the sides even up. And our bodies have an internal pressure that balances that out. The Earth also has liquid water on its surface, unique among the planets. The continental crust is higher than oceanic crust, so water flows down to fill those huge basins. The Earth's surface is about 70% covered in water. Most likely, some of this water formed when the Earth itself formed, and some may have come from comet and asteroid impacts billions of years ago. The exact proportion of locally sourced versus extraterrestrial

water, is still a topic of argument among scientists. Earlier, I mentioned trace molecules of gas in the atmosphere. One of these is carbon dioxide, which only constitutes about 0.04% of the lower atmosphere, but it's critical. Sunlight heats the ground, which emits infrared light. If this infrared light were allowed to radiate into space, the Earth would cool. But carbon dioxide traps that kind of light and the Earth doesn't cool as efficiently. This so-called greenhouse effect warms the Earth. Without it, the average temperature on Earth would be below the freezing point of water. We'd be an ice ball. This is why climate scientists are concerned about carbon dioxide. A little is a good thing, but too much can be very dangerous. Since the industrial revolution, we've added a lot of the gas to our atmosphere, trapping more heat. By every measure available, the heat content of the Earth is increasing, upsetting the balance.

It's melting glaciers in Antarctica and Greenland, as well as sea ice at the North Pole. Sea levels are going up and some of the extra CO2 in the air is absorbed by the oceans, acidifying them. There's an old concept in science fiction called terraforming, going to an uninhabitable alien planet and engineering it to be more Earthlike. I don't know what the opposite process would be called, but it's what we're doing to Earth right now. The Earth is the only habitable planet in the solar system. And you know what? We should keep it that way. Today, you learn that the Earth is a planet with a hot core, a thick layer of molten rock called the mantle, and a thin crust. The outer core generates a strong [music] magnetic field, which protects the Earth's atmosphere from the onslaught of the solar wind. Motion in the mantle creates volcanoes, and the surface is mostly covered in water. The Earth's atmosphere is mostly [music] nitrogen, and it's getting warmer due to human influence.

The moon is pretty hard to miss when it's up at night. It's big and bright and really very pretty. Even cooler, it's the closest astronomical object in the universe to us and arguably the one we know best at least for mankind. [music] >> I hate to break it to you, but we humans are pretty egotistical. Even though there are over 160 known moons of decent size in our solar system, we call ours the moon with a capital M, like it's the only one or the most important one. And I guess to us it is the most important one. It's only the fifth largest moon in the solar system in absolute size. But compared to Earth, it's pretty big, fully a quarter of the diameter of the planet. That's a way bigger ratio than for any other moon orbiting a major planet in the solar system. By the numbers, the moon is about 3470 km in diameter and on average about 380,000 km from Earth. That means it looks pretty big in our sky, but probably not as big

as you think. When the moon rises, it can look huge and looming on the horizon, like you could fall into it. Ah, but that's not really the case. It's easy to show it's the same size on the horizon as it is high overhead. Yet, people think it looks twice as big when it's on the horizon. This is the well-known moon illusion, and it's due to two factors. How we perceive the sizes of objects and how we perceive the sky. We don't really see the sky as a hemisphere over our heads, but more like a flattened bowl with the horizon farther away than the zenith. So, when the moon's on the horizon, our brains are convinced it's farther away. But if it's farther away, it must be physically bigger to appear that size, right? So, our brain interprets the moon size as huge. It's an illusion, but a convincing one. In fact, the moon is about the same size as a small pill held up at arms length. Our satellite is a lot smaller than you think. The internal structure of the moon is roughly similar to the Earth. It has a solid iron inner core, a liquid outer core, a thick mantle, and a crust of lighter material on top. The core is small, probably 350 km in

radius, and still hot, though not as hot as Earth's core. The lower mantle may be a thick fluid, but unlike Earth's mantle, the upper part is solid. The easiest part of the moon to observe is its surface, so of course we know the most about that. The near side, the side we see from Earth, is divided into two distinct types of regions. Highlands, which are heavily cratered, and Maria, Latin for seas, which are darker and smoother. The craters in the highlands are from countless impacts over the eons from asteroids and comets. Rocks from these regions have been dated to be well over 4 billion years old, nearly as old as the moon itself. The Maria, on the other hand, don't have nearly as many craters. They're younger, but that's relative. Rocks from those areas appear to be roughly 3 to 3.5 billion years old on average. The Maria are made of darker basaltic material, which means they probably formed from lava flooding older areas. For most of human history, the far side of the moon was hidden from us.

But in 1959, the Soviet Union flew the Luna 3 space probe around the moon, photographing the far side for the first time. Everyone expected it to look a lot like the near side, but shockingly, it's vastly different. There were almost no Maria. That's weird. The near side is covered with them. And not only that, observations from subsequent lunar probes showed the crust on the lunar far side is thicker than it is on the near side as well. The moon is starting to stack up mysteries. Why is it so big compared to Earth? Why are the near and far sides so different? It turns out the composition of the crust has a lot of similarities to Earth, but also a lot of differences as well. Why? It turns out the answers to these questions are all related to how the moon formed 4 1/2 billion years ago. A lot of ideas have been proposed to figure out how the moon formed, but the best one going these days is called the giant impact hypothesis. When the solar system formed, there were a lot more objects orbiting the sun than there are now. These ranged in size from grains of dust up to objects the size of planets, and that meant there were lots of

collisions. No doubt the Earth suffered many such impacts. But one happened late in Earth's history, relatively speaking, roughly 50 million years after things started to settle down. A Mars-sized planet given the unofficial name Thea slammed into our young planet. But it wasn't a head-on impact. It was a grazing collision. The impact was colossal, blasting a huge amount of material from both planets into space. Most of it came from the outer layers of the Earth. Since the collision was a grazing one, this material rapidly coalesed into the moon. And that explains why there's some but not total similarity in composition to Earth. It used to be part of the Earth, but some of it came from the other planet, too. And we may even have some evidence of it. Some rocks from the moon show a peculiar ratio of different kinds of oxygen atoms called isotopes. It's possible some of the more exotic isotopes of oxygen came from the other planet. Actually, this idea has been around for a long time, ever since the Apollo missions in the 1970s. There's a lot of evidence to support it, too. But a new twist on it goes farther. When the

moon formed from the debris of the collision, it was close to Earth, probably 20,000 km away. The collision was so violent that it completely melted the Earth and the moon was molten too. Tidal forces rapidly synchronized the moon's spin and orbital period so that one face always pointed toward Earth. If you were on that side of the moon, the Earth would dominate the sky, appearing 80 times larger than the moon appears in our sky today. And the Earth was hot. It hung there like a blast furnace and would have seriously heated the near side of the moon. The far side would have been much cooler. the material on the near side would have vaporized and a lot of it would have condensed on the far side. Over time, this created the thicker farside crust we see today. Then there was another seriously terrifying event, the late heavy bombardment, a period of intense collisions with comets from the outer solar system, probably triggered by the movement of outer planets. This reign of millions of comets left scars on all the inner worlds, including the moon. That's when most of the highland cratering occurred. Some of these collisions were massive,

carving gigantic craters on the moon. Later, lava bubbled up through cracks in the moon's surface, flooding these craters, creating the maria we see today. Ah, but the far side's crust was thicker. Impacts there couldn't get as deep, and it was harder for lava to break through and bubble up. That explains why there are so few Maria there. This idea that the Earth cooked the moon and formed the crust anomaly is pretty new and is still being argued over. There are also competing hypotheses about this as well. As we gather more evidence, we'll eventually get a better understanding of just why the moon's crust is so weird. After all that, I have to say it's a little unfair to divide the lunar surface today into just highlands and maria. For example, craters are pretty diverse. Some are small and simple bowl shapes. Others are huge, dozens of kilometers across and more, punctuated with a central peak, a mountain in the middle that formed as material from the gigantic impact that formed it splashed back up in the middle. Double craters are common, too.

probably formed when binary asteroids hit the moon. There are also crater chains, long lines of small craters that may have formed when a nearby large impact splashed out long streamers of material. Rays are common from big craters, too. These are extremely long splash marks pointing radially out from the impact, probably formed when plumes of material were ejected. Some of the brightest are from the crater Tao in the moon's southern hemisphere, and they stretch for 1,500 km. The material is a bit more reflective than the surrounding terrain, so they appear bright in contrast and are one of the most outstanding features visible near the time of the full moon. And there's so much more. Long sineuous rills like dry river beds wind their way across the surface. Actually, the sightes of ancient lava flows. There are lava tubes as well where the top of a lava flow cooled and formed a tunnel through which lava could flow for long distances. Sometimes the roof as it's called collapses and forms a skylight and we can see into the otherwise hidden tunnel. There are also cliffs, mounds, ancient and quite dead volcanoes, even

mountain chains. There's no tectonic activity on the moon, but mountain ranges formed at the edges of giant impacts where the huge forces unleashed pushed up the rocks at the crater rim. And new observations show there's something else on the moon, too. Water. Deep craters near the moon's poles have floors that never see sunlight. Comet impacts can distribute water all over the moon. But most of it is destroyed by sunlight, but it can collect in these dark craters. And studies show there could be over a billion tons of water there in the form of ice. That would be a huge boon to colonization. Water is heavy and really expensive to transport from Earth. If it's already on the moon, that makes it a lot easier to put people there. And that's in our future. I have no doubt. We're starting to get serious about going back to the moon. NASA has plans of returning and other countries like China and India are looking moonward as well. It's not a matter of if, but when. And it could be soon. So soon that I'd bet there's a really good chance the next person to step foot on the moon is already alive. Perhaps some teenage student in some country is just

now taking an interest in science, math, engineering, an interest that will one day lead to another giant leap for humanity. Today you learn that the capital M moon is a lowercase M moon. It's big compared to Earth and is thought to [music] have formed when a small planet impacted Earth at a grazing angle. It's heavily cratered and has huge flood planes on it called Maria. [music] There's water there, too, just waiting for us to thaw it out and drink it. Mercury is the closest planet to the sun, as you might expect. That makes it pretty hot, but also it's pretty cool.

>> [music] >> There are seven naked eye solar system objects in the sky. Mercury, Venus, Mars, Jupiter, Saturn, the sun, and the moon. Seven. Each of them was associated with a god in ancient times. Mercury was the Roman messenger of the gods, fleet of foot. Literally, he had wings on his shoes and a rapid traveler. To anyone who's seen Mercury in the sky, this affiliation with the swift god is no surprise. Mercury, the planet, moves pretty quickly, visibly changing its position relative to the background stars, even after a single night.

Despite its speed, the planet never gets very far from the sun. At best, it can reach a separation of about 28°. That's about three times the apparent size of your fist held at arms length. In 1639, the Italian astronomer Giovani Supi used a telescope to observe Mercury, and he discovered it undergoes a complete cycle of phases over time, just like the moon does. The only way that can happen is if Mercury orbits the sun and not the Earth. Another check mark in the column for helioentrism, which was starting to look better and better all the time. And of course, that's the way things really are. Mercury is the innermost of the planets in the solar system. It orbits the sun at an average distance of about 58 million km, roughly a third the distance of the Earth from the Sun.

That's why we never see it stray far from the sun. From our viewpoint, its smaller orbit keeps it huddled closer to our star. That's why we see it move so rapidly, too. It's closer to the sun, so the gravity it feels from the sun is stronger, and therefore its orbital velocity is faster than Earth's. It orbits the sun once every 88 days. And that's also why we see it undergo phases. When it's between us and the sun, we're looking at its dark side, and when it's on the other side of the sun, we're looking at its fully illuminated half. In between, it goes through the same phases as the moon. Crescent, half full, gibbus, and so on. Not that this is such an easy observation to make because it never gets far from the sun. It's always low to the horizon after sunset or before sunrise. When we observe it, we're looking through all of the muck and turbulence in our air. So, it's usually pretty fuzzy. Making matters worse, it's a dinky planet, only about 4,900 km in diameter, about a third the Earth's width. One upside to all this is that because it's close to the sun, it's illuminated fiercely and can be pretty bright even near the horizon. If you ever get a chance to see it, you really should. It's pretty cool.

Mercury's orbit is weird. It has the most elliptical orbit of any planet, ranging from 46 to nearly 70 million kilometers from the sun. When it's closest to the sun, it receives more than twice as much light and heat as when it's furthest. Mercury is too small and difficult to observe to see surface features on it, which for a long time made it impossible to figure out how long its day is. Astronomers assumed that the tides from the sun had locked Mercury's spin so that its day was equal to its year, just like our moon spins once for every time it goes around the Earth. However, in 1965, astronomers used Doppler radar to observe Mercury and directly measure its spin, and they got a surprise. Its day was only 59 Earth days long, not 88. But that's a significant number as well. To be more exact, the actual length of Mercury's year is 87.97 days, and the actual length of its day is 58.65 Earth days. If you divide those two numbers, you see their ratio is almost exactly 2/3. It turns out there's more than one way to tidly lock the rotation

of a planet to its orbit. Remember earlier when I said Mercury's orbit is highly elliptical. The tides from the sun are far stronger on Mercury when it's at perihelion, the closest point in its orbit to the sun, than when it's at ahelon, the farthest point in its orbit. After Mercury first formed, tides from the sun slowed its rotation, just like the Earth's tides on the moon slowed the moon down as well. But at some point, Mercury's spin slowed to where it was 2/3 of its orbital period. So at one perihelion pass, one side of Mercury faces the sun. Then 88 or so days later, it approaches perihelion again, but it's spun one and a half times. And this means the exact opposite side of Mercury faces the sun at this closest approach. 88 days later, Mercury is spun one and a half times again, and the whole thing repeats. It turns out that's a perfectly legitimate stable configuration, just like the one spin orbit setup. The way the physics works out, tides like simple multiples. Once the day became twothirds the period of the year forced by Mercury's elliptical orbit, the tide stopped slowing it and things have been that way ever since. Mercury's

elliptical orbit together with the 2 to3 spin to orbit ratio make for a very very weird day on Mercury. If you stay in one spot, it takes the sun two Mercury years, 176 days, for the sun to go around the sky once. That's because if you're on the side of Mercury facing the sun at one perihelion, the other side will face it one year later. It'll only be after the second year ends that you're facing the sun again. But it gets weirder. Mercury's spin is constant. It doesn't speed up or slow down. However, its motion around the sun is faster at perihelion than aphhelion. At ahelion, Mercury's spin is a bit faster than its orbital speed. So, the sun moves rapidly westward across the sky. But at perihelion, Mercury's motion around the sun actually more than compensates for its spin. So, the sun appears to stop in the sky and actually move backwards for a few days. Then, as Mercury pulls away from the sun, its orbital velocity slows down and the sun starts to move west once again as the planet's rotation dominates. If you're at just the right spot on the planet surface, this means

you could actually watch the sun rise, slow, stop, set again, then rise again, and you think time zones on the Earth are a pain. Mercury is hard to observe from Earth, and much of what we know about it is due to observations from space probes sent there. Mariner 10 made three flybys of Mercury in the 1970s and mapped about half the surface. We learned that it had almost no atmosphere and was therefore unsurprisingly covered in craters. In 2011, the Messenger probe entered orbit around Mercury after making a series of close flybys. The pictures it returned were breathtaking and revealed a world that has seen a lot of pummeling over the eons. It's covered in craters pole to pole, some hundreds of kilometers in diameter. The largest is called Basin, a whopping huge impact feature 1,600 km across. There are some smoother planes on the planet surface, too, which appear to be older than the cratered regions. These planes are covered in cracks called rupes. These are compression folds, like wrinkles on a fruit rind that's dried out. Apparently, as Mercury's interior cooled after it formed, the planet shrank and the crust cracked as it tried to shrink

as well. Several of the craters have extensive ray systems, like on our moon. These are formed when impacts fling out long plumes of material that then settle down on the surface. One of my favorite things of all about Mercury. Craters are named after artists, musicians, writers, painters, and more. So we have craters like Bacelli, Czechov, Debusi, Dega, Okio, Sibelius, Vivaldi, and Zola. There's even one named Tolken. Dipping below the surface, we can only infer what Mercury's internal structure is like. But the planet's dense, nearly as dense as Earth. We know the surface is rocky. So to be as dense as it is, it must have a large iron core, far larger in proportion to the planet than Earth's. Mercury's core may reach 3/4 of the way to the planet's surface. Why does it have such a high proportion of iron? Mercury may have formed as a larger planet, then got blasted in a huge grazing impact that blew away the lighter materials that had risen to the surface, leaving behind the denser part.

Or maybe the heat of the still forming sun vaporized the lighter materials off its surface. Mercury has a measurable magnetic field, which is a bit surprising since it rotates so slowly. Rotation plays a big part in the sun's and Earth's magnetic fields, but that fits with so much of its interior being molten iron. The bigger core may allow for a stronger field despite its slow spin. It doesn't have much of an atmosphere, but there is a trace of one, mostly due to its magnetic field trapping the solar wind and to material flung up from the surface after violent impacts from comets and asteroids. A lot of this material blown off the surface escapes the planet and gets blown away by the solar wind and pressure from sunlight. It forms a long comet-like tail that is tens of millions of kilome long. This tail is comprised of elements like sodium, calcium, and magnesium, material that's known to be abundant on the surface. Speaking of which, here's a fun fact. Poundforound, impacts on Mercury are more violent than they are on Earth. Mercury has weaker gravity, so it doesn't pull in impactors as hard as the Earth does. but it orbits the sun

far faster than Earth does. So, asteroids and comets tend to hit at higher velocity. That makes the explosive energy higher, making craters bigger. And there's one more surprise Mercury has. And it's really surprising. Despite being so close to the sun and having a surface temperature that can reach 430° C, 800° F, astronomers have found water ice on Mercury. It exists in the bottoms of deep craters near Mercury's poles where sunlight never reaches. These are called cold traps, and temperatures there don't get above -70° C. It's not known for sure where the water comes from, but it's likely to be from comets and asteroids that impacted the planet, scattering the water across the surface. Of course, in the harsh heat, that water just goes and goes away. But in those deep craters, it can persist, accumulating over the eons. There may be billions of tons of it there. It's bizarre to think that in one of the hottest places in the solar system, there can be conditions so cold ice can exist. But one thing we've

learned about nature over and over again. It has a lot more imagination than we do. Today, you'll learn that Mercury is the closest planet to the sun. It's airless and dense and is covered with craters. Its rotation is locked to its orbit in a 2:3 ratio and together with its elliptical orbit makes a day on Mercury very long and very weird. And [music] despite being very hot, there's actually water ice in deep craters at its poles. Venus was the Roman goddess of love. And honestly, if you've ever seen it after sunset, hanging like a diamond in the twilight, the planet Venus is stunningly beautiful. The only problem, Venus is hell.

>> Venus is the second rock from the sun. closer to the sun than Earth is. It's the third brightest natural object in the sky after the sun and moon. And it's shockingly bright when you see it. In fact, a lot of the time when people see it, they think it's an airplane. Some folks, and this is true, even report it as a UFO. Like Mercury, it never gets far from the sun. At best, it can reach about 45° away. If you want to see it, the best time to look is after sunset or before sunrise. Through a telescope, Venus can be seen to undergo phases just like the moon and Mercury do, too. When it's on the far side of its orbit from Earth, we see the illuminated face, so it looks full. As it rounds the sun and approaches Earth in its orbit, it gets bigger from our point of view because it's getting closer. But it also waines its gibbus, then half full, then a crescent. When it gets between the sun and Earth, it's as close as it can get to us, about 45 million kilometers away. It's a very thin crescent at that point, but close enough to us that you can see the phase easily with just binoculars.

Ironically, even though we see only a small portion of it illuminated, then it appears so big that it's actually brighter than when it's full but far away. The orbit of Venus around the Sun is tipped with respect to the Earth's by a little over 3°. So, like the moon is seen from Earth, Venus usually passes above or below the Sun in the sky as it laps the Earth and pulls ahead in its orbit. But again, just like with the moon, sometimes things line up just right and Venus passes directly across the Sun's face. This event is called a transit. During a solar eclipse, the moon can completely block the sun. But Venus is much farther away, so it can only block about a tenth of a percent of the sun. We see the planet as an inky black circle against the sun's face.

Transits were very important centuries ago. Scientists realized that by precisely timing the start and end of a Venus transit, and applying a little geometry, they could figure out how far away Venus was and use that like a meter stick to measure the size of the entire solar system. Grand expeditions were sent to remote parts of the Earth where the transits were visible. But it turns out Earth's atmosphere blurs the disc of Venus just enough that exact timing of the transit isn't possible. Scientists had to wait until we could use radar and other methods to get the exact distances to planets in the solar system. But transits are still pretty amazing to watch. They're very rare. The way the orbits of Earth and Venus align, they happen in pairs separated by 8 years, but then don't happen again for over a century. The last pair of Venus transits happened in 2004 and 2012. I was fortunate enough to see both the 2004 and 2012 transits and they were totally cool. But the next one, December 10th, 2117. If you missed the last two, sorry about that. Venus is actually so bright sometimes that it can be seen in broad

daylight. I've done this myself. So, here's an obvious question. Why is Venus so bright? Telescopic observations show it to be a nearly featureless white disc, completely covered in a thick cloud layer, reflecting almost all of the sunlight that falls on it. Okay, great. So, that's why it's bright. But then, why are its clouds so thick? Well, it turns out that Venus might be nice to look at from a distance. But up close, yikes. In some ways, Venus is the planet in the solar system most similar to Earth. It's only a wee bit smaller, 12,100 km across, about 95% the width of Earth, and has 80% the Earth's mass. But if it's a twin of Earth, it's the evil twin. Venus is closer to the sun. At a distance of about 110 million kilometers, 2/3 the distance of the Earth from the Sun, you'd expect it to be warmer. What you might not expect is that its surface temperature is 460° C. That's 860° F, hot enough to melt lead.

And it gets worse. Its air is almost entirely composed of carbon dioxide. And atmospheric pressure on Venus is a crushing 90 times that of Earth's. As if that's not bad enough, it rains sulfuric acid, too. But it's so hot the drops evaporate before hitting the ground. Yep, Venus is hell. Why? How did it get so hot? It turns out Venus is a victim of a runaway greenhouse effect. A long time ago, Venus may have been more temperate and might even have had liquid water oceans. But the sun was cooler when it was younger, and as it aged, it got hotter. At some point, the thinking goes, the sun warmed Venus enough that its oceans started to evaporate. Water vapor is an excellent greenhouse gas.

So, Venus's heating accelerated. It got so hot the oceans boiled away. Any carbon dioxide dissolved in the water was released to the atmosphere, heating the planet even further and thickening the atmosphere hugely. It got so hot that the CO2 even got baked out of the rocks, which was like, well, throwing gasoline on a raging fire. The greenhouse effect raged out of control at that point. Venus lacks a magnetic field, so it has no protection from the solar wind blasting past it. Over billions of years, this stripped a lot of the lighter elements from the atmosphere as well as all of the water.

The resulting atmospheric chemistry is pretty screwy compared to Earth. Sulfur dioxide is a popular molecule there, and clouds form from sulfuric acid. Those clouds are high off of the surface and very white and reflective. It's pretty incredible to think about. The sun warmed, so Venus warmed, and then a cascading series of events led to it being a ridiculously hostile environment due to the runaway greenhouse effect. Even though Mercury is closer to the sun, Venus is way hotter. Given that, the last thing you might expect on Venus is snow. Well, possibly. Observations show that there's something shiny on the mountain tops and snow fits the bill, but it's not water ice like on Earth. As the thinking goes, at lower elevations, minerals like bismthanite and galina would be vaporized by the heat. They'd then circulate up into the atmosphere and deposit onto the surface at mountain tops where it's cooler. If this is correct, then Venus is so bizarre it literally snows metal there. There's other weird stuff going on at Venus, too. For one thing, it has an incredibly slow rotation rate. One day on Venus is about 243 Earth days. It rotates so

slowly that at its equator, you could jog faster than the planet spins. Its slow spin is probably why it has no magnetic field, too. Since rotation is critical in generating the magnetic fields of objects like the Earth and Sun, not only that, but the planet spins backwards. This is called retrograde motion. And in a sense, it means Venus's north pole got swapped with its south pole. It flipped over. It's not clear why that happened. A lot of ideas have been proposed, including a giant collision that skidded the planet spin to a halt. But as of right now, Venus's lazy rotation is still not completely understood. If you could survive it, living on Venus would be really odd. The thick clouds mean it's about as bright as twilight on Earth, despite being closer to the sun. The thick atmosphere also means surface temperature is about the same everywhere on the planet pole to equator. Due to its backwards rotation, the sun would rise in the west and due to the slow rotation, its day is actually longer than its year. Calendars on Venus would be a mess. But calendars would probably be the least of your worries if you had to live there.

Incidentally, Venus has no moon. Being closer to the sun, it's possible that any moon it may have once had would have been perturbed by the sun's gravity and was eventually tossed out of Venus's grasp. It's also possible it simply never had one to begin with. And here's another fun fact. Venus is the most spherical of all the planets. Because it rotates so slowly, it doesn't bulge out at the equator from centrifugal force like the Earth does. The diameter from pole to pole is almost exactly the same as it is through the equator. We don't have a lot of data about the interior of Venus, though given its similar composition and size to Earth, it's not too crazy to think it has a core, mantle, and crust like we do. But even given the weird atmosphere, the surface of Venus is really different than ours. Venus doesn't have tectonic activity like Earth does. It's thought that water helps drive that, and Venus long ago lost its water to the greenhouse effect. Still, the surface appears to be very young. About a thousand impact craters have been found, and they're evenly distributed around the planet. The majority appear to be in pretty good shape. They haven't been eroded much, indicating they aren't terribly old. Not

only that, but the erosion we do seems to be about the same for all these craters, indicating they're all roughly the same age. That's weird. This all points to some sort of catastrophic event that resurfaced the planet roughly half a billion years ago. Any features older than that were wiped out and then big impacts over time created the craters we see now. But what could repave an entire planet? A big clue can be found by counting volcanoes on Venus. There are 167 bigger than 100 km in diameter. A huge number. These volcanoes could over time pump out enough lava to cover the entire surface of Venus.

There's also a lot of indirect evidence that volcanic activity is ongoing now today. One mountain on Venus called Idon Mons is seen to be abnormally warm, for example, indicating it might have magma under its peak. Also, sulfur dioxide levels dropped a lot in the 1980s, which may indicate a big volcanic event happened in the 1970s, blasting out lots of the gas, which then subsided. It's possible, though somewhat speculative, that the entire planet is a super volcano, a ginormous pressure cooker of barely constrained magma. Every few hundred million years, the whole surface lets go in a colossal eruption covering everything in lava. Yikes. Some volcanic features on Venus are different than on Earth. Without tectonics, slow bubbling leaks of lava from the interior of the planet can continue in one spot for a long time. This creates what are called pancake domes. Huge, flat, low domes. Many are dozens of kilometers across, but less than a kilometer high. The lava that formed them was probably really

viscous, which is why the domes are flattened and spread out. The searing surface temperature of Venus probably kept the lava hot and helped it spread out more, too. Oh, and one final note. Under international agreement, surface features on Venus, mountains, plains, craters, and so on are all named after women or goddesses of various cultures. This is an homage to the planet itself, being named after a goddess, of course, but also it's just cool. Today, you learn that Venus is the same size as Earth, but with a super thick atmosphere. A runaway greenhouse effect makes it the hottest planet in the solar system. It has the slowest rotation of any planet and spins backwards.

Tremendous volcanic activity ages ago resurfaced the entire [music] planet, and it still may be active today. Hey folks, I'm Phil Plate and this is Crash Course Astronomy. You know, I can't think of any object in the sky that's captured our imagination like Mars has. The red planet was once thought to be the god of war and in more modern times has been the setting for a zillion science fiction novels, movies, TV shows, and more. And now that we've gone there, landed there, roved there, it's become far more than a simple background for an alien invasion story. It's become a world, a place, and maybe one day a destination.

Mars is the fourth planet from the sun and the outermost of the terrestrial planets. That is ones that are rocky and similar in size to Earth. It orbits on average a little over 200 million km from the sun. That puts it outside Earth's orbit and it never gets much closer to us than about 60 million km. It's colder than us, too, with an average surface temperature of about 60 below 0 C. And it's not a terribly big planet. It's only about half the size of Earth. Still, when it's closest to us in space, it shines brightly in our night sky. A red beacon glowering like an angry eye. No doubt this is why ancient civilizations associated it with war. But it turns out Mars isn't red because it's bloody. It's red because it's rusty. Literally, that red color you see comes from fine grain dust on the surface, almost like ruddy talcum powder. The dust is rich in iron that's oxidized, forming rust. The dust coats a lot of the surface giving it a butterscotch or ochre coloring and also gets blown into the atmosphere. A lot of the surface of Mars is also gray volcanic rock called basaltt and together with other minerals gives Mars

its overall reddish look. We can get a decent view of Mars from Earth using telescopes. But small features are maddeningly difficult to tease out. The idea of a clement environment on Mars stuck with us though at least until the 1960s and 70s when we started sending probes to the planet. They did not see a lovely habitable world. Instead, what we got were photos of a dry, dead, forbidding planet. The surface geography of Mars is weird. There's a huge dichotomy between the northern hemisphere, which is mostly smooth plains, and the southern, which is cratered hilly highlands. Apparently, Mars suffered a tremendous impact from an object hundreds of kilometers across eons ago. It left behind a vast basin near the north pole of the planet, which filled with lava. Topographic maps show the northern hemisphere has much lower elevations than the southern and can be depressed by several kilometers. Walking from the south to the north pole is essentially all downhill. Another large feature is the Tharsus bulge, a huge plateau that's home to the four biggest volcanoes on Mars and the largest volcano in the solar system, Olympus

Mons. Mars doesn't have plate tectonics today, but there's evidence it once did. Tharsus was probably over a hot spot, a plume of hotter material rising up through the planet's mantle. That's what may have created the bulge. And as the plate slowly moved, the plume punched through the crust to create the chain of three smaller, but still huge volcanoes. But the grandest of the surface features on Mars is easily Val Marinys, a canyon discovered when the Mariner 9 probe orbited Mars in the 1970s. It's a gigantic crack in the surface of Mars.

4,000 km long, 200 km wide, and 7 km deep. That's 10 times longer, and 10 times wider than the Grand Canyon. Unlike the Grand Canyon, it wasn't carved by water. It may have formed when the Tharsus bulge rose up, creating the valley as a radial crack in the surface. Mars, like Earth, has polar ice caps. Both are mostly water ice several kilm thick, but they get seasonal coatings of dry ice, frozen carbon dioxide, that covers them from 1 to 8 m thick. This happens in their respective winters. In the summer, sunlight thaws the CO2, turning it directly into a gas, which then blows away from the pole, generating fierce winds. Speaking of which, Mars has an atmosphere, but it's thin. Pressure at the surface is less than 1% of Earth's, and the air is mostly carbon dioxide. In fact, as much as a third of the Martian atmosphere freezes out every winter to coat the polar ice caps. The air doesn't provide much of a shield from asteroid and comet impacts. So, the surface is heavily cratered and it still gets hit today. The space probe Mars Reconnaissance

Orbiter has been circling Mars so long, it's actually seen new craters from fresh impacts on the surface. However, the atmosphere is substantial enough to interact with the surface. Winds blow seasonally, filling craters with the ubiquitous dust. There's also sand on Mars made of eroded basaltic rock, so it's gray. The wind blows this into beautiful dunes, including series of parallel ridges and crater floors and bark or horseshoe shaped dunes. Mars also gets dust devils, towering vortices of winds similar to tornadoes. These dust devils have been seen from orbit, and when they blow the red dust around on the grayish basaltic plains, they can leave behind incredibly complex and beautiful curly cues. One of my favorite recent discoveries about Mars is that it has avalanches. There are lots of cliffs towering above the surface. And in the spring, when buried frozen carbon dioxide thaws, it can dislodge material, creating tremendous cascades of rock and dust. Several of these have been caught in the act by orbiting space probes. Mars has [music] two moons, small potato

shaped rocks named Daimos and Phobos. Both are tiny. Phobos is about 25 km across and Daimos just 15. Both look very much like asteroids and may indeed have been captured by Mars from the nearby asteroid belt. To be honest though, it's not clear what their origins are. Phobos orbits Mars only 6,000 km over the surface, and it moves so rapidly in its orbit that it orbits faster than Mars rotates. From the surface, it appears to rise in the west and set in the east. Tides from Mars are altering its orbit, slowly lowering Phobos closer and closer to the surface. In a few million years, it's expected to drop low enough to actually enter the atmosphere and impact the surface.

That'll be really exciting to watch from a safe distance. Incidentally, from some locations on the surface, the moons can be seen to transit the sun, passing directly across its face. The rovers on Mars have taken lots of pictures of these events, which is pretty cool. So, Mars has rocks, air, weather, volcanoes. But what about water? If it has frozen water at the poles, what about the rest of the planet? We know there's water ice at the mid latitudes of Mars. Some recent small asteroid impacts have white area around them, underground deposits of ice splashed out by the impact. There's no strong evidence of liquid water on the surface of Mars right now, though. Some crater walls collapse a bit and have dark channels running down them, which look like they could have been carved by flowing water. But there are other possible sources, too, so their cause still isn't certain. But in the past, things were different. Mars was once very wet. There's tons of evidence for flowing water on the surface long ago, including dry river beds, dry lakes, sedimentary layers, and minerals we know need water to form. There's even evidence Mars had oceans.

But that's all gone now. What happened? It's not clear. Billions of years ago, Mars was almost certainly warmer and had a thicker atmosphere. But for some reason, its internal dynamo shut down and its magnetic field disappeared. This left it vulnerable to the solar wind and over billions of years the Martian atmosphere was eroded away. The water went with it. This raises another obvious question. If it had air and water, could it have had life? That question wasn't taken terribly seriously just a few decades ago, but now we're very interested in it enough to spend a lot of money sending probes to Mars to look at what conditions for life are like now and once were in the past. It's still an open question and we know life on Earth got its start not long after the surface cooled. Mars is smaller and cooled more quickly after its formation.

So, it's not crazy at all to wonder if life formed there even before it did here. Intriguingly, the Curiosity rover detected simple organic molecules in a rock sample. This doesn't mean there's life, but it means the ingredients were and are there. It also detected a brief spike in methane, a volatile gas that can be produced by life, as well as by other geologic processes. The evidence we have right now is maddeningly vague, but we're just starting out. Give it time. Mars isn't the same planet it used to be, but that doesn't mean it's off limits. Getting to Mars is hard. Over half the missions sent there have failed, but not impossible. We've had amazing successes, including orbiters, landers, and rovers. I'd say we've learned as much about the red planet in the past couple of decades as we had in all the centuries of study before them.

Still, all we've sent are robots. They're good and relatively inexpensive, but they're slow. They can only cover so much ground. A human could do as much in a week as a rover could in years. But humans are fragile. We need water, air, food. We aren't terribly tolerant of radiation or vacuum. But the idea of sending people to Mars isn't as nuts as it might have once been. We're getting better at building rockets and the tech needed for human exploration of Mars is being developed now. There are even discussions about landing sites and where to build bases. One interesting idea is to use lava tubes, underground caverns created by ancient lava flows where the top of the flow cooled and created a roof. We know these exist on Mars because we've seen holes in the roofs called skylights, openings to the cavern below. This would provide shelter from solar radiation, protection from weather. Mars can get some pretty nasty dust storms in the spring and could be sealed up and filled with air. From there, a lot of the planet could be explored and in a few years we'd once again dwarf everything we've learned up to that point. I can't say when this

will happen. 20 years from now, maybe 30 or more, but it'll happen. Eventually, there will be life on Mars, and it'll be us. Today, you learn that Mars is smaller and colder than Earth. [music] It has polar ice caps and lots of rusty dust covering its surface. It also has the solar system's largest [music] volcano and valley. It's dry now, but once upon a time was much warmer and wetter with a thicker [music] atmosphere. It may even have had life. As we take our grand tour of the solar system here on Crash Course Astronomy, we're going to skip over the asteroids for now. We'll get to him, I promise, and instead pay a visit to the king of the planets, the big guy, the top dog, the big cheese, the head honcho, the one and only Jupiter.

>> [music] >> Jupiter is the largest planet in the solar system. It's not even close. All the other planets could fit inside it with room to spare. It's a gas giant, which means it's gasy and giant. And I do mean giant. It's 11 times wider than Earth. More than a thousand Earths could fit inside it. And it has a mass of over 300 times that of our planet. Despite its bulk, it rotates extremely rapidly. One day on Jupiter is a mere 10 hours long. That's the fastest spin of any of the planets in the solar system. Not surprisingly, a planet that big can reflect a lot of sunlight. And even though it orbits the sun on average at a distance of about 800 million kilometers, it's one of the brightest objects in the night sky. With binoculars or a small telescope, Jupiter is a wonder. You can easily see it as a disc, and its four biggest moons are readily visible. If they weren't hidden by the planet's glare, they'd be naked eye objects. Galileo himself discovered those moons. They're worlds in their own right, and so we'll dive into them literally in the next episode. When we

look at Jupiter, we're not seeing its surface. We're seeing the tops of its clouds, and they're a strange mix of permanence and change. The atmosphere of Jupiter is banded with multiple stripes running parallel to its equator. The lighter color stripes are called zones, and the darker ones belts. They're fairly stable, though their shape and coloring change over time. Belts and zones circulate around the planet in opposite directions. They form due to convection in Jupiter's atmosphere. Upwelling air cools and forms white ammonia clouds that creates the light colored zones. That air flows to the sides and sinks. And sunlight changes the chemistry in the clouds, forming molecules that color the air yellow, red, and brown. This is what causes the darker belts. In May of 2010, one of Jupiter's biggest belts sank so deeply it disappeared from view completely, covered by other clouds. Then a few months later, it popped back up and reappeared. None the worse for wear.

This has happened several times in the past, too. I saw one of these events once through a telescope, and Jupiter looked really weird. Lopsided. Turbulence in the regions between zones and belts can create storms. Gigantic vortices raging in the clouds. Dozens of them dot the face of Jupiter all the time. But there is one to rule them all. The Great Red Spot. A fittingly huge storm for a giant planet. It's actually a colossal hurricane, several times larger than our entire planet Earth with sustained wind speeds of 500 km/h. And it's old. It was first seen in the late 17th century. Imagine a storm on Earth lasting for more than 3 centuries. And it may be far older. The 1600s is just when we first spotted it. So why is it so stable? It turns out that a vortex, a local spinning region in a fluid, can persist if the fluid in which it's embedded is itself rotating. Jupiter's rapid spin is what keeps the red spot circulating, and the redness is probably due to cyanide like molecules that absorb blue light, letting redder light pass through. Weirdly, the red spot

appears to be shrinking. It was substantially bigger and more elongated just a few decades ago. It changes color over time, too, having gone from deep red to salmon and then back again. No one knows why its shape, size, and color change, but given how long the spot's been around, I doubt it's going to evaporate anytime soon. Remember, we're only seeing the tops of the clouds on Jupiter. Its atmosphere is thick, several hundred km deep. Like the composition of the sun, the air on Jupiter is mostly hydrogen and helium, but it's also laced with ammonia, methane, and other poisonous gases. As you dive into Jupiter's atmosphere, the pressure increases with depth, but you'll never reach the surface. The planet doesn't really have a proper surface. The gas gets thicker and hotter and eventually just sort of smoothly changes into a liquid over a several hundred km range below the clouds. Below that is where things get really weird. Instead of a mantle like terrestrial planets, Jupiter has a huge region made up of liquid metallic hydrogen. We think of hydrogen as being a gas or if it gets really cold, a liquid. But under the

ridiculous pressures generated deep inside Jupiter, hydrogen underos this strange transformation. Individual atoms don't hold on to their electrons, but instead share them. This means the hydrogen can conduct electricity and has other properties more like a metal. This substance is hot, too. about 10,000 degrees C, hotter than the surface of the sun. If we could see it, it would glow tremendously bright. Finally, at its center is most likely a dense core of material, probably composed of rock and metal. We really don't know because it's incredibly difficult to understand the physics and chemistry of material locked in at those pressures and temperatures. What's weirder is that we're not even sure if Jupiter has a core. If it did, it's possible it was eroded away by currents of hot metallic hydrogen early on in Jupiter's formation process. It's also possible it never had a core in the first place. The solar system formed from a flat disc of gas and dust. The center of this disc is where the sun was born, and it's thought that the planets formed as smaller particles of material stuck together during random collisions farther out in

the disc. As they got bigger, much bigger, these protolanets eventually started to grow even faster by drawing in material around them due to their gravity. Jupiter formed where the disc was thick, rich with material. It's possible that several large protolanets were forming, collided, and stuck together to really kickstart Jupiter's growth. If that were the case, it started out with a rocky metallic core, and once it got big enough, it drew in that gas that made it the giant we see today. Another idea is that Jupiter didn't grow from the bottom up, but from the top down. The disc itself collapsed in several places to form huge distended clumps. These then would have collided, stuck, and created the planet. If that's the case, then Jupiter might not have a core at all. These two different mechanisms make different predictions about Jupiter's structure. And that means that hopefully we can eventually figure out which is correct by studying Jupiter more carefully. But at the moment, we still don't know. Either way, Jupiter grew immense and it's mostly liquid under all that atmosphere. Couple that with its rapid rotation, and you

can see it's noticeably flattened. It's wider at the equator than through the poles by about 6% due to centrifugal force. So, Jupiter is a big bruiser of a planet. But how close was it to becoming a star? Sometimes people ask me if Jupiter is a failed star. In other words, as it formed, it almost got massive enough that nuclear fusion could start in its center, turning it into a star. I see this a lot on TV shows and in articles, and it really burns me up. When a star forms, hydrogen fusion starts when the star gathers so much mass that its gravity can compress atoms together in its core hard enough to get them to fuse. This happens when a star has roughly one 12th of the sun's mass.

In fact, the smallest stars we see do have about that mass. What about Jupiter? The mass of Jupiter is about 1/ 1,000th the mass of the sun. Far too little to undergo fusion in its core. If you want to turn Jupiter into a star, you'd have a lot of work ahead of you. You'd have to take Jupiter and then add about 80 more Jupiters to it. Saying Jupiter is a failed star is really unfair. It's not a failed star. It's a really successful planet. Even though Jupiter isn't a star, it does have another funny property. It emits more heat than it receives from the sun. The Earth and other terrestrial rocky planets are in a heat balance with the sun. We emit pretty much the same amount of heat that we receive. But Jupiter's different. After it formed, it started to cool by radiating away heat from its upper atmosphere. A large fraction of the planet is a gas, remember? And when you cool a gas, it contracts. So, the atmosphere cools and contracts, but this increases the pressure inside the planet, so it heats up. That heat works its way out of Jupiter and gets radiated away as infrared light. In the end, the amount of heat Jupiter gives off is more

than it receives from the sun. It's still actively cooling 4 1/2 billion years after it formed. Oh, and hey, remember the belt and zones, the stripes we see in Jupiter's atmosphere and all the storms that pop up, those are driven in large part by Jupiter's internal heat. On Earth, our weather is powered by heat from the sun, but on Jupiter, they get their energy from the planet itself. Jupiter has a very strong magnetic field. No doubt due to all of that metallic hydrogen inside it coupled with its rapid rotation. Like Earth, it has aurori at its poles as the solar wind is funneled down to the cloud tops.

As we'll see next week, Jupiter's moons affect the magnetic field and Aurora on Jupiter as well. Jupiter also has a ring, though it's not nearly as grand as Saturn's. It wasn't even discovered until we sent space probes of the planet. The ring is made of dust, probably thrown into orbit around the planet due to meteorite impacts on its smaller moons. Speaking of impacts, we know that Earth gets hit by interplanetary debris all the time. Go outside for an hour and you're bound to see a few meteors. Jupiter being larger and with more gravity gets hit a lot more, a lot more, a lot. And sometimes it gets hellaciously whacked. In 1994, the comet Shoemaker Levy 9 impacted Jupiter multiple times. Jupiter's fierce gravitational tides had ripped the comet into dozens of pieces and each slammed into the planet one after the other with the force of millions of nuclear weapons. The scars left in the upper atmosphere from the plumes of material that exploded outward lasted for months.

Several smaller impacts have been seen in Jupiter's atmosphere since then, and it may suffer an impact large enough to see from Earth every year or so. And while that sounds scary, it might actually be our savior. There's an idea that Jupiter's gravity tends to take comets that fall toward the inner solar system and fling them away into interstellar space. Over the eons, this has cleaned out a lot of otherwise dangerous objects that could have eventually hit Earth. On the other hand, Jupiter has a tendency to warp the orbits of some other comets so that they do swing by the Earth. It's hard to say if Jupiter's influence is a net benefit or not. But either way, it's clearly the two septillion ton gorilla in the solar [music] system. Today, you learn that Jupiter is really, really big. It's the biggest planet in our solar system, [music] a gas giant. It has a dynamic atmosphere, including belts and zones, and a gigantic red spot that's actually a persistent hurricane. [music] Jupiter is still warm from its formation and has an interior that's mostly metallic hydrogen and it may not even have a core. It has the fastest spin of any

planet and it's not a failed star. As we saw in the last episode, Jupiter is by far the largest and most massive planet in the solar system. That means it has a very strong gravitational field, which also means it can hold on to a lot of moons. A lot. Right now, as we record this episode, there are 67 that have been confirmed. And how many it really has depends on how small an object you're willing to call a moon. [music] >> In 1610, Galileo pointed his telescope at Jupiter and witnessed a revolution.

Oh, hey. Literally, he saw three little stars lined up on either side of Jupiter. Stars he could not see with his naked eye, and they moved. A week later, he saw a fourth one, and he knew he was seeing objects revolving, orbiting around Jupiter. It was proof that not everything in the solar system revolved around the Earth. That was a pretty big deal. Those four moons are now called the Galilean moons in his honor. Not bad for a week's work. All four are really big, too. If Jupiter weren't there drowning them out with its glare, they'd be visible to the naked eye. In that case, we might even call them planets, too. The biggest of Jupiter's moons is Ganymede at 5,270 km across. It's the biggest moon of any planet. It's even bigger than the planet Mercury. In fact, in size, it's halfway between Mercury and Mars. Size isn't the only planet-like characteristic of Ganymede, either. It's mostly rock and ice, but it probably has a liquid iron core. It even has a magnetic field, likely generated by that liquid core. The surface is similar to our own moon in that there's very old crater terrain, as well as smoother, younger areas.

Ganymede is also crisscrossed with large grooves. It's not clear what the origin of those grooves is, but it may be related to stress and strain on the surface caused by the tides from the other large moons as they orbit Jupiter and pass each other. Ganymede has a surprise well below its surface, too. Oceans of water. Measurements of Ganymede's magnetic field made during multiple passes by the Galileo spacecraft in the 1990s, combined with Hubble observations of the moon, indicate Ganymede has quite a bit of salty liquid water deep beneath its surface. As we'll see in a sec, it's not alone in that regard. The next biggest moon is Kalisto at 4,800 km in diameter.

In many ways, it's similar to its big brother Ganymede, mostly rock and ice. It probably has a rocky core, then a layer of mixed rock and ice above that. The surface is mostly ice, but mixed with darker material as well. It has a magnetic field, too, but it probably doesn't have a metallic core. The surface is heavily cratered, and there's no indication of any volcanoes or tectonic activity. That means the surface is very old, maybe as old as Kalisto itself. It even has an atmosphere, but it's a tad thin, roughly 1/100 billionth the pressure of Earth's air at the surface. Kalisto orbits Jupiter farthest out of the four biggies, almost 2 million kilometers away. That's too far to gravitationally interact with the other three. When I talk about the moons affecting each other, it's really the other three interacting. Next up is Io. It's only a little bit bigger than our own moon and orbits Jupiter so tightly it only takes about a day and a half to go around the planet. When the Voyager 1 space probe passed Io in 1979, it revealed a surface that was really weird. It was yellow and orange and red and black and didn't seem to have any obvious impact craters. An engineer, Linda Morabido, noticed that

in one image there appeared to be what looked like another moon behind Io, partially eclipsed by it. But that was no moon. It was a volcano on Io erupting, its plume shooting up from the surface and opening up into a wide arc. Io is the most volcanic object in the entire solar system with over 400 active volcanoes. Quite a few of them are erupting at any given time and images taken even a few months apart show changes in the surface due to ejected material. A lot of the eruptive material is rich in sulfur which is why the surface has all those odd colors on it. The energy for all this activity comes from the other moons. As they pass Io in their orbits, they flex it via tides, heating its interior through friction. A lot of that sulfur ends up as a very thin atmosphere around Io. And some of those sulfur atoms are then picked up by Jupiter's powerful magnetic field as it sweeps past Io and accelerates them to very high speeds. This has created a tremendous donut-shaped radiation belt around Jupiter, like Earth's Van Allen belts, but far more powerful. The radiation there is so intense it would kill an unprotected human in minutes. Of

course, if you're floating in space near Jupiter unprotected, you might have some more immediate concerns. Oh, one more thing. Both Ganymede and Io are magnetically connected to Jupiter. Charged particles flow from those moons along the lines of magnetism to Jupiter, which then slams them down at Jupiter's poles, just like the Earth does with the particles from the solar wind. On Earth, this creates the auror, the northern and southern lights, and it does at Jupiter, too. You can even see the ultraviolet glow where each of the moons connects to Jupiter, their magnetic footprints in the planet's atmosphere. And now we come to Europa, the smallest, but perhaps most exciting of all the Galilean moons.

Slightly smaller than our moon, it was known for decades to be very reflective, meaning its surface was probably loaded with water ice. But even so, the Voyager observations were shocking. They showed a surface completely lacking in craters, meaning something had resurfaced the moon, like Io or Venus, but Europa has no volcanoes. Even more intriguing, the surface was covered in long cracks, dark streaks all over the moon, as well as complex ridges. These and other features appear to be due to material from the interior of Europa welling up and forming the new surface, kind of like the way lava does on Earth. But in this case, the material is water. It's now thought that Europa has an entire ocean of water sealed up under a solid crust of ice several kilometers thick. Water welling up and moving under the crust causes it to shift, creating all the various surface features. The amount of water that may be locked up on Europa is staggering, easily more than all the water in all the oceans on Earth. Like Ganymede and Io, the interior of Europa is kept warm by tidal flexing from the

other moons, keeping the ice melted. Now, get this. A lot of Europa's material is silicut rock like on Earth and other terrestrial planets located in a layer under the ocean. If this interacts with the ocean in the same way Earth's oceans interact with the seafloor, this could make the subsurface Europan water salty. In fact, those dark cracks on the surface have been found to be rich in salt and organic materials. In other words, carbon-based compounds. This is pretty exciting. We think Earth's life originated in salty ocean water. If there are carbon- based molecules actually in Europa's water, it's not too crazy to wonder if the same spark that occurred here also happened there. We think Europa has everything it needs to spawn life. We just don't have any direct evidence of it yet. Some people have proposed sending a space probe to Europa specifically to look for life. It would land near a crack in the ice where the crust is thinner and somehow penetrate it, perhaps melting its way down. Chemical sampling could then look for signs of biological activity. That's amazing to me. The idea of life in Europa, even if it's just

microbial life, is taken very seriously by astrobiologists, scientists who study the possibility of life in space. It used to be science fiction. Now it's a topic of scholarly research. Astronomers have a concept called the habitable zone, the distance a planet can be from its parent star, where the temperature on the planet's surface can support liquid water. It's a fuzzy concept. Venus and Mars are both technically in the sun's habitable zone, but Venus is too hot and Mars too cold for liquid water. Atmospheres make a big difference, but it's still a useful concept as a rule of thumb for potential habitability. But Europa changed that. Jupiter is way, way outside the sun's habitable zone. Yet, there's Europa all wet. It's a great example that we need to let our ideas breathe a bit sometimes. Let them relax and flow outside the boundaries we set for them. When we look for signs of life on planets orbiting other stars, I bet we'll have to keep our minds open to types of life we've never considered before. Those are just the four big moons of Jupiter, each thousands of kilometers across. They probably formed

along with Jupiter, coalesing from the eddies and whirls around the proto Jupiter as it formed billions of years ago. But the planet has dozens of other moons, too. About the only thing they all have in common is that they're tidily locked to Jupiter. They all rotate once for every time they go around the planet. Jupiter's tides are hundreds of times stronger than Earth's. So, no surprise there. The next biggest moon after the big four is way smaller. Named Amthea. It's an irregular lump about 250 km across its longest dimension. It was discovered in 1892 and it's red, probably polluted by sulfur from Io. It orbits just over 100,000 km from Jupiter's cloud tops. If you stood on Amaltheia's surface, Jupiter would fill half the sky. The moons get smaller and more irregularly shaped from there with Himalayia and Thibbe and Allar and Pacé down to Hegman and Kala and Calachore which are no bigger than hills. Many of the irregular distant moons of Jupiter orbit the planet backwards relative to the others in what are called retrograde orbits. They may be captured asteroids from the nearby asteroid belt. Many of the moons have

orbital characteristics that are very similar, too, which may indicate they were once a single object that broke up. Several such families of moons orbit Jupiter. The smallest moons we've seen are roughly a kilometer across. If they were sitting on Earth, they might be hard to pedal up on a bicycle, but orbiting Jupiter, they hardly rate as more than debris. There are probably thousands of moons the size of houses circling the planet. And who knows, maybe millions the size of tennis balls. Should we even call those moons? Maybe. But I don't really worry about that kind of thing. The important thing to remember is that these are worlds big and small. Each fascinating, rich, and diverse, and there's still a lot more left to explore about them. Today, you learn that Jupiter has lots of moons and four big [music] ones. They're mostly rock and ice, though Ganymede, the biggest, may have an iron core. Io is riddled with volcanoes, and Europa has an under surface ocean that is the object of intense [music] study for scientists looking for life in space.

Io, Europa, and Ganymede are close enough to interact gravitationally, [music] providing a source of heat for their interiors. There are lots and lots of littleer moons, but at the moment, we really don't know much about them. Someday. So, now we've come to the last of the naked eye planets in the solar system, Saturn. It's the second largest planet, but ironically, the least dense. In fact, if you could find a bathtub big enough, Saturn would float in it. But it would leave a ring. >> [music] >> Okay, that's the oldest joke in astronomy, but it's true. Saturn is a gas giant, which means it has a thick atmosphere. Like Jupiter, it probably has a rocky core several times the mass of Earth, surrounded by layers of ice and weird metallic hydrogen. On top of all that is its atmosphere. Hundreds of kilometers deep, composed mostly of hydrogen with a smaller amount of helium and a whole bunch of trace gases like methane and ammonia. Overall, its

density is about 2/3 that of water. It wouldn't really float, though. Its gravity would draw in all the water from the giant bathtub. Although, actually, come to think of it, a bathtub filled with water big enough to put Saturn in would have more mass than the planet. So, the water would pull Saturn in. Astronomy is fun. Saturn spins really quickly, completing a day in just 10 and 1/2 hours. Its rapid rotation and low density means it's really oblate or squished. It's 10% wider through the equator than through the poles. That's really noticeable through a telescope. When we look at Saturn, we only see its cloud tops. These clouds are mostly ammonia ices with water clouds lower down. The atmosphere is banded like Jupiter's, but the pattern is much fainter, probably due to Saturn being farther from the sun and therefore colder, as well as the atmosphere being deeper than Jupiter's, too. That's not to say Saturn doesn't have storms. Oval hurricanes pop up every now and again, and in 2010, a huge storm erupted in Saturn's northern hemisphere. It was discovered by an amateur astronomer, and Cassini spacecraft pictures showed it

getting so big, it literally wrapped around the planet, reaching a length of over 300,000 kilometers. Traces of it persisted for years. One of the most striking features on Saturn is a huge hexagonal vortex at its north pole. Vortices like this are common on planets. Earth and Venus have them, too. That well-defined six-sided shape is pretty freaky, but it's actually a natural outcome of a spinning fluid, the air circulating in Saturn's atmosphere, which itself is in a spinning system, in this case, the planet itself. It's not that different than Earth's jetream, but it's a lot bigger, about 20,000 km across, wider than Earth. And right at its center over Saturn's north pole is a ridiculously beautiful storm system about 2,000 km across. Wind speeds there have been clocked at 500 kilometers per hour. Saturn as a planet is of course an amazing place, but come on. When you think Saturn, you think rings, as well you should. Saturn's rings were seen by Galileo, but his telescope lacked the

resolution to really figure out what they were. He thought Saturn might be three objects in close proximity. Some decades later, the Dutch scientist Christian Huygens correctly interpreted their appearance as a ring surrounding the planet. They're not solid. If they were, they'd get torn apart since the inner edge would orbit the planet way faster than the outer edge. They're actually made of countless small chunks of very nearly pure water ice. Each particle orbits the planet independently. So, I guess Saturn wins in the which planet has the most moons category. What really gets me about Saturn's rings is their scale. They're 250,000 km across. If you put Saturn between the Earth and Moon, the rings would cover 2/3 that distance. And yet, despite their size, they're incredibly thin. They average about 10 m thick. And no, I did not misspeak. I meant 10 m. 10 m. That's amazing. That's thinner to scale than a sheet of paper. Way thinner. If you shrank Saturn down to the size of a piece of paper, that paper would be 10,000 times thicker than the

rings. So, yeah, Saturn's rings are flat, but why? Well, that's tied to their origin, though the specifics of how they came to be aren't exactly known. One idea is that they were originally an icy moon that got disrupted by a huge collision. The shattered moon particles would have spread around Saturn, and any of them that were on a tilted orbit would quickly collide with others, and in the end, that would average out all their orbits to form a flattened ring. Or maybe there was once a large moon near Saturn that was big enough to be differentiated. That means the rocky bits fell to the moon's core while the ice stayed near the surface. As it plowed through the material around the proto Saturn, the moon's icy layer got stripped off forming the rings. And again, that would form a very flat disc.

However they formed, collisions would quickly flatten the ring out. And remember, Saturn is oblate. So, as the rings formed, they would have felt a gravitational tug from Saturn's equatorial bulge, centering them right over the planet's midline. Saturn has three main rings, give or take. In classic imaginative astronomer fashion, they're given the names A, B, and C in order of their discovery. B is the broadest, roughly 26,000 km wide. A is outside B, while the C ring is fainter and inside the B- ring. When the rings were first observed, the A and B rings were thought to be one ring. But then the Italian astronomer Giovani Dominico Cassini discovered a gap between them, and we now call that the Cassini division. It's about 5,000 km across, but it's not empty. There are quite a few very narrow rings inside of it. The gap is due to one of Saturn's moons, Mus. If you're an ice particle in the Cassini division, you orbit Saturn twice for every one time Mus goes around. This is called a resonance, and it means that the particles at that location get tugged repeatedly by the gravity of Mus, which in essence yanks them out of their

orbit. There are quite a few gaps in the rings caused by moons. In fact, there are hundreds of ringlets in the main rings. Even weirder, the F- ring is outside the A- ring and is very narrow, just a few hundred kilometers across. It's kept that way by two moons, Prometheus and Pandora, orbiting just inside and outside the ring. They tug on the ring particles in such a way that keeps them confined to very strict orbits. If a particle wanders out, the moon's gravity drags it back in. As the moons and ring particles interact, they can also create weird rippling waves in the rings. And this is really cool. When small moons orbit Saturn on slightly inclined orbits, it can create waves in the rings, tremendous vertical excursions that can reach several kilometers high. Saturn is weird, and so are its moons. I already mentioned Mus, an icy moon that carves out the Cassini division with its gravity. It also suffered a massive impact long ago that created a single huge crater, making it look somehow familiar. But Saturn has two moons specifically you should know

about. The first is Titan, a monster satellite that's actually bigger than Mercury, not that much smaller than Mars, and second in moon size ranking only to Jupiter's moon Ganymede. Titan not only has a substantial atmosphere, the only moon to have one, but it's thicker than our air on Earth. I wouldn't suggest breathing it, though. It's mostly nitrogen with a smattering of methane and hydrogen. It's also 180 below zero Celsius there. Definitely don't stick your tongue on a flag pole on Titan. Titan's atmosphere has a thick layer of haze that prevents us from seeing the surface using visible light but using infrared and radar. Though the Cassini spacecraft, which has been orbiting Saturn since 2004, is able to map the surface. It also deployed a probe named Huygens to land on Titan surface, the first time humanity had ever landed on an outer solar system moon. What they saw was an odd world, Earthlike in some ways and very alien in many others. Titan has wind sculpted dunes, but they're made of hydrocarbon grains, not sand. There are hints of volcanoes that spew liquid water instead of lava. These are called cryo

volcanoes, cold volcanoes, a term I love. This together with measurements of Titan's gravitational field have scientists thinking that Titan may have an underground ocean of liquid water. Intriguingly, Titan also has long winding channels indicating flows of some kind of liquid on its surface, but it can't be water. It's far too cold on the surface for that. So, what kind of liquid could it be? Well, Cassini also spotted quite a few flat regions near Titan's north and south poles. These have since been confirmed to be lakes of liquid methane, the first time any liquid has been seen on the surface of another world. The shorelines of the lakes appear to change over time, too, hinting very strongly that Titan has weather driven by methane as opposed to water here on Earth. Weather, atmosphere, surface liquids, and an abundance of organic carbon-based molecules. Could there be life on Titan?

The conditions there are interesting. We obviously don't know yet, but it's an exciting possibility. And it's not the only Saturnian moon scientists are looking at for signs of life. Enceladus is a smaller icy moon about 500 km across, about the size of my home state of Colorado or the southern part of England. Enceladus is incredibly shiny, reflecting nearly all the light that hits it. A strong indicator it's covered in water ice. Parts of it are cratered, but other parts are very smooth, indicating some event resurfaced the moon. The surface also has an extensive series of cracks in it, much like Jupiter's moon Europa. Scientists suspected cryovalkcanism, and that was confirmed quite dramatically in 2005 when Cassini spotted active geysers of water spewing from the south pole of Enceladus. The water is erupting from a series of cracks nicknamed tiger stripes and is coming from deep under the surface. Like Europa, Enceladus has an under surface ocean kept liquid by Saturn's ferocious tides, which squeezed the moon as it orbits the planet. The

plumes reach several hundred kilometers from the moon's surface, and Cassini even flew directly through the plumes and detected organic molecules. Could Enceladus be yet another harbor for life in the solar system? Again, no one knows, but I'd put it in the top three places to look, including Europa and Titan. It's weird, isn't it? We've always looked for planets where conditions were like conditions here on Earth, but it turns out extremely cold moons may be the best places to seek out new life. Saturn has a huge retinue of other moons, too, and they're all pretty weird. Ius is shaped like a walnut with a tall ridge of mountains encircling its equator. No one's quite sure how that formed. Hyperion looks like a huge piece of styrofoam that's been hit repeatedly with a BB gun. And that's not a bad analogy since it has such a low density, it's likely to be extremely porous, like a sponge. A lot of Saturn's moons orbit the planet backwards, retrograde, similar to several moons of Jupiter.

These may be captured asteroids or even Kyper belt objects, giant chunks of ice that we'll learn about very soon in a future episode. All in all, Saturn has over 60 moons we've discovered so far. I want to add a personal note. When you ask astronomers or astronauts what got them started, what was the very first thing that happened to them to inspire them to do what they do? Overwhelmingly, they will tell you, "I saw Saturn through a telescope." It's true. It's true for me. I remember my parents buying a cheap department store telescope when I was 5 or six, setting it up in our driveway, and aiming it at Saturn. I still remember seeing Saturn in the eyepiece, a perfect jewel. The ring small, but vivid and sharp. It's hard to believe it's real when you see it. And even after all these years, it's my favorite target when I'm stargazing.

I'll never get tired of watching people's faces when they see it for the first time. They gasp, their eyes get wide, their mouths drop open. It's honestly one of the greatest joys of life. Saturn is the crown jewel of the solar system. It's the best kind of astronomical object. Visually stunning, artistically gorgeous, and filled to overflowing with incredible science. Today you learn that Saturn is a gas giant and has a [music] broad set of rings made of ice particles. Moons create gaps in the rings via their gravity. Saturn has dozens of moons, including Titan, which is as big as Mercury and has a thick atmosphere, and lakes of methane, [music] and Enceladus, which has an under surface ocean and eruptions of water geysers. And though we don't know for sure, it's possible either or both moons may support [music] life. Uranus. Uranus.

Uranus. Uranus. Ur Uranos for mankind. [music] >> There are five planets in our solar system you can see without a telescope. Well, six if you include the one you're sitting on. But there are two more big ones out there orbiting the sun out in the cold depths of the outer solar system. Uranus and Neptune. They're a lot alike in many ways, but of course, they have their individual quirks. Uranus was the first planet to be discovered, and by that I mean it wasn't known in ancient times. It took an astronomer with a telescope to find it. In 1781, William Herschel was mapping the skies through his telescope when he spotted a greenish object that was clearly a disc and not a dot like a star. He noted its position and moved on. But when he went to observe it again sometime later, he was astonished to discover it had moved. It was quickly determined to be a planet more distant from the sun than Saturn. In true brown-nosing style, he named it

Gayorgium Cidus or George's star after the reigning king George III. Yeah, happily that name didn't stick. And keeping with the nomenclature of Roman gods, the new planet was dubbed Uranus. Interesting tidbit, it actually is visible to the unaded eye if you have very sharp eyesight and very dark skies. It's right on the thin hairy edge of visibility. What's funny is that several people had observed it before Hershel, but none had noticed its movement. Some even cataloged it on their maps as a star. The planet is massive, about 14.5 times the mass of Earth, but not terribly dense. This means its interior must be made of lightweight stuff. Scientific models of the planet based on physics and chemistry of the outer solar system indicate its interior is probably made up of three general layers. There's a small rocky core, smaller than Earth, surrounded by a very thick layer of materials like water, ammonia, and methane. This mantle makes up the bulk of the planet in fact, and is dense, hot, and under a lot of pressure. In many ways, it's more like an ocean than anything else. However, somewhat confusingly, I'll admit, outer solar

system planetary scientists refer to water, ammonia, and methane as ice. So, even though it's not ice like we usually think of it here on Earth, we say that the mantle of Uranus is icy. To distinguish it from the gas giants Jupiter and Saturn, Uranus is called an ice giant. Speaking of ice, here's a really weird thing. Studies have shown that the pressure inside Uranus can break up methane molecules, squeezing the carbon in them so tightly that it actually forms diamonds. These would then fall down to the base of the mantle like sparkly hailstones, except it's dark. But still, down there in the depths of Uranus, there may even be an ocean of liquid diamonds where solid winds float like uh diamondbergs. Not that we'll ever see that. When we observe the planet, we're only seeing the top of its atmosphere. Besides hydrogen and helium, the air there is about 2% methane. Methane is really good at absorbing red light, which means the light we see reflected from Uranus is mostly green and blue, making the planet look distinctly cyan or aquamarine. And it's pretty striking through a telescope. But in visible light, the

planet looks almost featureless. It doesn't have that deep banding like Jupiter or even the pale winds of Saturn. Though, when you look in the infrared, some banding can be seen. There are clouds, but again, they're difficult to see in visible light. The clouds are made of methane, ammonia, and hydrogen sulfide. That last one is what makes rotten eggs smell so bad. I'd avoid breathing through your nose, Uranus. But then, the atmosphere there is about -220° C, so that might be a better reason not to inhale. In late 2014, a bunch of storms popped up in Uranus's atmosphere.

So big and bright, they were easily visible from Earth. The storms may have dredged up very reflective methane ice from lower down in the atmosphere. And this time, I do mean icy ice, which is why they were bright. The northern hemisphere of Uranus is approaching summertime, which may be why these storms formed. And that brings us to the weirdest thing about this planet. It's sideways. If you were above the Earth's north pole looking down, you'd see our planet spinning counterclockwise west to east. The sun spins that way and all the planets do as well, except Venus and Uranus. While Venus is flipped all the way over, Uranus is tilted by about 98°. That means that in the summer, its axis is pointed almost directly at the sun. So, seasons on Uranus are pretty extreme by outer solar system standards. Weird. No one knows why Uranus is tipped so much. An obvious thought is that it got whacked hard by an impact long ago. If it were a grazing collision by a big object that could have pushed hard enough on the planet to tip it over. Unfortunately, Uranus is very far away

and has only been visited by spacecraft once, Voyager 2 in 1986. And even then, it was a quick flyby. Uranus's weird tilt is just one of those many mysteries that astronomers are trying to solve with limited data. Uranus has a magnetic field, but it's truly odd. Its axis is tipped over by 50 degrees from the planet's spin axis, and it's way off center. The center of the magnetosphere is about 8,000 kilometers from the planet's center. It may be that the magnetic field is generated in the icy mantle, or that the core somehow interferes with the magnetic field, throwing it off. Truthfully, no one really knows why. Uranus has more than two dozen moons, five big ones, and a bunch of dinkier ones. Cool fact, the moons are named after characters in Shakespeare plays. So, we have Ariel, Umbreel, Titania, Oberon, and Miranda.

Even Puck. Of them all, I think the most interesting one is Miranda. When Voyager 2 flew past, it revealed an icy world that looks like it was put together by Dr. Frankenstein. A patchwork of jumbled terrains all crammed together, crisscrossed by canyons and grooves. It's possible a giant impact in its past actually disrupted the moon somewhat, and it settled back together into this weird mishmash. But the reason I like it so much is a feature called Verona Rupes. It's the tallest cliff in the solar system, 5 to 10 kilometers high.

If you jumped off the top, it would take you 6 minutes to fall to the surface. That would be a fantastic ride. Like Jupiter and Saturn, Uranus has a ring system, too. They were discovered by accident in 1997. Astronomers were observing Uranus pass directly in front of a star. They were hoping to use this to gather information about the planet's atmosphere as starlight passed through it. But they saw several dips in starlight before the main event, which they realized were from rings around the planet. The rings are made of dark particles, probably ice and reddish organic molecules. There are 13 rings known. Most of them are very faint and narrow. They may have been created by an impact completely shattering a small moon orbiting Uranus. But as for now, the ring origins are unclear. And then finally, we have Neptune, the guardian of the solar systems nether regions. Neptune is an ice giant like Uranus and has a lot of similarities. Like its green brother, it probably has a rocky core surrounded by a thick icy mantle of water, ammonia, and methane. Above that is an atmosphere of hydrogen, helium, and methane. But there are differences,

too. Neptune is more massive than Uranus, 17 times Earth's mass versus just 14 1/2 for Uranus. Neptune is slightly smaller than Uranus, which means it's a lot denser. Also, while Uranus is teal, Neptune is a deep, rich azure. I like to call it the other blue planet. The first one being, you know, Earth. Through a telescope, Neptune's color is quite lovely. It has roughly the same amount of red light absorbing methane in its atmosphere as Uranus does. So, its deeper blue hue is something of a mystery. That may have to do with its active atmosphere. Unlike blander Uranus, Neptune has clouds of methane, ammonia, and hydrogen sulfide lying in the skies at different depths.

and white streaky clouds were seen during the Voyager 2 flyby in 1989. They looked whipped by wind, and for good reason. Sustained wind speeds in Neptune's atmosphere have been clocked at over 2,000 kilometers per hour, faster than the speed of sound on Earth. It's thought that the low temperatures in the atmosphere reduce friction, allowing the winds to gather to such amazing speeds. Voyager saw a huge storm marring Neptune's face called, for some reason, the Great Dark Spot. A few years later, when Hubble was used to observe the planet, the spot was gone. But others had appeared. They're probably vortices, cyclones, which allow us to see through the upper atmosphere and peer farther into Neptune's depths. Neptune has a magnetic field, and like Uranus, it's offset from the planet center. Perhaps that icy mantle is at work, somehow interfering with the generation of the magnetic fields in both planets. Neptune has rings, too. But shocker, they're weird. There are three main rings, two narrow and one broad. They're clumpy and have bright stretches that make the rings look more like incomplete arcs. It's possible

those arcs are being constrained by small moonletits near the rings. Speaking of which, Neptune has over a dozen known moons. Most are quite small, but one Triton is by far the largest. At 2,700 km across, it's smaller than our own moon, but the rest of them are really dinky. Triton orbits around Neptune backwards retrograde. As we'll learn in a future episode, there's a repository of giant ice balls out past Neptune. So, Triton was probably one of those that got too close to Neptune and was captured by its gravity. Most of what we know about Triton came from a single flyby of Voyager 2 in 1989. And only about 40% of the surface was seen, but this quick glimpse revealed a weird little moon. The surface is covered in nitrogen ice, as well as water and carbon dioxide ice. It's really flat and has very few craters, meaning something resurfaced it in geologically recent times. Most likely, this was from cryo volcanoes, cold volcanism. That is volcanoes where water and ammonia take

the place of lava there. Also, Triton has been seen to have active geysers of nitrogen erupting from its surface. They're probably due to warming from the sun, and they make Triton one of the few objects in the solar system seen to be geologically active. It also has a very thin atmosphere of nitrogen, probably due to evaporation from the surface. After all this, Neptune is special in another [music] way, too. Neptune is faint and can only be seen telescopically. It was discovered in 1846, and it wasn't an accident. Over the decades, astronomers observed Uranus and found something weird. It wasn't where it was supposed to be. Over time, its predicted position was off from where it actually was. The French mathematician Urban Lever concluded that this was due to an unseen planet and was able to use the mathematics of orbital mechanics to predict where the new planet would be. He sent a letter with the predicted position to the Berlin Observatory. Astronomer Johan Gala read the letter, went right out, and found the planet that very night. Neptune was

within a degree of the predicted spot. Amazingly, another mathematician, Englishman John Couch Adams, had also worked on the math and had made a similar prediction, but Levier beat him by 2 days. Two days of such tight races are fame made in science. Interestingly, over time, Neptune seemed to wander from its predicted position as well. A ninth massive planet was predicted, leading to a grand search that resulted in the discovery of Pluto. But Pluto was far too small to affect Neptune. When Voyager passed both Uranus and Neptune, it found the masses of the planets were different than what had been measured from Earth. When the new masses were used in the orbital equations, Uranus and Neptune were right where they were supposed to be. It helps to have the right numbers to plug into your equations. Pluto, therefore, was found by accident. That means Neptune is the only planet in the solar system found via math. See, your algebra teacher was right. Someday this stuff [music] will be important. Today you learn that Uranus and Neptune are ice giants with

small rocky cores, thick mantles of ammonia, water, and methane, [music] and atmospheres that make them look greenish and blue. Uranus has relatively dull weather, while Neptune has clouds and storms whipped by tremendous winds. [music] Both have rings and moons, with Neptune's Triton probably being a captured iceball that has active geology. When you look at a diagram of the solar system, you'll see a big gap between Mars and Jupiter. A few centuries ago, that gap bugged astronomers. They really wanted there to be a planet in there. On the first day of the 19th century, January 1st, 1801, they got their wish. Kind of. Italian astronomer Jeppi Piazi found a point of light moving at just the right speed to be the desired planet, but it was just a dot and too faint to physically be a terribly big object. He suspected it might be a comet, but follow-up observations showed it wasn't fuzzy. The object was given the name series, but was it really a planet? Well,

by for mankind. [music] >> Hopes were high that series was the wished for planet between Mars and Jupiter. But then something rather amazing happened. A little over a year later in 1802, another one was found. Then in 1804, astronomers spotted a third one and a fourth in 1807. It was becoming clear that a new class of solar system object had been discovered. Given that they were all just dots in the telescopes of the time, points of light like stars, they were given the name asteroids, which literally means star-like. By the end of the 19th century, more than 450 had been found in total. The rate of discovery has accelerated over the years, and now today, we know of hundreds of thousands.

There are probably billions, yes, billions of them, larger than 100 meters across in the solar system, and over a million larger than 1 kilometer in size. So, what are we dealing with here? What are these asteroids? There's not really a hard and fast definition of what's an asteroid and what isn't, but generally speaking, it's a class of smaller bodies that are rocky or metallic that orbit the sun out to Jupiter. Objects past Jupiter have special designations that we'll get to in the next episode. Over the centuries, we've learned a lot about them by scrutinizing them with telescopes. Asteroids come in a few basic flavors. Most of them, about three/arters, are carbonacious, which means they have lots of carbon in them. About a sixth are silicacious, heavy on the siliconbased materials, you know, rock. The rest are lumped into one catchall category, but are dominated by metallic objects, literally loaded with iron, nickel, and other metals. So many of them orbit the sun between Mars and Jupiter that this region is now called the main belt. The main belt has structure. For example, there are very few asteroids about 425 million km from

the sun. An asteroid at that distance would have an orbital period of about 4 years, a simple fraction of Jupiter's 12-year period. Any asteroid there would feel a repeated tug from Jupiter's mighty gravity pulling it out of that orbit. The resulting gap is called the Kirkwood gap, and there are several such asteroid deserts, all with simple multiples of Jupiter's period. In this way, the main belt is like Saturn's rings, whose gaps are carved out by the gravity of the orbiting moons. Another way to group asteroids is by orbit. Some have similar orbits and may have formed from a bigger parent asteroid that got disrupted by an impact. These groups are called families and there are a few dozen known. For example, the Unomia family has over 400 members and are silicacious rocky asteroids and probably all formed from a parent body that was about 300 km across. When you watch movies, they always show spaceships dodging and swooping through asteroid belts trying to evade the bad guys. But in reality, our asteroid belt is mostly empty space. On average, decent-sized asteroids are millions of kilometers apart. So far that if you stood on an

asteroid, odds are good you wouldn't even be able to see another one with your naked eye. And despite their huge numbers, they don't add up to much. If you took all the asteroids on the main belt and lumped [music] them together, they'd be far smaller than our own moon. Series is the biggest at about 900 km across. It's round, nearly spherical due to its own gravity crushing it into a ball. A funny thing about series, as we write and record this episode, it's being visited for the first time by a spacecraft named Dawn. That means everything I tell you about this asteroid is probably about to be obsolete. But we do know a few things. Siri probably has a rocky core surrounded by a water ice mantle. The amount of water in it is staggering, probably more than all the fresh water on Earth. It may even be liquid under the surface, like the oceans of Enceladus and Europa. Early images by dawn as it approached the asteroid show its surface is heavily cratered and some craters are very bright. They may be exposing ice under the surface or just fresher, brighter material. There are tantalizing observations of localized

water vapor on the surface which may be from sublimation ice turning directly into a gas due to the sun's heat or it might indicate cryo volcanoes. Don also visited Vesta which is the third largest but second most massive asteroid known. Vesta is roundish, what's called an oblate spheroid, flattened a bit like a ball someone's sitting on. The southern hemisphere got hammered by impacts long ago, leaving a huge basin there. Several other main belt asteroids have been visited by spacecraft, mostly via flybys. Luteia, Gasper, Stein, Matilda, Ida is another, and was discovered to have a small moon orbiting it. In fact, a lot of asteroids have moons or are actually binary with two similarsized bodies in orbit around each other. Cleopatra, a weird dog bone-shaped rock, has two moons. You might think asteroids are just giant versions of rocks you might find in your garden, tough, solid, singular bodies.

But it turns out that's not the case. A few years ago, scientists realized that asteroids have spent billions of years whacking into one another. Sometimes in high-speed collisions, sometimes more slowly. Slower hits can disrupt the asteroid, crack it, but not necessarily be strong enough to actually disrupt it so that it breaks apart over time. Enough hits like that can leave behind what's called a rubble pile. Individual rocks held together by their own gravity, like a bag of gravel or a car window that's been cracked and still holds its overall shape. This became more clear when the Japanese Hayabusa spacecraft visited the asteroid Ida and saw what can only be described as a jumbled mess. The asteroid had no craters on it and was littered with rubble and debris. It was also very low density, just what you'd expect for a loosely bound rock pile. It's weird to think of some asteroids as being not much more than free floating bags of gravel. But the universe is under no obligation to adhere to our expectations. It's full of surprises, and we need to keep our minds flexible. So, here's a question. Why is there even

a main asteroid belt at all? The solar system formed from a dis of material, and over time, that material started to clump into bigger and bigger pieces. As planets formed, they swept up and pulled in lots more stuff and grew large. Jupiter consumed a lot of the material around it, but not all, and left a lot of debris inside its orbit. Some of this clumped together to form middling sized objects, probably smaller than the planets we have now, but big enough to undergo differentiation. Heavy stuff like metals sank to the middle, and lighter stuff formed a mantle in crust. Collisions broke almost all of them apart, though, and that's why we see asteroids with different compositions.

Some are from the denser core, others from the lighter crust. There was probably a lot more material between Mars and Jupiter billions of years ago, but it either got eaten by Jupiter or the planet's immense gravity altered the asteroid's orbits, flinging them away. This may be why Mars is so small, too. Jupiter robbed it of all of its food as it formed. While most asteroids live in the main belt, not all of them do. Some have orbits that cross that of Mars, taking them closer to the sun. We call those, wait for it, Mars crossing asteroids. Some have orbits that take them even closer to the sun, crossing Earth's orbit. We call those Apollo asteroids. Ah, gotcha. They're named after the asteroid Apollo, the first of its kind to be found. Some have orbits that are almost entirely inside Earth's orbit called Aten asteroids. Atton and Apollo asteroids can get pretty close to Earth, so we call them near Earth asteroids. Now, while they get close to us, that doesn't mean they'll hit us because, for example, their orbits may be tilted. So, their orbits and the orbit of the Earth don't actually ever physically cross, but some do have paths

that literally intersect Earths. That doesn't mean they'll hit us every pass either. After all, you can walk across a street without getting hit by a car. The problem comes when you try to occupy the same volume of space as a car at the same time. Astronomers, unsurprisingly, are very concerned about asteroids that can hit us. That's why we have surveys, observatories, scanning the skies, looking for them. This is a pretty important topic, and I'll go into it in more depth in a future episode. There's another category of asteroid that exists due to a quirk of gravity. When a planet orbits a star, there are points along the planet's orbit and near it in space where the gravitational forces are in balance. If you place an object there, it tends to stay there, like an egg in a cup. These are called lrangee points. One of them is along the same orbit as the planet, but 60 degrees ahead. Another is 60 degrees behind. The first such asteroid found was orbiting 60° ahead of Jupiter and was named Achilles after the Greek hero in the Trojan War. As more were found, the naming convention stuck. Asteroids ahead of Jupiter were named after Greek figures

in the Trojan War. And those behind Jupiter were named for Trojans, and now we just call them all Trojan asteroids. Trojan asteroids have been spotted for Jupiter, Mars, Uranus, Neptune, and even Earth. Earths was found in 2010 using observations by an orbiting observatory called WISE, which scans the skies in infrared light where asteroids glow due to their own heat. 2010 TK7, as it's called, is about 300 m across and 800 million km away, orbiting the sun ahead of Earth. There are also asteroids that have orbits that are very similar to Earth's, but are slightly elliptical and tilted with respect to ours. Because of this, they can stay relatively near the Earth in space, but don't really orbit us. Instead, they sometimes get closer and sometimes recede. It's pretty weird, but a natural outcome of orbital mechanics. Some people say these asteroids are moons of Earth, but it's better to say they're co-orbital with us. Only a few are known, the most famous being Cuinia, which can get as close as 12 or so million kilometers from us. Oh, one more thing. Originally, asteroids were named after female goddesses, Siri, Vesta, Juno, and so on.

But as hundreds more were found, and then thousands, we ran out of names. Eventually, astronomers who discovered asteroids were allowed to name them through a lengthy proposal and acceptance process governed by the International Astronomical Union. They also get a number assigned to them as well. A lot of astronomers have asteroids named after them, including astronomers who study asteroids, like my friend Amy Mer, who works on the Wise mission. Hers is 234750 Amy Mer and Eleanor Helen, who discovered quite a few asteroids and comets. Hers is 3267 glow for her nickname. And this one, it's a 1 km wide rock in the main belt and goes by the name 165347 fill plate. Must be coincidence. Today you learn that asteroids are chunks of rock, metal, or both [music] that were once part of smalish planets but were destroyed after collisions. Most orbit the sun between Mars and Jupiter, but some get near the Earth. The biggest [music] series is far smaller than the moon, but still big enough to be round and have undergone differentiation. Hey, Phil

Plate here, and this is Crash Course Astronomy. Since humans have been human, we've looked to the skies for portance of the future. The sun, the moon, the planets, the stars. They've all been used for prognostication, and so have comets. A fuzzy blob moving slowly across the stars. How could soothsayers resist? But now we know a lot more about comets. They're beautiful, fascinating, and can bring both life and death upon our little world. [music] >> [music] >> Comets have been seen in the sky since antiquity. Comet Halley, for example, is shown in the Bayu Tapestry, which depicts the Norman invasion of the British Isles in the year 1066. It was seen by ancient Chinese and Greeks, too.

In general, and like everything else in the sky, comets were considered omens or harbingers of human events. Sometimes they were good omens. William the Conqueror liked his chances in 1066 after seeing one. and sometimes bad. That same comet didn't do so well for King Herald II. A comet bright enough to see with the naked eye shows up in the heavens every few years or so, and some can get spectacularly bright. In 2007, I saw comet McNot very near the sun in broad daylight. When you think of a comet, you probably picture a fuzzy blob and a long tail stretching away from it. Fair enough, but there's a bit more to them than that. Comets are in many ways similar to asteroids. They're roughly huned chunks of stuff left over from the formation of the solar system. Unlike asteroids, which are mostly rock with a dash of ice and maybe metal, comets are a more balanced mixture of ice and rock. And by ice, I mean frozen water, but also frozen carbon dioxide, carbon monoxide, methane, ammonia, things we normally think of as gases on Earth. And by rock, I do mean rocks, but also

gravel, and dust. In fact, astronomers sometimes call comets dirty snowballs, which isn't a half bad term. It's that ice that makes comets, well, comets. When they're way out in deep space, they're so cold that they're basically inert lumps of ice and rock. But many are on elliptical orbits that take them from those subfreezing depths into our neck of the woods where the sun can warm them. As they heat up, the ice turns directly into a gas, a process called sublimation. The gas then flows away from the comet, creating a cloud around it. This makes the comet look fuzzy and actually in the past they've been called hairy stars. I like that term too and in a sense we still use it. The solid part of the comet is called the nucleus and the gaseous cloud around it is called the coma, Latin for hair. In fact, that's why we call them comets. As the ice sublimates, the bits of rock and gravel and dust embedded in it can be freed and leave the nucleus as well. This material is what forms the comet's tail, but how that happens depends on which material you're talking about. The

gas and the dust from [music] comets form two different tails. Gas molecules emitted by the comet get ionized by the sun's ultraviolet light. That means they lose electrons becoming charged and charged particles are highly susceptible to magnetic fields. The solar wind is a stream of charged particles blown out by the sun and carries a magnetic field with it. As the wind hits the ionized gas from the comet, it picks up those particles and carries them downstream away from the sun. The solar wind is usually moving far far faster than the comet. So this ion tail winds up pointing directly away from the sun. The dust on the other hand is influenced more by sunlight. Light from the sun exerts a small but inexurable pressure and this pushes on the dust particles.

The dust streams away but because the pressure isn't as intense as the solar wind is on the gas tail. The dust tail blows away more lazily and tends to lag behind the comet in its orbit. That means the two tails usually point in two different directions. In some comets, like 1997's incredibly bright and gorgeous comet hail bop, this is pretty obvious. The dust tails look white or a teeny bit yellowish due to reflected sunlight, while the ion tail glows blue or green depending on the primary constituents of the gas. Carbon monoxide tends to emit blue light, while carbon molecules glow a ghostly green. A comet's tail can stretch for tens of millions of kilometers. But despite their length, tails are incredibly low density, as low as a few hundred atoms per cubic centimeter. The air you breathe is a million billion times denser. In 1910, Earth passed through the tail of comet Halley. This caused some public fear because cyanogen, a deadly gas, had been detected in the tail. But of course, nothing happened. It turns out getting a gazillionth of

the toxic dose isn't that big of a problem. Broadly speaking, comets are classified by their orbits. If they have orbital periods less than 200 years, they're called short period comets. These tend to orbit the sun in the same plane as the planets and go around the sun in the same direction as well. From Earth, we see them sticking near the ecliptic, the line across the sky that marks the annual path of the sun. Comets that take longer than two centuries to go around the sun are called long-term comets and have orbits that are tilted every which way. That means they can appear anywhere in the sky. But this raises an interesting point. Comets go away. Every time they get near the sun and start outgassing, they lose mass. Over time, they get smaller. Eventually, they should evaporate. Some do this all at once because they dive into the sun, skimming our stars surface. We call those sun divers or sun grazers. Many of those may actually be pieces from a bigger comet that broke up in space nearly a thousand years ago. But besides those, we know of some comets with orbits that can be short, some with periods of just a few years. Even a

century is like a single flap of a mosquito's wing compared to the lifetime of the solar system. How can comets be billions of years old if their orbits bring them close to the sun all the time? Ah, astronomers wondered about this very thing. Over the years, they came up with an idea. Maybe out past Neptune, there's a repository of comets, chunks of dirty ice, millions of them, billions, orbiting the sun where it's perpetually cold. They could have orbits that last for millennia or more. But then something tweaks them, makes them fall toward the sun. In fact, there may be two such regions since we have both short period and long-term comets. Turns out this idea is correct. We now know enough about those distant regions of the solar system that they deserve their own episode. So, we'll dive into that topic later. So, what do comets look like up close? Like really close. Studying them from Earth is hard. The coma obscures the nucleus, making it nearly impossible to see it directly.

Ah, from Earth. If you instead send your telescope to a comet, things change. We first did that in the 1980s, the last time comet Halley came around. Several nations sent spacecraft to fly past the comet, and the Soviet mission Vega 1 was the first to successfully get pictures of the nucleus. The lowresolution images revealed a dark lump highlighted with two bright spots. later determined to be jets of gas streaming away. These images were used to better determine the position of the nucleus and a few days later the European probe Gato zipped past the nucleus at an incredibly close distance of just 600 km. Those pictures were more detailed and showed us a flying mountain in a regular chunk 15 by 8 km in size and it was dark reflecting only 4% of the light that hit it. That makes the nucleus as black as asphalt. You might think that all that ice would be shiny, but it's not that simple. Most of Halie's nucleus is covered in thick dust laced with darker molecules with only a few spots emitting gas. Most likely, there are deposits of ice under the surface, and only some of them

receive enough heat from the sun to sublimate and blow out gas. This has been seen with other comets as well. The gas is emitted from specific spots on the comet, venting out from cracks in the crusty surface. The surfaces of comets must be inhomogeneous, different in different places. That fact was brought home magnificently in 2014 by another European mission, Rosetta. It went into orbit around the comet 67P Churimoff Garasenko and found it to be a bizarre little object measuring about 4 km end to end. 67P has two loes connected by a narrow neck looking very much like a cosmic rubber ducky. The surface is completely devoid of craters. Clearly, the surface is very young.

Images show jets of gas emitted from very specific places on the surface. And there are wide circular pits here and there, which may be gas vents growing wider over time as the ice below is depleted. Surprisingly, the surface is fairly tough and hard when some scientists expected it to be fluffier. The comet has a very low density, similar to rubble pile asteroids, so it was expected that the surface would be soft. Rosetta sent down a lander named Fel to set down on the surface using harpoons to anchor itself. but instead the lander bounced, unable to penetrate the tougher than expected material. One idea to explain this is that the ice is porous and fluffier inside the comet.

But as it nears the sun, the ice at the surface warms and changes its structure, forming that harder crust. As for the double lobe thing, well, that's a bit baffling. We see some asteroids shaped that way as well. It's possible 67P used to be two separate comets that had a low-eed collision and stuck together. Or maybe it used to be one big lump, but over the eons, the ice in the middle sublimated more, leaving behind the two loes. Rosetta is the first time in human history we've had a probe orbiting a comet, studying it up close and long term. We're still learning, still figuring this stuff out. Incidentally, I mentioned earlier that a comet have lots of ice in them, and two, they also get really close to Earth sometimes. In fact, they can hit us. Now, not to get all technical and scientificy, but that is what we would call bad, as we'll discuss in an upcoming episode. But billions of years ago, lots of comets hit the Earth not long after our planet formed. Together with asteroids, many of which are also rich in water ice, they may have brought a significant amount of

water to Earth. Scientists are still wrestling over the details of this, and it may be a while before the actual numbers are nailed down, but it's an intriguing thought. Even more interesting, in 2004, NASA's Stardust Space Probe physically passed through the coma of comet Vilt 2, collecting samples that were returned to Earth. Careful analysis of the material found the presence of organic carbon-based molecules in them. And not just any random molecules, but complex ones, including amino acids. These are the building blocks of all life on Earth. Amino acids are what our bodies use to create proteins. It's possible that the ingredients of life on Earth didn't start here, but instead were brought to our planet from comet impacts. Or at least there was a mix of the two. If that's the case, then in a sense, all life on Earth is part alien. How about that? But what gets me are the philosophical ramifications of this. When we look into space, when we examine our celestial neighbors, when we send probes to comets and survey what we

find, we're looking at our own origins. Comets are like time machines, allowing us to investigate our past 4 billion years back, hinting at the secrets of the origin of life itself. And you thought astronomy was just lying out in a field and looking up. Well, it is, but if you let it, it's also a whole lot more. Today, you learn that comets are chunks of ice and rock that orbit the sun. When they get near the sun, the ice turns into a [music] gas, forming the long tail, and also releases dust that forms a different tail. We visited comets up close [music] and found them to be lumpy with vents in the surface that release the gas as ice sublimates. Eons ago, comets and asteroids may have brought a lot of water to Earth, [music] as well as the ingredients for life.

Well, look where we are now. With our backs to the sun and the planets, asteroids, and comets behind us, we face deep space. There's nothing between us and the stars so terribly, terribly far away. [music] Or is there? The empty space past Neptune isn't exactly empty. In episode 21, I mentioned that comets come in two varieties. Those with orbital periods of less than 200 years, which tend to orbit the sun in the same plane as the planets, and those with longer periods, which have orbits tilted every which way. This is something of a problem. Comets lose material when they get near the sun. Over the course of millions of years, these comets should evaporate.

And yet, here we are 4.56 billion years after the solar system's birth. And comets still appear in our skies. So, where are they coming from? To see, we'll have to turn the clock back a wee bit, like 4 1/2 billion years. Behold our forming solar system coalescing out of a flat disc of material around the sun. The inner planets were warmer, smaller, and rocky, while the outer planets were in a region that was colder and grew huge. Out there in the chillier part of the solar system, water came in the form of ice mixed in with dust and other stuff. These bits would collide and stick together, growing bigger. Some grew to hundreds of kilometers across. But there was a problem. Those outer planets, they had a lot of gravity, and any chunk of ice getting too close would either fall into the planet and get assimilated or get kicked into a different orbit. It could then plunge in toward the sun or get flung out into deep space. Trillions upon trillions of such ice balls got tossed around by the planets. Even though they were small compared to the planets, they did have a

little bit of mass and gravity. So every time the planet pulled hard on them, they also pulled a little bit on the planets, too. It wasn't much per chunk, but after trillions of events, this adds up. A current model of what happened called the Nice model after the city in France where it was proposed says that the overall effect of all these encounters was that Saturn, Uranus, and Neptune slowly moved outward from the sun while Jupiter moved inward. Neptune would have had the biggest effect on these ice balls because it bordered the biggest volume of space where they lived. As Neptune migrated outward, close encounters with these chunks of ice flung lots of them into crazy orbits, highly elliptical and tilted with respect to the planets. repeated more distant encounters tended to more slowly increase the sizes of the orbits of the ice balls, too. We think that this shuffling around of the outer planets is what caused the late heavy bombardment, the intense shower of objects that came screaming down from the outer solar system, scarring planets and moons a few hundred million years after the planets themselves formed.

It's not known for sure, but all the pieces fit together really well. In the end, today, there are three rather distinct populations of these objects. One is a region shaped like a puffy disc or a donut aligned with the plane of the planets. Icy objects there have stable orbits unaffected by Neptune. We call this the Kyper belt, named after the Dutch astronomer Gerard Kyper, one of many who initially speculated about the existence of this region. The Kyper belt starts more or less just outside Neptune's orbit, extending from about 4.5 to 7.5 billion km from the sun. The second region is called the scattered disc. This is composed of the ice balls sent by Neptune into those weird, highly tilted orbits. This overlaps the Kyper belt on its inner edge and extends out to perhaps 150 billion km from the sun. That's 25 times farther out than Neptune. Finally, outside those two zones, there's a spherical cloud of icy objects which starts roughly 300 billion km out, 70 times farther out than

Neptune, a staggering 2,000 times the distance of the Earth from the Sun. And that's just where it starts. It extends way farther out than that, perhaps as much as a lightyear, 10 trillion kilometers. This is called the Orort cloud after astronomer Yanort who first proposed it. The Orort cloud is the origin of long period comets. Since they orbit the sun on a sphere with no preferred orientation, they come in toward the inner solar system from random directions in the sky. Many newly discovered comets fall into this category. Their orbits can be extremely long. They start their fall from so far away they swing around the sun at nearly escape velocity and their orbits are close to being parabolic. The scattered [music] disc is the source of short period comets. They can still be affected by Neptune which can alter their orbits to drop them down closer in. They can orbit the sun on paths between Jupiter and Neptune, meaning eventually they'll have a close encounter with Jupiter. This can send them in closer to the sun and they become short period comets. Ta. That's how comets are made. So how do we know

all this? Well, until 1930, it was pretty much just conjecture. But then an American astronomer, Clyde Tomba, discovered the first Kyper Belt object, Pluto. Pluto orbits the sun on an elliptical, mildly tilted path. Its orbit actually brings it closer to the sun than Neptune. So, how come it never collides with the larger planet? Pluto's orbit crosses Neptune's more or less. Because the orbit is tilted, they never actually physically cross. When Pluto is at perihelion, closest to the sun, it's well above the plane of the solar system, far from Neptune's orbit. Not only that, but Pluto orbits the sun twice for every three times Neptune does. Because of this, whenever Pluto is closest to the sun, Neptune is always 90° away in its orbit. That's many billions of kilome distant, way too far to affect Pluto. This is mostly coincidence. We've seen how orbital resonances can be forced by tides and by gravity, but in this case, it's due to attrition. Once upon a time, billions of years ago, there were probably a lot of objects out by Pluto with orbits of all

different shapes and tilts. But the ones that got too close to Neptune got gravitationally tweaked into different orbits, turning them into comets or flinging them deeper into space. The only ones that could survive just happen to have orbits with that 3:2 or 2:1 resonance with Neptune, keeping them far from Neptune's influence. Today, those are the only kinds of objects we see with orbits near Neptune. We call these objects Plutinos. They're not really a separate class of object. They're still Kyper belt objects, but a fun and interesting subset of them. Once Pluto was found, astronomers wondered if it might herald a new class of icy objects past Neptune. However, it took more than six decades to find the next one. 1992 QB1 was discovered in 1992, and that opened a sort of gold rush of Kyper belt discoveries. We now know of more than a thousand Kyper belt objects. One called AIS is very close to Pluto's size and is more massive. It's probably rockier than icy Pluto. Pluto is an interesting object. A moon was discovered in 1978 named Kiron. It's actually about 1/8 the

mass of Pluto itself. While Kiron orbits Pluto, the moon has enough mass that it can be said that Pluto noticeably orbits Kiron, too. In reality, both circle around their mutual center of mass located between the two. Four more moons were discovered in Hubble images of Pluto in 2005 and 2012. There may be more. Pluto is so small and distant that we don't know much about it. But that may be about to change. [sighs] And now I have to admit to being in a tough spot. As I record this episode of Crash Course, a space probe called New Horizons is heading toward Pluto. It'll fly by the tiny world in July 2015. There's no doubt our view of Pluto will change. There may be more moons discovered. We'll see surface features for the first time and much more. But right now, I can't tell you about any of that because we don't know yet. So, I think the best thing to do is leave little Pluto alone for now. But there is a point I want to bring up. Pluto was found in 1930, long before any other Kyper belt object because it's much brighter than any other. When it

was discovered, it was thought to be about the size of Earth. But over the years, better observations showed it to be far smaller than first thought. In fact, it's smaller than Earth's moon. Its surface is unusually reflective, shiny, making it look much bigger than it seems. Most other Kyper Belt denisonens are far less reflective and so are far fainter. If Pluto is king of the Kyper belt objects, it has a lot of loyal subjects. We think the Kyper Belt may have 100,000 objects in it larger than 100 km wide. If that sounds like a lot, get this. The Orort cloud surrounding the solar system may have trillions of icy bodies in it. Trillions. While we know lots of Kyper Belt objects, we don't know of any Orort cloud objects for sure. Two very interesting bodies have been found. Sedna and VP13. Sedna's orbit takes it an incredible 140 billion kilometers from the sun, while VP13 gets about half that far out. Both are on very elliptical orbits. Neither, however, gets close to Neptune. So, it's not at all clear how they got where they are.

They may be orcloud objects that were disturbed by passing stars long ago, dropping them closer into the sun, but no one knows yet. Speaking of which, we can calculate how many Ort cloud objects there should be left over from the formation of the solar system, and it's about 6 billion. However, calculating how many there are using long period comet observations, you wind up getting about 400 billion. That's a big discrepancy. Now, get this. One idea to solve this discrepancy is that the sun has stolen comets from other stars.

Seriously, comets should form wherever stars do and sometimes the sun passes near other stars. When we see a long period comet gracing our skies, could we be seeing an object from an alien solar system? Maybe. There is another explanation, but it's highly speculative. Perhaps there's another planet in the solar system well beyond Neptune. It's possible. Some very preliminary studies have shown that some long period comets aren't coming in randomly, but instead have their orbits aligned in a way you might expect if a very distant planet perturbed them. There are a handful of Kyper belt objects aligned in a similar way. NASA's Wise Observatory scan the skies in infrared and would have seen anything as big as Jupiter or Saturn out to tremendous distances. So, any hypothetical planet would have to be smaller and very distant, probably tens of billions of kilometers out. We've seen other stars with planets this far out, so it's physically possible. But is there really one there? We can't say either way, yes or no. At least not yet.

This region of the solar system is seriously underexplored. It's distant, difficult to reach, and above all else, extremely huge, and numbingly empty. You could hide a whole planet out there, and it would be pretty hard to find. The point? There's still lots of solar system left to explore. We've barely dipped our toes into these dark, frigid waters. Today you learned that past Neptune are vast reservoirs of icy bodies that can become comets if they get poked into the inner solar system. The Kyper Belt is a donut shape [music] aligned with the plane of the solar system. The scattered disc is more eccentric and is the source of short period comets. And the Orort cloud, which surrounds the solar system out to great [music] distances, is the source of long period comets. These bodies all probably formed closer into the sun and got flung out to the solar system suburbs by gravitational interactions with the outer planets. I love astronomy, you may have noticed. But there's one really frustrating aspect of it. Everything we study is really far away. Nearly everything we understand about the universe comes from light

emitted or reflected by objects. It'd be nice if we could get actual samples from them, physical specimens we could examine in the lab. Well, sometimes the universe can be accommodating and allows us to hold it in our hands. Cambot, can we get this up on still store? If you go outside on a clear, dark, moonless night, and you really should, chances are pretty good that within a few minutes you'll see a shooting star.

It'll zip across the sky, a fiery dot leaving a long glowing trail behind it. They're one of the most exciting and fun things you'll see when you look up, and they always get a gasp and a squeal of delight from someone who's stargazing. What you're actually seeing is a tiny bit of interplanetary debris. Rock, ice, or metal ramming through the Earth's atmosphere, heated to incandescent. Most are faint, but some can be astonishingly bright. I saw one once that left an after image on my eye. Obviously, shooting stars aren't really stars. So, what do we call them? Sometimes it seems like astronomers use different names for objects to keep things as confusing as possible, but really we do that to separate out different things. In this case, the actual bit of solid stuff coming from space is called a meteoroid. The phenomenon of the meteoroid getting hot and blazing across the sky is called a meteor. And finally, that hits the ground, we call it a meteorite. I think the second best way to tick off an astronomer is to mix up meteor and meteorite. Sometimes astronomers can be

pretty pedantic about such things. Now, the best way to tick off an astronomer, ask them, "Hey, what's your sign?" Amazingly, a typical meteor that you'll see is due to a meteoroid that's tiny, probably smaller than a grain of sand. How can that be? It's because they're hauling mass. You heard me. The meteoroid is orbiting the sun, probably at speeds of a few dozen kilometers/s. As it approaches the Earth, our planet's gravity accelerates at an additional 11 km/s, Earth's escape velocity. And when it enters our atmosphere, it's moving incredibly fast, up to 70 km/s or more. The energy of motion is called kinetic energy. If you want to get something moving, you have to give it energy. And if you want it to stop, you have to take that energy away. This kinetic energy depends on the mass of the object and how fast it's moving. In fact, it depends on the square of the velocity. Double its speed, and it'll have four times the kinetic energy. Meteoroids may usually be small, but they're screaming fast and have a huge amount of kinetic energy. As they hit our atmosphere, they

slow from their ridiculous orbital speed to nearly a standstill. And all that energy has to go somewhere. It gets converted into light and heat. And that's what we see as a meteor. A big misconception about meteors is that they get hot due to friction with air. Actually, a far bigger contributor to their heat is compression. One of the most basic laws of physics is that when you compress a gas, it heats up. And a meteoroid coming in at hypersonic speeds compresses the air in front of it a lot, heating it hugely. The gas can reach temperatures of thousands of degrees C for a few seconds. The air radiates away this heat, in turn, heating up the meteoroid. The material on the surface vaporizes and blows away, a process called ablation. That ablated material leaves a glowing trail behind the meteor, which we call a train. Sometimes it can glow for several minutes getting twisted up in high altitude winds leaving behind an eerie ghostlike persistent train. This all happens high above your head about 90 to 100 km above the ground. Typically from any one

location you can see a few meteors per hour. It may not seem like much but when you add them up all over the planet you find the Earth is getting pelted to the tune of about 100 tons of material a day. But again most of these meteoroids are teenytiny. Those random meteors are called sporadic meteors. They tend to be rocky in composition and generally come from asteroids. If two asteroids smack into each other, the collision can eject little bits of material that then orbit the sun on their own. If their orbit crosses the Earth, then you have a potential meteor. It may take a few million years, but at some point the Earth and the meteoroid are at the same place at the same time and boom. But sometimes meteoroids travel in packs. When that happens, we can get meteor showers, many dozens or even hundreds of meteors per hour. with one exception.

Those don't come from asteroids. They come from comets. When a comet orbits the sun, the ice on it turns to gas, dislodging dust and gravel mixed in. This material leaves the comet and tends to stay more or less in the same orbit as the comet itself. Over time, that material gets scattered all along the orbit, creating a puffy ribbon of tiny pieces of space debris around the sun. When the Earth plows through that cloud of debris, we get a meteor shower. From our viewpoint on Earth, we see meteors shooting across the sky, apparently radiating away from a single point. That's a perspective effect. It's like driving through a tunnel and seeing the tiles on the wall and ceiling flying past you, all apparently coming from a point ahead of you. The point on the sky where the meteors come from is called the radiant. And the shower is named after the constellation the radiance in. So we have the Percied meteor shower, the Leonids, the Camelopartids or the Camelopardalids.

And since the Earth hits a specific comet stream around the same time every year, the showers are annual. The Perciads are in August and the Leonids in November. Watching a meteor shower is easy. Just go outside and look up. Generally, they're better after local midnight. The Earth plows into the meteoroids. So, facing the direction of Earth's orbital motion means more meteors, just like you get more raindrops on the front windshield of your car than on the back when you're driving through a storm. After local midnight, you're on the part of the Earth facing into the orbit, so you see more meteors. By the way, if you happen to be on the International Space Station, you have to look down to see a meteor. In 2011, astronaut Ron Garin photographed a Perciad burning up below him. But don't worry, the odds of the space station getting hit are extremely low. Space is big. Oh, and that one exception I mentioned before, that's the annual Geminid shower, which occurs in December. that comes from the asteroid 3200 Fthon, which is on an orbit that takes it very close to the sun. It's possible it gets so hot that the rock

vaporizes, making it act like a comet. The vast majority of meteoroids are small and tend to burn up in our atmosphere, but they can be bigger. A bolide or fireball is an extremely bright meteor, and those can be about the size of a grapefruit. Those happen pretty often somewhere over the Earth. I've seen a few myself. Very rarely, an incoming meteoroid will survive all the way to the ground and become a meteorite. Sometimes the immense pressure of ramming Earth's air causes the incoming meteoroid to crumble or even explode, raining down dozens or hundreds of smaller pieces. Typically, they slow rapidly after their blaze of glory and simply fall the rest of the way to the ground. The air up there is cold and their interiors are cold from being in space so long. So, despite what you might think, meteorites don't cause fires when they hit the ground. In fact, they can be quite chilly. Meteorites are classified into three broad categories. Stony, which are mostly rock, iron, which are mostly metal, and stony iron, which are a mixture of the two. The majority of meteorites we find are stony. The stony meteorites are

themselves subdivided into two kinds, condrites and aonditrites. Condrites contain condrils, small grains of minerals. These are very primitive and are thought to have condensed out of the original disc of material that formed the solar system. Their ages can be found by looking at ratios of elements in them formed from radioactive decay. The oldest known meteorite formed 4.568 billion years ago before the Earth itself formed. Aondrites don't have condrils in them. Most likely they came from a bigger asteroid, one that was once molten all the way through mixing the minerals. A big collision disrupted the parent body creating the aondritic meteoroids. Iron meteorites most likely come from the center of a large asteroid, one big enough that metals fell to the center via gravity. Again, a big impact blew the asteroid up, scattering its material around the asteroid belt, and with some on orbits that eventually intersected Earth. Stony irons are the rarest. Some have green or orange crystals of a mineral called olivine embedded in a web of metal

called palocytes. They may be the most beautiful of all meteorites. I actually collect meteorites. It's fun, but can be a somewhat pricey hobby. If you're interested, make sure you get them from a licensed dealer. We have links to some in the doobly-doo. Of course, on occasion, the meteoroid coming in can be a tad bigger. And when that happens, well, all hell can break loose. On February 15th, 2013, residents of the Russian city of Chelabinsk got a rude awakening. At 9:20 a.m. local time, a rock about 19 m across came in at a low angle. It got nearly as bright as the sun as it slammed into the atmosphere, and the pressure of its passage broke it up into several chunks, which broke up again. In a moment's time, the sudden energy released was equivalent to the detonation of a half million tons of TNT, as much as a small atomic bomb.

While no one was killed, over a thousand people were injured by flying glass shattered by the explosion. No doubt they were at their windows gawking at the huge vapor trail in the sky when the shock wave hit. There was no warning for this event. The asteroid was essentially too small to detect while it was out in space. Well, for now at least. Telescopes are coming online soon that should be able to find smaller asteroids and give us some warning. Astronomers are more worried about ones roughly 100 m across or bigger. These can do serious damage on a citywide scale or larger, but at the moment aren't easy to spot much in advance. And what do we do if we do see one headed our way? As of right now, there's not much we can do. Studies have been done to determine the best course of action. Maybe lobbing a nuke at it or simply ramming it with a space probe to change the orbit and make sure it misses Earth. These ideas look good on paper, but they haven't been tested yet. We're still a few years from that.

The good news is that objects that size hitting the Earth are rare. Maybe once every century or three, but if we do nothing, it will happen eventually. As science fiction writer Larry Nan points out, the dinosaurs went extinct because they didn't have a space program. Hopefully, we're smarter than they were. Today, you learn that meteors are small bits of interplanetary [music] debris sloed off by asteroids and comets. When the Earth plows through the stream emitted by a comet, we get a meteor shower. Meteors burn up about 100 km above [music] the Earth, but some survive to hit the ground. Most of these meteorites are rocky, some are metallic, [music] and a few are a mix of the two. Very big meteorites can be a very big problem, but there are plans in the works to prevent us from going the way [music] of the dinosaurs. Hey, Phil Plate here and this is Crash Course Astronomy. In last week's episode, I mentioned that nearly all the information we have about the universe comes in the form of light. But how does that light get made? What can it tell us about these astronomical objects? And honestly, what is light?

Here's a hint for mankind. [music] Light is a wave. It took centuries of thought and experiments to figure that out and to also figure out that at its most basic, light is a form of energy. It travels in waves similar to waves of water in the ocean. Except with light, the things doing the waving are electric and magnetic fields. Literally, light is a self-contained little bundle of these two fields intertwined. That's why we call light electromagnetic radiation. The details of this are very complex, but we can make some pretty good overall observations about light just from thinking of it as a wave. If you're floating in the ocean, you'll move up as a wave passes you, then back down, then back up again when the next wave rolls by. The distance between these crests in the wave is called the wavelength. Since light is a wave, it has a wavelength as well. And this may be its single most

important feature. That's because the energy of light is tied to its wavelength. Light with a shorter wavelength has more energy and light with a longer wavelength has less energy. And our eyes have a really convenient way of detecting these different energies. Color. What you think of as the color violet is actually light hitting your eye that has a short wavelength. Red light has a longer wavelength about twice the distance between crests as violet light. All the colors in between, orange, yellow, green, blue, have intermediate wavelengths. This spread of colors wavelengths is called a spectrum. Over millions of years, our eyes have evolved to detect the kind of light the sun emits most strongly. Well, that makes sense. That makes it easier for us to see. We call this kind of light visible light. But that's just the narrowest sampling of all the different wavelengths light can have. If light has a slightly shorter wavelength than what our eyes can see, it's invisible to us, but it's still real. We call that ultraviolet light. Light with shorter wavelengths than that fall into the X-ray part of the spectrum. And light waves with the shortest wavelengths of

all are called gamma rays. At the other end, light with slightly longer wavelengths than the reddest color we can see is called infrared light. Light waves longer than that are called microwaves. And those with the longest wavelengths of all are called radio waves. These different groups don't really have hard and fast definitions. Just think of them as general guidelines. But together, we call all of these different kinds of light the electromagnetic or EM spectrum. And remember, energy goes up when the wavelength gets shorter. So ultraviolet light has a higher energy than violet.

X-rays have a higher energy than that. And gamma rays have the highest energy of all. Infrared light has lower energy than red light. Microwaves lower than that. And radio waves have the lowest energy. When you look at the whole EM spectrum, you'll probably notice that we really do only see a teeny little sliver of it. Most of the universe is invisible to our eyes. That's why we build different kinds of telescopes to detect the kind of light our eyes can't detect. They let us see a lot of stuff that otherwise we'd never notice. So, you might be asking, how is light made? Well, one of the most basic properties of matter is that when you heat it up, it gains energy and then it tries to get rid of that energy. Since light is energy, one way to get rid of energy is to emit light. Another important property of matter is that the kind of light an object emits depends on its temperature. An object that's hotter will emit light with a higher energy, that is a shorter wavelength. Cooler objects give off light with a longer wavelength. You may have seen this in action. Heat up an iron bar and it starts to glow red, then orange, then

yellow as it gets hotter. The color, the wavelength of light emitted changes as the bar heats up. Astronomers use a shorthand for this. We say that light with a shorter wavelength is bluer and light with a longer wavelength is redder. Don't take this literally. We don't really mean more blue or more red, just that the wavelengths are decreasing or increasing. So in this lingo, ultraviolet light is bluer than blue and x-rays are bluer than ultraviolet. So objects that are more energetic that have a higher temperature are bluer than cooler, redder objects. This rule of thumb works really well for dense objects like iron bars and stars, even humans. You emit light, but it's in the far infrared, well beyond what our eyes can see. There are less dense objects in space too like gas clouds and the way they emit light is different. To understand that we have to zoom in on them way way in and look at their individual atoms. And to understand that we need to take a brief diversion into atomic structure. Atoms are the building blocks of matter. In general, atoms are

made up of three subatomic particles. Protons, neutrons, and electrons. Protons have a positive electric charge. Electrons a negative charge. And neutrons are neutral. Protons and neutrons are much more massive than electrons and occupy the centers of atoms in what's called the nucleus. Electrons whiz around the nucleus. Their negative charge attracted by the protons positive charge. The type of atom depends on how many protons it has in the nucleus. Hydrogen has one proton, helium 2, lithium 3, and so on up the periodic table of elements. It's common to think of the electron as orbiting the nucleus like a planet orbits the sun. But that's not really the case. The real situation is fishly complex and involves pretty hairy quantum mechanics. But in the end, the electron is only allowed to occupy very specific volumes of space around the nucleus. And those depend on the electrons energy. Think of these like stairs on a staircase where the landing is the nucleus. When you walk up the stairs, you have to use energy to go up. And when you do, you have to go up a

whole step at a time. If you don't have the energy to get to the next step, you can't move. You can be on the first step or the second step, but you can't be on the first and a half step. There isn't one. Electrons are the same way. They whiz around the nucleus with a very discreet amount of energy. If you give them an additional precise amount of energy, they'll move up to the next energy level, the next step. But if you give them the wrong amount, they'll just sit there. The opposite is true as well. Electrons can be in a higher energy state, up on a higher step, and then give off energy when they jump down. The amount they give off is exactly the same amount needed to get them to jump up in the first place. How do they get this energy? Light. If light hitting the atom has just the right amount of energy, the electron will absorb it and jump up. It can also jump down and emit light at that energy, too. An electron can also jump two steps or three or whatever, but it needs exactly the right energy to do it. But, as I said earlier, energy and wavelength are the same thing, and that's equivalent to color. So when an electron jumps up or down, it absorbs or

emits a very specific color of light. Not only that, but the steps are different for different atoms. To stick with our analogy, it's like different atoms are different staircases with different heights between the steps. So when an electron jumps down a step in a hydrogen atom, it emits a different energy, a different color of light than an electron jumping down in a helium or a calcium atom. And this this is the key to the universe because different atoms emit different colors of light. If we can measure that light, in principle, we can determine what an object is made of, even if we can't touch it, even if it's a bazillion light years away. And we can. Can you tell the difference between these two squares? They're a very slightly different shade of red. Your eye probably can't tell the difference, but a spectrometer can. This is a device that can precisely measure the wavelength of light and can, for example, distinguish light emitted by a hydrogen atom from light emitted by helium. When you hook one of these spectrometers up to a telescope, you can figure out what astronomical objects are made of. In the case of thin gas clouds

in space, the atoms are basically floating free, rarely bumping into one another. The atoms emit those individual colors of light, allowing us to identify them. Unlike dense stars, the color of the thinner gas depends more on what's in it than its temperature. And this is how we learned what the universe is made of. Stars and gas clouds in space are mostly hydrogen with some helium and heavier elements thrown in. Jupiter has methane in its atmosphere. Venus carbon dioxide. Everything in the universe has its own mix of ingredients like cakes at a bakery. With spectroscopy, we can taste them. But wait, there's more.

You're probably familiar with the Doppler effect, the change in pitch when, say, a motorcycle goes by. In sound, the wavelength defines the pitch. Higher tones have shorter wavelengths and lower tones [groaning] longer wavelengths. When the motorcycle is headed toward you, the sound waves get compressed causing the pitch to rise. After it passes you, the pitch drops because the wavelengths get stretched out. The same thing happens with light. If an object is headed toward you, the wavelength of light from the source gets compressed, shorter. We say the light is blueshifted. If it heads away, the wavelength gets longer and it's redshifted. apply that to a spectrum and by measuring that shift, we can tell if an object is moving toward or away from us. Here's a teaser. This becomes super important later when we talk about galaxies. Spoiler alert, the universe is expanding. And it's this red shift that allowed us to figure that out. And that's still not the end of it. With other spectroscopic techniques, we can determine if an object is spinning and how fast, whether it has a magnetic

field and how strong it is, and even how massive and dense an object is. A vast amount of the fundamental properties of astronomical objects can be found just by dissecting their light into individual colors. Almost everything we know about the universe comes from the light objects in it give off. Pictures of astronomical objects show us their structure, their beauty, and hint at their history. But with spectra, we can examine their blueprints. [music] Today, you learn that light is a form of energy. Its wavelength tells us its energy and color. Spectroscopy allows us to analyze those colors and determine an [music] object's temperature, density, spin, motion, and chemical composition.

Oh, hey, sorry. I don't mean to be rude. I'm just trying to figure out how far away my thumb is. How parallax for mankind. [music] >> Centuries ago, people thought the stars were holes in a huge crystal sphere letting through heavenly light. It wasn't clear just how big the sphere was, but it was pretty dang big. I have some sympathy for them. By eye and for all intents and purposes, the stars are infinitely far away. If you drive down a road, you'll see nearby trees flying past you. But distant mountains move more slowly. The moon is so far it doesn't seem to move at all compared to nearby objects. And it's easy for your brain to think it's much closer, smaller, and actually following you, which is a bit creepy. Sometimes people even think it's a UFO tailing them. Finding the distance to something really far away is tough. It's not like you can just pace off the distance. Or can you?

The ancient Greeks knew the Earth was round, and there are lots of ways to figure that out. For example, ships sailing over the horizon seem to disappear from the bottom up, as you'd expect, as they slip around the Earth's curve. But how big is the Earth? Over 2,000 years ago, the Greek philosopher Aeritosines figured it out. He knew that at the summer solstice, the sun shone directly down a well in the city of Sin at noon. He also knew that at the same time it was not shining straight down in Alexandria and could measure that angle.

There's a legend that he paid someone to pace off the distance between the two cities so he could find the distance between them. But more likely he just used the numbers found by earlier surveying missions. Either way, knowing the distance and the angle and applying a little geometry, he calculated the circumference of the Earth. His result, a little over 40,000 km, is actually amazingly accurate. For the very first time, humans had determined a scale to the universe. That first step has since led to a much, much longer journey. Once you know how big the Earth is, other distances can be found. For example, when there's a lunar eclipse, the shadow of the Earth is cast on the moon. You can see the curve of the Earth's edge as the shadow moves across the moon.

Knowing how big the Earth is and doing a little more geometry, you can figure out how far away the Moon is. Also the phases of the moon depend on the angles and distances between the earth, moon and sun. Using the size of the earth as a stepping stone, Aristocus of Samos was able to calculate the distances to the moon and the sun as well as their sizes. That was 2200 years ago. His numbers weren't terribly accurate, but that's not the important part. His methods were sound and they were used later by great thinkers like Hipparus and Tommy to get more accurate sizes and distances. They actually did pretty well and all over a thousand years before the invention of the telescope. And I think it also says a lot that these ancient thinkers were willing to accept a solar system that was at least millions of kilometers in size. But at this point things got sticky. Planets are pretty far away and look like dots. Our methods for finding distances failed for them for a while at least. In the 17th century, Johannes Kepler and Isaac Newton laid the mathematical groundwork of planetary

orbits and that in turn made it possible in theory to get the distances to the planets. Nah, but there was a catch. When you do the math, you find that measuring the distances to the other planets means you need to know the distance from the earth to the sun accurately. For example, it was known that Jupiter was about five times farther from the sun than the Earth was, but that doesn't tell you what it is in kilometers. So, how far away is the sun? Well, they had a rough idea using the number found by the Greeks. But to be able to truly understand the solar system, they needed a much more accurate value for it. To give you an idea of how important the distance from the Earth to the Sun is, they gave it a pretty high fallutin name, the astronomical unit or AU. Mind you, not an astronomical unit, the one. That's how fundamental it is to understanding everything. A lot of methods were attempted. Sometimes Mercury and Venus transit or cross the face of the sun, timing these events accurately could then be used to plug numbers into the orbital equations and get the length of an AU. Grand expeditions were sent across the globe

multiple times to measure the transits and didn't do too badly, but our atmosphere blurs the images of the planets, putting pretty big error bars on the timing measurements. The best they could do was to say the AU was 148,510,000 km, plus or minus 800,000 km. That's good, but not quite good enough to make astronomers happy. Finally, in the 1960s, astronomers used radio telescopes to bounce radar pulses off of Venus. Since we know the speed of light extremely accurately, the amount of time it takes for the light to get to Venus and back could be measured with amazing precision. Finally, after all these centuries, the astronomical unit was nailed down. It's now defined to be 149,597,870.7 km. So there, the Earth orbits the Sun on an ellipse. So think of that as the average distance of the Earth from the Sun. Knowing this number unlocked the solar system. It's the fundamental meter stick of astronomy and the scale we use

to measure everything. Having this number meant we could predict the motions of the planets, moons, comets, and asteroids. Plus, it meant we could launch our probes into space and explore these strange new worlds for ourselves, see them up close, and truly understand the nature of the solar system. And it's even better than that. Knowing the astronomical unit meant unlocking the stars. We have two eyes, and this gives us binocular vision. When you look at a nearby object, your left eye sees it at a slightly different angle than your right eye. Your brain puts these two images together, compares them, does the geometry, and gives you a sense of distance to that object. And you thought your teacher lied when she said math was useful in everyday life. We call this ability depth perception. You can see it for yourself by doing the thumb thing.

As you blink one eye and then the other, your thumb appears to shift position relative to more distant objects. That shift is called parallax. The amount of shift depends on how far apart your eyes are and how far away the object is. If you know the distance between your eyes, we'll call this the baseline, then you can apply some trigonometry and figure out how far away the object is. If the object is nearby, it shifts a lot. If it's farther away, it shifts less. It works pretty well, but it does put a limit on how far away we can reasonably sense distance with just our eyes. Stars are a bit beyond that limit. If we want to measure their distance using parallax, we need a lot bigger baseline than the few centimeters between our eyes. Once astronomers figured out that the Earth went around the Sun rather than vice versa, they realized that the Earth's orbit made a huge baseline. If we observe a star when the Earth is at one spot, then wait 6 months for the Earth to go around the Sun to the opposite side of its orbit and observe the star again, then in principle, we can determine the distance to the star, assuming we know the size of the Earth's

orbit. That's why knowing the length of the astronomical unit is so important. The diameter of Earth's orbit is about 300 million km, which makes for a tremendous baseline. Hooray! Except oops. When stars were observed, no parallax was seen. Was helioentrism wrong? No. It's just that stars are really and truly far away, much farther than even the size of Earth's orbit. The first star to have its parallax successfully measured was in 1838. The star was 61 Signney, a bit of a dim bulb, but it was bright enough and close enough for astronomers to measure its shift in apparent position as the Earth orbited the Sun. 61 Signney is about 720,000 astronomical units away. That's a soulc crushing distance, well over 100 trillion km. In fact, that's so far that even the Earth's orbit is too small to be a convenient unit. Astronomers came up with another one, the lightyear.

That's the distance light travels in a year. Light's pretty fast and covers about 10 trillion kilometers in a year. It's a huge distance, but it makes the numbers easier on our poor ape brains. That makes 61 Signi a much more palatable 11.4 light years away. Astronomers also use another unit called a parseek. It's based on the angle a star shifts over the course of a year. A star one parseek away will have a parallax shift of one arcsec, 136 hundredth of a degree. That distance turns out to be about 3.26 lightyear. As a unit of distance, it's convenient for astronomers, but it's a terrible one if you're doing the Kessle run. Sorry, Han. The nearest star to the sun we know of, Proximus Centuri, is about 4.2 lighty years away. The farthest stars you can see with the naked eye are over a thousand lightyears distant, but the vast majority are within 100 lightyear. Space-based satellites are used now to accurately find the distance to hundreds of thousands of stars. Still, this method only works for relatively nearby stars, ones that are less than about a

thousand light-years away. But once we know those distances, we can use that information on more distant stars. How? Well, like gravity, the strength of light falls off with the square of the distance. If you have two stars that are the same intrinsic brightness, giving off the same amount of energy, and one is twice as far as the other, it will be one quarter as bright. Make it 10 times farther away, it'll be 1/100th as bright. So, if you know how far away the nearer one is by measuring its parallax, you just have to compare its brightness to one farther away to get its distance. You have to make sure they're the same kind of star. Some are more luminous than others. But thanks to spectroscopy, we can do just that. A star's distance is the key to nearly everything about it. Once we know how far it is, and we can measure its apparent brightness, we can figure out how luminous it is, how much light it's actually giving off, and its spectrum tells us its temperature. With those in hand, we can determine its mass and even its diameter. Once we figured out how far away stars are, we started to grasp their true physical nature. This led to even more methods of

finding distances. The light given off by dying stars, exploding stars, stars that literally pulse get brighter and dimmer over time. All of these and more can be used to figure out how many trillions of kilometers of space lie between us and them. And we see stars in other galaxies, which means we can use them to determine the actual size and scale of the universe itself. And all of this started when some ancient Greeks were curious about how big the earth was. Curiosity can take us a great great distance. Today you learned that ancient Greeks were able to find the size of the earth and from that [music] the distance to and the sizes of the moon and sun. Once the Earth's sun distance was found, parallax was used to find the distance to [music] nearby stars. And that was bootstrapped using brightness to determine the distances to much farther stars. Twinkle, twinkle, little star. Oh, I know who you are.

[music] >> At first glance, stars pretty much all look alike. twinkly dots scattered across the sky. But as I talked about in episode two, when you look more closely, you see differences. The most obvious is that some look bright and some faint. As I said, then sometimes that's due to them being at different distances. But it's also true that stars emit different amounts of light, too. If you look through binoculars or take pictures of them, you'll see that they're all different colors, too. Some appear white, some red, orange, blue. And for a long time, the reason for this was a mystery. In the waning years of the 19th century, astrophotography was becoming an important scientific tool. Being able to hook a camera up to a telescope and take long exposures meant being able to see fainter objects, revealing previously hidden details. It also meant that spectroscopy became a force, say it with me now, for science. A spectrum is the result when you divide the incoming light from an object into individual colors or wavelengths. This reveals a vast amount of physical data about the

object. But in the late 1800s, we were only just starting to figure that out. Interpreting stellar spectra was a tough problem. The spectrum we measure from a star is a combination of two different kinds of spectra. Stars are hot, dense balls of gas. So they give off a continuous spectrum. That is, they emit light at all wavelengths. However, stars also have atmospheres, thinner layers of gas above the denser inner layers. These gases absorb light at specific wavelengths from the light below depending on the elements in them. The result is that the continuous spectrum of a star has gaps in it, darker bands where different elements absorb different colors. At first, stars were classified by the strengths of their hydrogen lines. The strongest were called A stars, the next strongest B, then C, and so on. But in 1901, a new system was introduced by spectroscopist Annie Jump Cannon, who dropped or merged a few of the old classifications and then rearranged them into one that classified stars by the strengths and appearances of many different absorption lines in their spectra. A few years later, physicist Max Plank solved a

thorny problem in physics, showing how objects like stars give off light of different colors based on their temperature. Hotter stars put out more light at the blue end of the spectrum, while cooler ones peaked in the red. Around the same time, Bengali physicist Magnod Saha solved another tough problem. How atoms give off light at different temperatures. Two decades later, the brilliant astronomer Cecilia Payne Gaposkin put all these pieces together. She showed that the spectra of stars depended on the temperatures and elements in their atmospheres. This unlocked the secrets of the stars, allowing astronomers to understand not just their composition, but also many other physical traits. For example, at the time it was thought that stars had roughly the same composition as the earth. But Pay Gaposkin showed that stars were overwhelmingly composed of hydrogen with helium as the second most abundant element. The classification scheme proposed by Canon and decoded by Payne Kaposkin is still used today and arranges stars by their temperature, assigning each a letter. Because they were rearranged from an older system,

the letters aren't alphabetical. So the hottest are O type stars. Slightly cooler are B followed by A, F, G, K, and M. It's a little weird, but many people use the pneummonic O be a fine guy, kiss me, or O be a fine girl, kiss me to remember it, which was dreamed up by Annie Jump Cannon herself. Each letter grouping is divided into 10 subgroups, again according to temperature. We've also discovered even cooler stars in the past few decades, and these are assigned the letters L, T, and Y. The sun has a surface temperature of about 5,500° C and is a G2 star. A slightly hotter star would be a G1 and a slightly cooler star, a G3. Sirius, the brightest star in the night sky, is much hotter than the sun and is classified as an A0. Beetlejuice, which is red and cool, is an M2. Stars come in almost every color of the rainbow. Hot stars are blue, cool stars red. In between there are orange and even some yellow stars. But there are no green stars. Look as much as you want and you won't find any. It's

because of the way our eyes see color. A star can put out lots of green light, but if it does, it'll also emit red, blue, and orange. And our eyes mix those together to form other colors. A star can actually emit more green light than any other color, but we'll wind up seeing it as white. How do I know? Because if you look at the sun's spectrum, it actually peaks in the green. Isn't that weird? The sun puts out more green light than any other color, but our eyes see all the mixed colors together as white. Wait, what?

White? You may be thinking the sun is actually yellow. Not really. The light from the sun is white, but some of the shorter wavelengths like purple and blue and some green get scattered away by molecules of nitrogen in our air. Those appear to be coming from every direction but the sun, which is why the sky looks blue. The sun doesn't emit much purple, so the sky doesn't look purple and the green doesn't scatter as well as blue. That gives the sun a yellowish tint to our eyes. And looking at the sun is painful anyway, so it's hard to accurately gauge what color it appears. That's also why sunsets look orange or red. You're looking through more air on the horizon to see the sun. So all the bluer light is scattered away. So we can use spectra to determine a lot about a star. But if you combine that with knowing a star's distance, things get amazing. You can measure how bright the star appears to be in your telescope.

And by using the distance, you can calculate how much energy it's actually giving off. What astronomers call its luminosity. An intrinsically faint light looks bright if it's nearby, but so does a very luminous light far away. By knowing the distance, you can correct for that and figure out how luminous the objects actually are. This was no exaggeration the key to understanding stars. A lot of a stars physical characteristics are related. Its luminosity depends on its size and temperature. If two stars are the same size but one is hotter, the hotter one will be more luminous. If two stars are the same temperature but one is bigger, the bigger one will be more luminous.

Knowing the temperature and distance means knowing the stars themselves. Still, it's a lot of data. A century ago, spectra were taken of hundreds of thousands of stars. How do you even start looking at all that? The best way to understand a large group of objects is to look for trends. Is there a relationship between color and distance? How about temperature and size? You compare and contrast them in as many ways possible and see what pops up. I'll spare you the work. A century ago, astronomers Inar Herzrung and Henry Norris Russell made a graph in which they plotted a star's luminosity versus its temperature. When they did, they got a surprise, a very strong trend. This is called an HR diagram after Herdsprung and Russell. It's not an exaggeration to call it the single most important graph in all of astronomy. In this graph, really bright stars are near the top, fainter ones near the bottom. Hot blue stars are on the left and cool red stars on the right. The groups are pretty obvious. There's that thick line running diagonally down the middle, the clump to

the upper right, and the smaller clump to the lower left. This took a long time to fully understand, but now we know this diagram is showing us how stars live their lives. Most stars fall into that thick line, and that's why astronomers call it the main sequence. The term is a little misleading. It's not really a sequence per se, but as usual in astronomy, it's an old term, and we got stuck with it. The reason the main sequence is a broad long line has to do with how stars make energy. Like the sun, stars generate energy by fusing hydrogen into helium in their cores. A star that fuses hydrogen faster will be hotter because it's making more energy. The rate of fusion depends on the pressure in a stars core. More massive stars can squeeze their cores harder, so they fuse faster and get hotter than low mass stars. It's pretty much that simple. And that explains the main sequence. Stars spend most of their lives fusing hydrogen into helium, which is why the main sequence has most of the stars on it. Those are the ones merrily going about their starry business of making energy. Massive stars are hotter and more luminous, so they fall on the

upper left of the main sequence. Stars with lower mass are cooler and redder, so they fall a little lower to the right, and so on. The sun is there, too, more or less in the middle. What about the other groups? Well, the stars on the lower left are hot, blue, white, but very faint. That means they must be small and we call them white dwarfs. They're the result of a star like the sun eventually running out of hydrogen fuel. We'll get back to them in a future episode. The stars on the upper right are luminous but cool. They must therefore be huge. These are red giants, also part of the dying process of stars like the sun. Above them are red super giants, massive stars beginning their death stage. You can see some stars that are also that luminous, but at the upper left, those are blue super giants. They're more rare, but they too are the end stage for some stars. And again, we'll get to them soon enough in a future episode. But I'll just say here that it doesn't end well for them. But on a brighter note, we literally owe our existence to them. And this implies

something very nifty about the HR diagram. Stars can change position on it. Not only that, but massive stars versus low mass stars age differently and go to different parts of the HR diagram as they die. In many ways, the diagram allows us to tell at a glance just what a star is doing with itself. This difference between the way low mass stars like the sun and higher mass stars age is actually critical to understanding a lot more about what we see in the sky. So much so that they'll be handled separately in later episodes.

I'm sorry to tease so much about what's to come. But this aspect of stars, finally understanding them physically, was a major step in astronomy leading to understanding so much more. And don't you worry, we'll get to all that today. You learned that stars can be categorized using their spectra. Together with their distance, this provides a wealth of information about them, including their luminosity, size, and temperature. The HR diagram plots stars luminosity versus temperature, [music] and most stars follow along the main sequence where they live most of their lives. When you look up at the night sky, and if you happen to live far from city [music] lights, you can see thousands of stars. It seems like the sky is crammed shoulderto-shoulder with them, and you're only seeing the tiniest fraction of stars there are. Billions more exist that are too faint to see [music] with just your eyes. As you ponder this incredible number, a natural thought arises. Are there planets circling those stars, too? [music] And are any of them like Earth? People just like you and me have wondered about this for thousands of [music] years. And

right now, today, we can answer that question. And the answer is yes. Our [music] sun is orbited by a lovely array of planets and they're wildly diverse. Big, small, airless, rocky, gaseous, hot, cold, and more. That makes you think that maybe forming planets is easy with so many varieties to choose from. Even if making planets is hard, there are so many stars in the sky that it's hard to believe our sun is the only one that's been able to pull this trick off.

Astronomers have fredded over this for a long time, but trying to find such planets is hard. The biggest problem is any such planets would be faint, far away, and sitting right on top of their parent star. Being able to see one in a telescope would be like trying to spot a firefly sitting next to a search light. So, if you can't spot a planet like that directly, maybe you can spot it indirectly. Imagine two kids, one big and one small, facing each other. They clasp hands and start to spin around. As they do, the little kid who weighs less will make a big circle, and the bigger kid will make a small circle. The same would be true of a star and planet. As the planet orbits the star, it makes a big circle or ellipse. But the planet has gravity, too, and it tugs on the star. That means the star will make a small circle, what's called reflexive motion. For a long time, astronomers looked really hard for this motion in nearby stars. But it turns out that indirect effect is also too small to see. There were a few false alarms, but no real planets. Then in 1992,

everything changed. Astronomers Alexander Volshan and Dale Frail made a shocking announcement. They found not just one planet, but two, orbiting a pulsar, the dead remnant of a star that had exploded. This was really bizarre. When a star explodes, it's a catastrophic event that should destabilize any orbiting planets. It's the last place anyone would have thought to find them. However, follow-up work quickly confirmed that the planets did indeed exist, and in fact, a third one was found a few years later. The first true alien planets had been found. Officially, we call them exoplanets, [music] which you have to admit is pretty cool. While this was an incredible discovery, it was still a little unsatisfying. For one thing, pulsars are really weird. And for another, it looked like those planets may have formed around the pulsar after the supernova explosion from the material left over from the catastrophe. That's nothing at all like our own solar system. And that still left the question open. Are there exoplanets orbiting sun-like stars? We didn't have to wait long to find out. In 1995, Swiss

astronomers Michelle Mayor and Ddier Kaos made a big announcement. They had found a planet orbiting the star 51 PEG, a star very much like the sun just 50 lighty years away. How did they do it? Well, remember those two kids holding hands and circling each other? Even though the wiggling back and forth of the star is too small to measure, that doesn't mean the effect is undetectable. As the host star of the exoplanet makes its little circle, sometimes it's headed toward us and sometimes away. That means that its light will undergo a Doppler shift and that can be detected. It's not a big shift and takes some pretty fancy equipment to see it, but it's measurable. That's how Mayor and Keros found their planet. And the planet they found called 51 pegb is weird. For one thing, the orbital period turns out to be just a little over 4.23 days. That's right, I said days. That means the planet is seriously close to its parent star, just 8 million km out. Compare that to Mercury, which on average is 55 million km from the sun. Not only that, but the amount of Doppler shift in the

star is related to the mass of the planet. A more massive planet pulls harder on the star, making it move more quickly. They found the planet was at least half the mass of Jupiter and probably more. That was a problem. According to planetary formation models, that wasn't possible. You can't form a planet that big, that close to a star. Well, it turns out the models are probably right. The planet didn't form that close. It probably formed farther out, just like Jupiter did. And like Jupiter, it then moved, migrated inward toward the star as it interacted with the disc of planet forming material around the star. In our solar system, Jupiter didn't get very far in its inward motion. It's thought that interactions with Saturn put the brakes on that and pulled Jupiter out to where it is now. Apparently, 51 pegb didn't have its own version of Saturn pulling on it. Its inward spiral continued until it ran out of disc material to interact with, which was very close indeed to the star. We call planets like that hot Jupiters. Once 51 pegb was found, other teams began looking for short period planets. And within a few years, several

more had been found, many of them hot Jupiters just like 51 pegb. Now, mind you, at first, there was a lot of doubt and skepticism in the community about these exoplanet discoveries. A lot of other phenomena could masquerade as planets, like star spots, pulsating stars, or background stars messing up the measurements. Scientists discuss these possibilities viferously, as well they should. Science is all about not fooling ourselves. A good scientist wants other scientists to try to poke holes in their ideas. It's disappointing to be wrong, but if we are, we want to know. That all changed in 1999. A planet called HD 209458b had been discovered on a very short orbit around its star, taking just 3.5 days. As luck would have it, from Earth, we see the planet's orbit edge on. That means once per orbit, it passes directly in front of its star. This event is called a transit. And when the planet transits the star, it blocks a little bit of the stars light. And that means we can detect a dip in the stars brightness. And sure enough, that dip

was found. HD 209458b was the first independent confirmation of an exoplanet. And pretty much everyone was on the bandwagon after that. The beauty of transiting exoplanets is that the amount of starlight blocked tells you how big the planet is. A big planet blocks more light. If we know the planet's mass from the stars Doppler shift, we can use the planet's size to calculate its density. This is important. A gas giant like Jupiter has a low density, and a rocky metallic planet like Earth has a very high density. Without even being able to see the planet directly, we can already start to determine what it's like physically. In 2009, NASA launched a space-based telescope named Kepler, designed specifically to stare at 150,000 stars to detect that telltale dip in light, indicating exoplanet transits. And oh my, did it work. By early 2015, Kepler found its 1,000 confirmed exoplanet. And there are 500 more confirmed from groundbased telescopes. That's more than 1,500 planets. And we have well over 3,000

more candidates from Kepler awaiting confirmation. All these planets have been found using indirect methods. What about actually seeing them, getting photographs of them? That's hard because planets are so faint. But it's not impossible. In 2004, the first picture of an exoplanet was released. 2M1207b, a planet with five times the mass of Jupiter. It orbits a brown dwarf, a peculiar kind of low mass star that we'll learn more about in a future episode. It's a young system, which makes it easier to see. The planet is still glowing hot from its formation, and it appears a lot brighter using a telescope that can see in the infrared. About a dozen other planets have been seen this way, too. My favorite is the planet orbiting the star Beta Ptorus. It has seven times the mass of Jupiter and orbits the star in about 20 years, and we've actually seen it move. Images taken a few years apart actually show the planet in different positions around the star, confirming its orbital motion.

That is incredible. Taking photos of these planets is still a daunting task, which is why so few have been seen. But we're getting better at this, and as new technology comes along, we'll get more pictures of exoplanets and learn even more about them. The sheer variety of exoplanets is staggering. Hot Jupiters are the easiest to find because they're massive and fast, making their signal easier to detect. But as the techniques have improved, planets of lower mass have been seen. The smallest exoplanet found is smaller than Mercury and not much bigger than Earth's moon. We've seen planets bigger than Earth but smaller than Neptune called super Earths. About 500 multiple planet systems have been found too, including one with seven planets. We found them around every kind of star, too. Exoplanets have been detected around stars like the sun, as well as tiny cool red dwarfs, hot, massive blue stars, and even red giants, stars nearing the ends of their lives. One exoplanet system announced in 2015 is incredibly old. The host star is 11 billion years old. When

our solar system was just beginning to form, these planets were already over 6 billion years old, older than our solar system is now. We've even seen planets orbiting binary stars, making Star Wars seem a lot closer to home than being in a galaxy far, far away. We've seen so many exoplanets now that we can extrapolate a bit and get some numbers. The results are staggering. In our galaxy alone, there may be hundreds of billions of planets. In fact, planets may outnumber stars in the sky. Now, if we've seen planets as big as Jupiter and as small as Mercury, then we must have seen planets around the same size as Earth, right? Yes. Yes, we have. We've actually found hundreds of them so far.

It looks like making planets the same size as ours is pretty easy for stars to do. But Earthsized is one thing. Earthlike is another. How many of these planets might actually be habitable? That is at the right distance from their star to have Earthlike conditions where liquid water could exist on their surface. We're not sure, but from what we've seen so far, it looks like the galaxy may have more than 10 billion Earthlike planets. 10 billion and maybe a lot more than that. Now, I want to be careful here. We don't know what kind of atmospheres these planets will have or what they're composed of. Do the planets have magnetic fields strong enough to prevent solar wind from eroding away their atmosphere? Do they even have an atmosphere, let alone liquid water? We don't know. But still, there are a lot of planets out there. There could very well be a twin of Earth orbiting a star not too far away. And over the whole galaxy, we could be part of a very large family. After all this time, we finally have an answer to one of the biggest questions we've ever asked in astronomy.

The sky is filled with planets. Today, you learned that planets orbiting other stars exist and can be detected with a variety of methods. Nearly 2,000 have been found so far. The most successful method [music] is using transits, where a planet physically passes in front of its parent star, producing a measurable dip in the stars light. Another way is to measure the Doppler shift in a stars light due to reflexive motion as the planet orbits. Exoplanets appear to orbit nearly every kind of star. And we've even found planets that are the same size as Earth. We think there may be many billions of Earthlike planets in our galaxy. The sky, we now know, is full of stars and planets. Stars are massive enough to fuse hydrogen into helium in their cores, generating energy. The heat created by that process tries to expand them, but their gravity balances that outward force, creating an equilibrium. Planets, even gas giants like Jupiter, are far too small to generate fusion. The stuff inside them resists being squeezed, so their gravity is balanced by simple gas pressure.

Jupiter is only about 1% the mass needed to have fusion going on in its core. That's a pretty big gap between a big planet and a small star. It's natural to ask what would happen if we dumped more mass onto Jupiter. Eventually, it would become a star. The pressure in its core would get high enough to initiate hydrogen fusion. But what if we stopped just short of that? What if we have an object far more massive than a planet, but not quite massive enough to become a true star? What sort of thing would we have then? What indeed?

By the late 1950s, astronomers were starting to get a pretty good handle on how stars worked. The mathematical equations that govern the physical processes of fusing hydrogen into helium were being worked out and applied to what we knew from observing the stars themselves. In the 1960s, the idea that you could have a star with a minimum mass was becoming clear. If it had less than about 0.075 075 times the sun's mass, roughly 75 times the mass of Jupiter. It simply lacked the oomph needed to squeeze hydrogen together hard enough to generate fusion. What would such an object look like? Well, it might form like a star, collapsing from a gas cloud just like the sun did 4.6 billion years ago. But instead of turning on and becoming a star, it would simply sit there cooling. It might start off pretty hot due to the physical forces that made it, but it couldn't sustain that heat. Like a charcoal ember, it would radiate its heat away. After a few billion years, it would be cold, black, and for all intents and purposes, dead. As people started working out what such an object would be like, they tried to come up with a name for them. These things

were black and small, but the name black dwarf was already being used for another type of object. Some people called them substellar objects, but that wasn't terribly catchy. really low mass stars are red. And these new objects would be so cool that they'd emit light in the infrared and almost nothing at all in the visible. So they're somewhere between red and black. Jill Tarter, then a young astronomer working in the field, but who later made a name for herself looking for aliens, and oh boy, we'll get to that later, dubbed them brown dwarfs. And she didn't mean it literally. Stars can't be brown. But the name stuck. Work proceeded in figuring out what brown dwarfs were like, and a lot of progress was made despite there not being any actual examples of them found. But the hunt was on. Now, as I talked about in episode 26, astronomers classify stars by their temperature. The hottest are O stars, then B stars, which are slightly cooler down through A, F, G, K, and with the coolest stars being M. But then in 1988, astronomers found a star that was so cool it was distinct from even the M-class stars. It was the first of a new cooler class of stars, so

it was given the letter L. Why L? Because there wasn't any other astronomical object that used that letter, so why not? Many more such L stars were found. But still, these weren't true brown dwarfs. These stars were massive enough to initiate fusion in their cores. Worse yet, when brown dwarfs are first born, they're very hot. They can mimic higher mass L stars for a while, looking just like them, making it hard to distinguish between the two. But then a way out was found. A low mass brown dwarf. if it was determined would have lithium in it whereas normal stars wouldn't. Lithium is an element, the next one in the periodic table after hydrogen and helium. It can be fused much like hydrogen can and regular stars would quickly use up their supply of lithium when they were still young. Brown dwarfs lighter than about 65 times the mass of Jupiter wouldn't fuse lithium at all. Very careful observations of an object would be able to detect lithium if it were there. That would provide a test to distinguish brown dwarfs from regular stars. The lithium test isn't perfect, but it does work under a lot of circumstances.

Astronomers began using it to look for actual real brown dwarfs. And so they found one. In 1995, a group of astronomers was observing the Pleaides, a nearby cluster of stars that's visible to the naked eye. They were trying to find the faintest stars they could in the cluster to get a complete sample of its membership. The advantage of this is that the distance to the cluster was pretty wellnown, so a faint star in it must be very low mass. They found an oddball object which they named Tato 1. It was very red and cool and best of all, lithium was found in its spectrum. The best models of stellar mass showed that it had about 50 times the mass of Jupiter or 0.05 times the mass of the sun. It was clearly substellar. Huzzah! The very first true brown dwarf had been found. At just about the same time, astronomers found that another nearby star called Giza 229 had an extremely faint companion. spectra showed that it was even weirder than Tet 1. It also had lithium and so it was clearly a brown dwarf, but its spectrum showed it had methane in its atmosphere. Methane is a

delicate molecule and would break down even in the mild heat of T1's atmosphere. This new object called Giza 229b was even cooler than Tato 1. It was looking like we needed yet another letter to classify stars and so Tarfs became a thing. On a personal note, when I worked on Hubble Space Telescope, Giza 229B was one of our camera's first targets after it was installed on Hubble in 1997. I was lucky enough to work on the spectrum we took of it, and it was freaky. It emitted almost no light in the visible part of the spectrum and rocketed up in the infrared. I'd seen a lot of stellar spectra before, but nothing like this. Remember, Gleasa 229b had only been discovered 2 years before.

I became so intrigued by it, I wound up studying low mass stars and brown dwarfs for several years after. Well, it didn't take long before more brown dwarfs were found. In 2009, NASA launched the Widefield Infrared Survey Explorer, or WISE, an orbiting observatory designed to scan the entire sky looking at infrared light. It found hundreds of brown dwarfs. And now at least 2,000 are known with more found all the time. Some are so cool that they form yet another classification, Y dwarfs. So now we have O, B, A, F, G, K, M, L, T, and Y. You're on your own for an acronym here. So, if brown dwarfs aren't brown, what color are they? Some are so cool they don't emit visible light at all, so they'd be black. You could be right over one and you wouldn't see it. But some are still warm and so give off some visible light, feeble as it might be. What color would they look? Funny thing, they might be magenta. You'd think they'd be really red because of their temperature. But it's a bit more complicated than that.

Remember, they have molecules in their atmospheres that absorb specific colors of light. In some brown dwarfs, there are molecules like methane and even water. Well, steam at those temperatures that are pretty picky about what colors they absorb. Some of these molecules block more red light than blue, so that messes with their colors, making them look magenta. Wise takes pictures in the infrared, which our eyes can't see. To make pictures, astronomers map each infrared color to one our eyes can see. So, an image using the shortest wavelength infrared detector is displayed as blue, the medium wavelength one green, and the longest one red. Brown dwarfs put out a lot of light in the intermediate wavelength Wise sees. So, weirdly, they appear green in Wise pictures. That does make them easy to spot in those images, even when thousands of other stars are visible, too. The physical nature of brown dwarfs is just as weird as you'd expect. For one thing, they have a very unusual characteristic. As they get more massive, they don't get any bigger.

Usually, if you dump mass onto an object, it gets bigger. Take two lumps of clay and smush them together, and you get one more massive, slightly larger lump. Same with planets and stars. But brown dwarfs are different. At their cores, the density is very high and the physics is a bit different than what you'd expect. The details are complex, but the end result is that when you add more mass to them, they actually get denser, not bigger. This effect becomes important right around the mass of Jupiter, which means that a brown dwarf twice as massive as our biggest planet won't actually be a whole lot bigger.

So, what's the difference between a small brown dwarf and a really big planet? Well, not much. Nature isn't as picky as we are about having narrowly defined borders between classes of objects. Some people say a planet forms from a dis of material around a star, growing larger as it accretes stuff, while a brown dwarf collapses directly from a cloud of gas and dust. But then you could have two objects the same mass and which look exactly the same, yet one would be a planet and one a brown dwarf depending on how they formed. That strikes me as inconvenient. Astronomers are still debating this and it gets worse. For example, as I said before, brown dwarfs over 65 times Jupiter's mass fuse lithium. It turns out that ones more massive than about 13 times Jupiter can also fuse dutarium, an atom that's very similar to hydrogen, except it has a proton and a neutron in its nucleus. But neither of these fuses actual hydrogen, so they're not considered true stars. That's still a little arbitrary. So again, I don't make too much of a fuss about it. I think

it's best not to think of them as planets or stars, but something with characteristics of both. For example, the way their atmospheres behave depends a lot on how hot they are. In some, iron is vaporized, a gas. In others, they're just cool enough that iron condenses out of the atmosphere, which means it literally rains molten iron. One more thing, the nearest star to the sun is a red dwarf called Proximus Centuri, which orbits the binary star Alpha Centtory. It's about 4.2 lighty years away. In 2013, astronomers announced the discovery of a binary pair of brown dwarfs called Lumman 16. They're only 6.5 light years away and became the third closest known star system to Earth. You got to wonder, could there be an even fainter, cooler brown dwarf closer to us? We know there's none in our solar system, even out in the Orort cloud. It would have been seen by now by one of several different sky surveys.

But a lightyear or two out, maybe. Is Proximus Century really the closest star, or will we find one even closer? It seems unlikely, but no more unlikely than the existence of brown dwarfs themselves. Maybe sometime soon we'll have to rewrite astronomy textbooks again. Today you learn that brown dwarfs are objects intermediate in mass between giant planets and [music] small stars. They were only recently discovered but thousands are now known. More massive [music] ones can fuse dutarium and even lithium but not hydrogen distinguishing them from normal stars. Sort of. Stars in the sky look pretty. Flickering intense diamonds dotting the velvety night. But make no mistake, they are churning cauldrons of violence, barely constrained thermonuclear generators creating enough energy to vaporize the earth a thousand times over. Their lives depend on it. Heck, our lives depend on it. But in their case, how they live and how long they live depends on that creation of energy. And in this

universe, nothing lasts forever. >> [music] >> Very roughly speaking, we can divide stars into two groups. Low mass stars and high mass stars. That line lies around eight times the mass of the sun. And the reason for that is something we'll get to shortly. Things get very exciting at the high end, but for now, let's take a look at the low mass end of the scale. Things are simpler there. Stars make energy in their cores by fusing hydrogen into helium. This is actually a pretty complicated process with lots of steps, but in the end, four protons plus some other ingredients get turned into one helium nucleus. Each time this happens, a little bit of energy is released. But in the core of a star, it happens countless times every second. It adds up to a lot of energy, enough to power a star. In fact, the rate at which hydrogen fusion occurs depends on how much pressure the star

has in its core. A higher mass star squeezes hydrogen harder, so it fuses far more quickly. The rate is so much higher in fact that even though they're a lot bigger than low mass stars, high mass stars run out of hydrogen fuel in their cores more quickly. The lower mass a star is, the longer it lives. In the lowest mass stars, cool red dwarfs, the rate is so slow, they last a long, long time. But there's more to it than that. They fuse hydrogen only in their very centers, and their cores aren't very big. Outside of this small region, the gas is convective, which means the hot stuff rises all the way to the surface, cools, then falls back to the core. That's important because it means the entire star is available for fuel. Both hydrogen and helium rise up through the star and then fall back down. The star can't fuse the helium, [music] but the hydrogen that makes it back to the center is just more grist for the milk.

Even after a long, long time, these stars can still shine as the hydrogen mixed throughout the star eventually makes it down to the center. So, how long can a really low mass red dwarf last? A trillion years. A trillion. The universe itself is less than 14 billion years old. So, even the oldest red dwarf stars are basically infants. They're just barely getting a start in life, and they'll look pretty much the same for the next oh 990 billion years or so. Eventually though, they do run out of fuel. When one does, the star itself will be nearly pure helium plus whatever heavier elements it was born with. and fusion will cease. It'll then cool, which will take many more billions of years. Finally, it'll be a cold, black, dead star. And for them, that'll be pretty much that. Or so we think. The universe is far too young for any of these red dwarfs to have run out of fuel yet. That's true for stars at the lower end of the mass scale, up to roughly a third of the mass of the sun. But stars with more mass than that. Stars more like the sun itself work a little

differently. Their cores are bigger and hotter and denser. The different conditions inside them means the material in them doesn't convect. The hot stuff doesn't rise away from the core. What's in the core stays in the core. Outside the core, there's a very deep and active convection zone, but that doesn't interact with the material in the core itself. This means that not only do stars near the sun's mass use up their fuel more quickly than red dwarfs, they have relatively less fuel to work with as well. Depending on their mass, their lives aren't nearly as long. For the sun, that total lifespan is about 12 billion years, give or take. The sun is nearly 5 billion years old now, so it's approaching middle age. Its life is similar to other stars of similar mass.

So, let's take a look into the far future. Will our sun go gently into that good night? Ever since the sun was born, it's been fusing hydrogen into helium in its core. The helium is trapped there, unable to be used as fuel, and it's building up in the core like ash in a fireplace. As it does, the density in the sun's core slowly increases. When you compress a gas, it heats up. That means every day the sun's core gets a wee bit hotter. That extra heat works its way out through our star, heating up the outer layers, too. A hotter gas shines more brightly. So, the sun has been steadily increasing in brightness, too. It's an incredibly slow but inexraable process. The sun's increased in luminosity by about 40% since it was born. And it's not done. Helium is still building up in its core, and eventually it'll run out of hydrogen to fuse.

Without fuel, fusion in the core will stop. Helium can be fused into carbon and a little bit of oxygen and neon, but it has to be incredibly hot to do so. And the sun's core won't be anywhere near those conditions. Still, half the sun's mass is bearing down on the core. So, even though fusion stops, the core will continue to contract and heat up. It'll get so hot that the temperature outside the core will get to hydrogen fusion temperatures, like igniting a match by holding it near a flame. The hydrogen outside the core will start to fuse. That'll occur in a shell surrounding the core, adding its heat to the outer layers of the sun as well. When you heat a gas, it expands. The sun's outer layers will do just that, and our star will grow to well over twice its current size. Astronomers call a star like this a subgiant. Not only will it grow, but its color will change. The sun will be giving off more total energy than it does now, but its surface area will increase so much that the amount of energy radiated per square cime will go down. There will just be a lot more square cime. This means, ironically, that the surface will

actually cool. The sun's temperature will go down. Cooler stars are red, and so the sun will be, too. But the core still isn't done. The details are complicated, but the core continues to contract and heat up. It gets so hot that the outer layers swell even more, and the sun can bloat up to a fantastic 10 to 150 times its present size. It will then be a red giant. Red giants are fantastically bright because of all the energy percolating up from their interiors, coupled with their enormous size. When it turns into a red giant, the sun will increase its luminosity by an incredible 2,000 times. That huge increase in luminosity does more than just make the sun bright, though. As a red giant, the sun will be so swollen that the gravity at its surface will drop substantially from what it is now. The force of gravity holding it down is weak, but the force from all that light blasting out from below is strong, easily overpowering gravity. Material on the surface will get blown off like a leaf in a hurricane. Think of it as a super solar wind. That means the sun

will shed mass a lot faster than it does now. While it's a red giant, it'll lose fully a third of its mass. At some point, the core contracts and heats up so much that the conditions will finally be primed for helium fusion. Suddenly, helium is converted into carbon, releasing a lot of energy. In a weird twist, due to a lot of very complicated physics, the core itself winds up expanding, absorbing most of that energy. In the end, less energy is pumped into the outer layers. So, the sun's outer layers contract. The sun shrinks. The outer layers get dense again, which warms them up a bit, and the sun turns from red to orange. It's still pretty big, more than 10 times its current day size and puts out 20 to 50 times as much energy as it does now. But then we see a sort of reboot of what's happened in the core before. This time though, helium is fusing in the core and the carbon is building up like ash. Once again, the core starts to contract and heat up. This dumps more energy into the outer layers. They expand and cool, and the sun turns into a red giant again.

This time it's even brighter and bigger and loses even more mass, shedding over half of what's left. In the meantime, helium fusion isn't very stable to say the least. It swings wildly, increasing and decreasing its rate by huge amounts on very short time scales. The sun will probably undergo a series of tremendous peroxisms, epic eruptions as the helium fusion spikes, creating huge surges in energy production. It'll lose even more mass each time. Finally, though, we approach the endgame. The sun doesn't have enough mass to squeeze carbon atoms enough to fuse them. They build up in the core until there's no more helium and fusion stops. On top of that, well, there's nothing on top of that. By this time, the sun will have literally blown off all its outer layers. It's essentially a naked core, a hot, intensely bright, super compact ball, not much bigger than Earth. We call this a white dwarf. Its fade is sealed. It can't generate energy, so it'll cool and fade over the next few tens of billions of years. At that point, we can safely say the sun is dead. But that's not

always the case for all stars. Some with more mass than the sun still go through another phase before dying. They form what are called planetary nebula. And these are so amazing and beautiful that they rate their own episode, which along with more fun stuff about white dwarfs, we'll get to next time. So that's it for the sun. But what about us? Of course, the Earth won't fare very well during all this. It'll get cooked before the sun even becomes a subgiant. And by the time the sun is a red giant, we'll be good and fried. The average temperature on our planet will be so high that it'll melt rocks. And in a sense, it'll look much like it once did so many billions of years before when it first formed, a molten ball of rock. But it gets worse. When the sun expands, it'll reach a radius very close to the size of the Earth's orbit. Now, does that mean our planet itself will be consumed by the sun? Maybe not. As the sun expands, remember, it loses mass. That means its gravitational pole will weaken and Earth will move out from the sun into a bigger orbit. If things work out right for us,

the Earth will back off faster than the sun expands, and we'll avoid finding out what it's like to be inside a star. Due to a quirk of physics, if the sun loses more than half its mass, its gravity will decrease so much the planets will no longer be gravitationally bound to it, and they'll be flung into interstellar space. I'm not sure that's any better for us. Either way, the Earth will have been long dead, and hopefully, humanity will have figured out the trick to interstellar travel by that time. All those Earthlike exoplanets around younger stars will look pretty nice in a billion years or two. Mind you, this isn't happening anytime soon. The sun won't become a subgiant until it's nearly 11 billion years old, over 6 billion years from now. The first red giant stage happens when it's over 11.5 billion years old, and the second stage a half billion years later. This is a long, long time coming. If the sun had more mass, it could avoid this fate. A star with about eight times the sun's mass can. It has enough mass to squeeze those carbon nuclei hard enough to fuse

them together. Those highmass stars have a much different and far more explosive fate than lower mass stars like the sun. And trust me, we'll cover that in lots of fun detail soon. And while this all sounds very bleak, not all is lost. This is all part of the natural cycle of the universe. And in fact, we wouldn't be here at all if it weren't for the way stars live and die. Be thankful for what you have and that we can not only observe it, but understand it, too. Today, you learn that very low mass stars live a long time, fusing all their hydrogen into helium over a trillion years. More massive stars like the sun live shorter lives. They fuse hydrogen into helium and eventually helium into carbon and [music] also some oxygen and neon. When this happens, they expand, get brighter, and cool off, becoming red giants.

>> [music] >> They lose most of their mass, exposing their cores, and then cool off over many billions of years. Hey folks, Phil Plate here. In the last episode of Crash Course Astronomy, I talked about the eventual fate of the sun and other low mass stars like it. After a series of expansions and contractions, they blow off their outer layers, become white dwarfs, and fade away over billions of years. The end. Except not so much. First, white dwarfs are pretty awesome objects and worth investigating. And second, when a star becomes a white dwarf, it produces what is quite simply one of the most gorgeous objects in space.

[music] To recap, when the sun ages, it under goes a series of changes in its core. It's fusing hydrogen into helium now, today, and will eventually fuse helium into carbon, and it'll make a dash of oxygen and neon, too. But when it runs out of helium to fuse, it's in trouble. It doesn't have enough mass to squeeze the carbon nuclei together, so they can't fuse. Fusion is the sun's energy source. Once the core is nearly pure carbon, that power is switched off. By this time, nearly 8 billion years from now, the sun's outer layers are gone, expelled by all the shenanigans going on in the core. What's left of our star is just its core, exposed to the dark of space. Over the next few billion years, it'll cool and fade to black. That might seem like the end of the story, but I skipped a step, and it's a beauty. When helium fusion stops, the sun's core will have about half the mass of what the sun does today. The rest will have blown away into space around it. What remains is basically composed of electrons and

carbon nuclei mixed with a small amount of a few other elements. So what kind of an object are we left with here? You may know that light charges repel. Electrons have a negative charge and repel each other. The tighter you squeeze them, the stronger that pressure. There's also a second force called electron degeneracy pressure. It's a result of some of the weird rules of quantum mechanics. For you QM nerds, it's the polyexclusion principle. This describes how subatomic particles behave on teeny scales. One of these rules is that electrons really hate to be squeezed together above and beyond simple electric repulsion. This resistance is phenomenally strong and becomes the dominant force in supporting the core of a star once helium fusion stops. By the time this electron degeneracy pressure balances the core's immense gravity, the core is only about the size of Earth, 1% the original width of the sun, and it's called a white dwarf. Listing its characteristics is enough to melt your brain. Ironically, everything about it gets amplified as its size shrinks. It becomes

ridiculously dense. A single cubic centimeter of it, the size of a six-sided die, has a mass of a million g, one metric ton. An ice cream scoop of white dwarf material outweighs 60 cars. Because it's so dense, the gravity at the surface of a white dwarf screams up, easily topping 100,000 times the Earth's gravity. If you have a mass of 75 kilos, you'd weigh 7,500 tons if you stood on the surface of a white dwarf. Not that you can stand there. I mean, you'd be flattened into a greasy smear. But not for long. Newborn white dwarfs are hot. They can glow at temperatures of upwards of 100,000° C.

If you were on the surface, you'd be a vaporized and ionized smear. Their intense heat makes them white, and they're small, hence their name. They're so hot they also glow in the ultraviolet, even in X-rays. Weirdly though, because they're so small, they're actually quite faint. The closest one to us, Sirius B, can only be seen with a telescope, even though it's 9 light years away, one of the 10 closest known stars. Over 10,000 white dwarfs have now been found in our galaxy. Still, any gas near a newly formed white dwarf is likely to be affected by the intense radiation pouring out of it. And hey, wait a sec. When a star like the sun is in its final death throws, it expels its outer layers as a gaseous wind. You don't suppose?

Yep. By the time that white dwarf forms, the gas blown off hasn't gotten very far from it. At most a lightyear or two, that's plenty close enough to get zapped by the white dwarf radiation, causing that gas to glow in response. What does something like that look like? Why, it looks like this. This object is what we call a planetary nebula. It's a funny name and like so many other names, it's left over from when these objects were first discovered. The astronomer William Herschel, the same man who discovered infrared light and the planet Uranus, gave them that name because they appeared like small green discs through the eyepiece. The first planetary nebula was discovered in 1764 by the French astronomer Charles Mesier, who spent years scanning the skies looking for comets. He kept seeing faint fuzzy objects that he mistook for comets. So he decided to make a catalog of them, a sort of avoid these objects list. That list is now a staple of amateur astronomers because ironically it contains some of the best and brightest objects to observe. Among them is M27,

the 27th object on Messier's list. One of the biggest planetary nebula in the sky and one of my favorites. I love seeing it through my telescope when it's up high in the summer. Planetary nebula can be a bit tough to observe. Most are small and faint. On film, even with long exposures, they can appear to be little more than discs. For a long time, they weren't thought to be terribly complicated. When a star becomes a red giant and blows off its outer layers, it's rotating very slowly. So, the wind should blow away in a sphere surrounding the star. Many planetaries, as we call them for short, are round and look like soap bubbles. Pretty much what you expect when you look at an expanding shell of gas. But with the advent of digital detectors, their fainter structures became clearer and the true beauty of these phenomenal objects was revealed. Some are elongated. Some have spiral patterns. Some have jets shooting out on either side. Some have delicate tendrils streaming away from them. In fact, only a handful of the hundreds known are actually circular. Clearly, there's more to planetaries than meets [music] the eye. If the wind from a star

blows off in a sphere, how can planetaries come in all these fantastic shapes? It turns out the real situation is more complicated. As usual, when a star is a red giant, it spins slowly and blows off a dense but slow solar wind. If there's nothing else happening to the star, then that wind will blow outward in all directions spherically. However, as those outer layers of the star peel away, they expose the deeper, hotter part of the star. The star starts to blow a much faster, though far less dense wind. That wind catches up with and slams into the slower wind. When that happens, you get that idealized soap bubble nebula. But some stars are binary, two stars that orbit very close together. We'll go into detail on them in a later episode. But if the dying star has a companion, they may circle each other rapidly. That will shape the wind, forcing more of it outward in the plane of the stars orbits due to centrifugal force. The overall shape of the expanding gas is flattened like a beach ball someone sat on. When the fast wind kicks in, it slams into that stuff in the orbital plane and slows down. But there's less stuff in the polar

direction up and down out of the plane. It's easier for the wind to expand in those directions and it forms huge lobes of material stretching for trillions of kilome. That's a very common shape for planetary nebula. But to explain many of the shapes we see, the two stars would have to orbit improbably close. Most binaries aren't that tight. So what can cause these shapes? When I was in graduate school, my master's degree adviser Gnome Soaker came up with a nutty idea. Maybe the stars had planets like in our solar system. If the star expanded into a red giant and swallowed them, it would take millions of years or more for the planets to vaporize. And for all that time, they'd be orbiting inside the star, moving faster than the star itself. Like using a whisk to beat eggs. The planets inside the star would spin it up, causing it to rotate faster, fast enough to explain the shapes of planetaries. That was in the early 1990s. A few years later, the first exoplanets were found and we saw that massive planets orbiting very close to their stars were common. I suspect this

is why we see so many weird and fantastic shapes and planetaries. Their progenitor stars swallowed their planets. So, planetary nebula really may owe their existence to planets and we come full circle. The glow in a planetary nebula is due to the hot central white dwarf exciting the gas. Most of the gas is hydrogen which glows in the red. However, a lot of the gas is oxygen. There's not nearly as much oxygen as hydrogen. But kilo for kilo, oxygen glows more brightly than hydrogen due to the atomic physics involved. This oxygen glows green, giving planetaries their characteristic hue. Funny, when this green glow was first analyzed spectroscopically, astronomers couldn't identify the responsible element making it. They dubbed it nebulum, but eventually figured out it was just extremely tenuous oxygen. Other colors can be found, too. Nitrogen and sulfur glow red, and oxygen can emit blue as well. All adding to the beauty of these celestial bles. But these aren't just pretty pictures. The structure, color, and shape of a planetary nebula tells us

about the life of the star that formed it. We learn even more about stellar evolution by studying how stars die. Mind you, the gas in a planetary nebula is still expanding, cruising outward from its initial momentum of being thrown off the star. Eventually, the gas expands so much it thins out and stops glowing. That takes a few thousand years. So when you see a planetary nebula, you're seeing a very short snapshot of the life, the death of a star. And that's why we don't see many. Though there are billions of stars in the galaxy that die this way, this phase is very brief. Enjoy looking at them while you can. And what of the sun? Will it one day in the distant future be at the center of a planetary nebula it expels as it dies? Probably not. When the sun becomes a white dwarf, it most likely won't be energetic enough to make the surrounding gas glow. Most planetaries start off as stars more massive and hotter than the sun. When our sun dies, it'll go quietly and without a lot of visible fanfare. Alien astronomers, if they're out there in 8

billion years, may not even notice. But more massive stars do make quite the spectacle. And if they're really massive, more than about eight times the sun's mass, they really and truly make a scene when they die. They explode. But that's for next week. [laughter] Today you learned that when low mass stars die, [music] they form white dwarfs, incredibly hot and dense objects roughly the size of Earth. They can also form planetary nebula, huge, [music] intricately detailed objects created when the wind blown from the dying stars is lit up by the central white dwarf. [music] They only last a few millennia.

The sun probably won't form one, but higher mass stars do. Stars are in a constant struggle between gravity trying to collapse them and their internal heat trying to inflate them. For most of a stars life, these two forces are at an uneasy truce. For a star like the sun, the balance tips in its twilight years. For a brief glorious moment, it expands, but then blows away its outer layers, leaving behind the gravitationally compressed core. It goes out with a whimper. Well, a whimper from a two octillion ton barely constrained nuclearpowered fireball. But more massive stars aren't quite as resigned to their fate. When they go out, they go out with a bang. A very, very big bang.

[music] In the core of a star, pressure and temperature are high enough that atomic nuclei can get squeezed together and fuse. This releases energy and creates heavier elements. Hydrogen fusion makes helium. Helium fusion makes carbon. And each heavier element in general takes higher temperatures and pressures to fuse. Lower mass stars like the sun stop at carbon. Once that builds up in the core, the stars fate is sealed. But if the star has more than about 8 times the sun's mass, it can create temperatures in its core in excess of 500 million° C.

And then carbon will fuse. There are actually a lot of steps in this process, but in the end, you get carbon fusing into neon, magnesium, and some sodium. What happens next hearkens back to what we found goes on in the sun's core as it ages. Fuse an element, create a heavier one, then that heavier one builds up until the core contracts and heats up enough to start fusing it. So carbon fusion makes neon, magnesium, and sodium, and those accumulate. The core heats up, and when it reaches about a billion°, neon will fuse. Neon fusion creates more magnesium as well as some oxygen. These build up in the core. It shrinks, heats up to about 1.5 billion°, and then oxygen fuses, creating silicon.

Then that builds up until the temperature hits about 2 to 3 billion° whereupon silicon can fuse. Among a pile of other elements, silicon fusion creates iron. And that's trouble. Big, big trouble. Once silicon fusion starts, the star is a ticking time bomb. But before we light that fuse, let's take a step back. What's happening to the outer layers of the star? What do we see if we're outside looking back at it? Because the star was born massive, it spent its hydrogen fusing days as a blue main sequence star. Stars like this are extremely luminous and can be seen for tremendous distances. Like the sun though, a massive star changes when hydrogen fusion stops. Its core contracts and then helium fusion begins. It swells up just as the sun will, but instead of becoming a red giant, it generates so much energy, it becomes a red super giant. These are incredibly huge stars, some over a billion kilometers across, and they are luminous. For example, Beetlejuice and

Orion is a red super giant and one of the brightest stars in the sky despite being over 600 lighty years away. From that distance, you'd need a decent telescope to see the sun at all. And that's nothing compared to Vy Canis Majorus, the largest known star, which is a staggering 2 billion km across. We even have a special term for it, a hyper giant. As the core switches back and forth from one fusion reaction to the next, the outer layers respond by contracting and expanding. So, a red super giant can shrink and become a blue super giant. Riel, another star in Orion, is a blue super giant, putting out over 100,000 times as much energy as the sun. Okay, let's go back to the core. It now looks like an onion with multiple layers. Iron is building up in the center surrounded by fusing silicon. Outside that is a layer of fusing oxygen, then neon, then carbon, then helium, and finally hydrogen. You might think massive stars would last longer because they have more fuel than lower mass stars. But the cores of these monsters are far hotter and fuse

elements at far higher rates, running out of fuel more quickly. A star like the sun can happily fuse hydrogen into helium for over 10 billion years. But a star twice as massive as the sun runs out of hydrogen in just 2 billion years. A star with eight times the sun's mass runs out in only a 100 million years or so. And each step in the fusion process happens faster than the one before it. In an extreme case, like for a star 20 times the mass of the sun, it'll fuse helium for about a million years, carbon for about a thousand, and neon fusion will use up all its fuel in a single year. Oxygen lasts for only a few months. Silicon fuses at a ridiculously high rate. The star will go through its entire supply in, get this, a day. Yes, one day. The vast majority of a star's life is spent fusing hydrogen. The rest happens in the metaphorical blink of an eye. Silicon fuses into a bunch of different elements, including iron. That inert iron builds up in the core just like all those elements did before. And

just like before, the iron core shrinks and heats up. But there's a huge difference here. In all the previous fusion stages, energy is created. That energy transforms into heat and that helps support the soul crushing amount of stellar mass above the core. But iron is different. When it fuses, it actually sucks up energy instead of creating it. Instead of providing energy for the star, it removes it. This accelerates the shrinking, compressing the core, heating it up even more. Even worse, at these temperatures and pressures, the iron nuclei suck up electrons that are whizzing around, which are also helping support the core. It's a double whammy. Both major means of support for the star are removed in an instant. Silicon fusing into iron is happening so fast, this literally takes a fraction of a second once it gets started. The core gets its legs kicked out from under it. It doesn't shrink, it collapses. The gravity of the core is so mind-bogglingly strong that the outer parts crash down on the inner parts at a significant fraction of the speed of

light. This slams down on the central core, collapsing from several hundred km across down to a couple of dozen kilome across in just a few thousandth of a second. The star is doomed because all hell is about to break loose. Now, at this point, one of two things can happen. If the star has less than about 20 times the sun's mass, the core collapse stops when it's still 20 or so kilometers wide. It forms what's called a neutron star, which I'll cover in the next episode. If the star is more massive than this, then the collapse cannot be stopped by any force in the universe. The core collapses all the way down to a point. The gravity becomes so intense that not even light can escape.

A black hole is born. We'll cover black holes in a future episode as well, but for now, what happens when the core collapses and suddenly stops? The core of the star, whether it's a neutron star or a black hole, is now extremely small with terrifyingly strong gravity. It pulls on the stars matter above it hard. This stuff comes crashing down at a fantastic speed and gets hugely compressed, ferociously heating up. At the same time, two things happen in the core. While this stuff is falling in, a monster shock wave created by the collapse of the core moves outward and slams into the incoming material. The explosive energy is so insane, it slows down that material substantially. The second event is that the complicated quantum physics brewing in the core generates vast numbers of subatomic particles called nutrinos. The total energy carried by these little nutrinos is almost beyond reason. In a fraction of a second, they carry away 100 times as much energy as the sun will produce

over its entire lifetime. That's an incredible amount of energy. Now, these little beasties are seriously elusive and hate to interact with normal matter. One single nutrino could pass through trillions of kilometers of lead without even noticing. But so many are created in the core collapse and material barreling down on the core so dense that a huge number of them are absorbed. This vast wave of nutrinos slams into the oncoming material like a bullet train hitting a slice of warm butter. The material stops its infall, reverses course, and blasts outward. The star explodes. It explodes.

This is called a supernova, and it is one of the most violent and terrifying events the universe can offer. An entire star tears itself to shreds, and the expanding gas blasts outward at 10% the speed of light. The energy released is so huge they can be seen literally halfway across the universe. They outshine all the stars in the rest of the galaxy combined. The expanding material called the supernova remnant forms fantastic shapes. The most famous is the Crab Nebula from a star we saw blow up in the year 1054. The tendrils form as the material expands into the gas and dust that surrounded the progenitor star. As remnants expand and age, they become more tenuous. Some have bright rims as they push into material between the stars. Others form complex webs of filaments. I'm often asked if there are any stars near enough to hurt us when they explode. The quick answer is no. Even though supernova are incredibly violent, space is big. A supernova would have to be at least as close as 100 lighty years from us before we start feeling any real effects. The

nearest star that might explode in this way is Spya in Virgo, and it's well over a 100 lighty years away. I say might explode because it's at the lower mass limit for going supernova. It might not explode at all. Beetlejuice will certainly explode someday, but it's too far away to hurt us. We're pretty safe from this particular threat. I'll note that after all this, there is another kind of supernova involving white dwarfs, which we'll cover in our episode about binary stars. Happily, we're probably safe from them, too. Breathe easy. As terrifying and dangerous as supernova are, there's a very important aspect of them you need to know. Supernova are capable of great destruction, but they're also critical for our own existence. When the star explodes, the gas gets so hot and is compressed so violently by the blast that it underos fusion. What astronomers call explosive nucleiosynthesis, literally creating heavy elements explosively. New elements are produced in quantities that dwarf the Earth's mass. Calcium, phosphorus, nickel, more

iron, all made in the hellish forge of the supernova heat and flung outward into the universe. It takes millennia or longer, but this material mixes with the other gas and dust clouds floating in space. Sometimes these clouds will be actively forming stars. Sometimes the collapse of the cloud to form stars may even be triggered by the supernova slamming into it. Either way, the heavy elements created in the supernova will become part of the next generation of stars and planets. Supernova are how the majority of heavy elements in the universe are created and scattered. The calcium in your bones, the iron in your blood, the phosphorus in your DNA, all created in the heart of the titanic death of a star. That star blew up more than 5 billion years ago. But parts of it go on in you. Today you learn that massive stars fuse heavier elements in their cores than lower mass stars. This leads to the creation of heavier elements up to iron. Iron robs critical energy from the core causing it to collapse. The shock wave together with a huge swarm of nutrinos blasts through

the stars outer layers causing it to explode. The resulting supernova creates even more heavy elements, scattering them through space. Also, happily, we're in no [music] danger from a nearby supernova. When an 8 to 20 solar mass star ends its life, it does so with a bang, a supernova. And when it's all over, there's a couple of octillion tons of superheated plasma expanding away from the explosion site at a fraction of the speed of light. a whole mess of energy released in the form of light and neutrinos and a bizarre little ball of quantum nastiness in the center composed almost entirely of neutrons. The properties of this neutron star are almost as bizarre as things get in the universe. And if it all seems rather alien to you, well, that's okay. For a little while, astronomers wondered if aliens really were behind what they were seeing. No, I'm not saying aliens.

>> [music] >> When we last left the core of a highmass star, it was in a bad way. Milliseconds ago, it was fusing silicon into iron, but now it's collapsing under its own immensely powerful gravity. The collapse takes a fraction of a second, but a lot happens in that fraction of a second. In lower mass stars, the core supports itself via electron degeneracy pressure. The result of a rule in quantum mechanics that says electrons vehemently resist being squeezed together. But even electron degeneracy fails to stop the collapse if the core has a mass more than about 1.4 times the mass of the sun. That's just too much of a load to bear and the collapse continues. Under these huge pressures, a funny thing happens. Protons, electrons, and other subatomic particles get smashed together and they merge to form neutrons. And this happens to almost all of them. When the core collapses down to about 20 km in diameter, it's essentially a ball of neutrons with some protons and electrons here and there that survived and a crust of normal but highly compressed matter on top. When this happens, yet another

effect comes into play. Neutron degeneracy. Like electrons, neutrons resist being squeezed too tightly together. But this time, the strength of the pressure is far, far stronger than for electrons. If the core is less than about 2.8 8 times the sun's mass, the collapse runs into a wall. It stops. This generates a huge shock wave which along with a flood of energetic subatomic particles called neutrinos blasts outwards blowing up the star. What's left of the core after the metaphorical smoke clears is a neutron star. One of the most bizarre objects in the universe. Such a star would be extremely weird or really just extreme. its mass would be more than that of the entire sun all packed into a sphere maybe 20 km across. Now, let's just stop there for a sec and let that sink in. The sun has a mass 300,000 times the Earth. Imagine packing that all into a ball the size of a small city. Too mind-boggling? Okay, think of it this way. You are mostly empty space. Every

atom in your body has a nucleus made of tightly packed neutrons and protons and electrons whizzing around outside them. If you could magnify an atom to be 100 meters across, the nucleus would be roughly the size of a marble. Imagine all that empty space between the nucleus and the electrons. That's a normal atom. But in a neutron star, all of that space would be filled with neutrons. All of it. Every nook and cranny inside the neutron star has matter in it. all the way down to the scale of an atomic nucleus. This is what gives a neutron star its mind crushing properties. I'm now going to barrage you with very large numbers. So, take a deep breath and you might want to sit down. Neutron stars are ridiculously dense. A single cubic centimeter of neutroneum, as neutron star stuff is usually called, has a mass of about 400 million tons. Want some perspective on that number? Well, very roughly, that's the total mass of every single car and truck in the United States. Imagine a couple of hundred

million vehicles crushed down until they could all fit inside this six-sided die. That's neutroneum. It's so dense that as far as it's concerned, normal matter is a slightly polluted vacuum. If you set it on the ground, it would fall right through the Earth. Now, anything that dense has a huge gravitational pole. If you were on the surface of a neutron star, well, you'd be very dead, obviously, like immediately flattened down to a thickness of just a few atoms. And that's because a typical neutron star has a surface gravity 100 billion times stronger than Earth's. I have a mass of about 77 kilos. And here on Earth, I weigh about 170 lb. On a neutron star, I'd 17 trillion lb. That's 23,000 times the weight of the Empire State Building. But wait, there's more.

In our introduction to the solar system, I mentioned that when you take a spinning object and shrink it, the spin will increase. The usual example is an ice skater drawing in his arms, increasing his rotation until he's a blur. The same is true for the stars core when it collapses into a neutron star. It may have had a very slow spin before the supernova, maybe even taking weeks to spin once. But when it shrinks down to just 20 km across and becomes a neutron star, that rotation will increase by a huge factor. A freshly minted neutron star might spin several times per second. The magnetic field increases as well. A star like the sun has an overall magnetic strength not too different from the Earth's. But when that core collapses, the strength of the field skyrockets. And a neutron star can easily have a magnetic field several trillion times stronger than the sun's. That's strong enough to erase your credit card from a 100,000 km away. See? Ridiculous. All of these properties are brainmelting. But are they real? Could an object like this really exist? Oh my.

Yes. The first neutron star was detected in 1965, though not recognized for what it was at the time. A couple years later, another one was found and this time was correctly identified as a neutron star. But then things got weird. In 1967, Joselyn Bell was a graduate student helping build a radio telescope. There was a persistent noise in their data they couldn't seem to fix. Belle studied it night after night, finally figuring out that the pattern wasn't a problem with their data. It was from an actual astronomical object. She had discovered the first known pulsar. What's a pulsar, you ask? Pulsars are neutron stars. In a nutshell, their rapid rotation coupled with their incredibly strong magnetic fields launch twin beams of energy away from the star like the beams from a lighthouse. The beams sweep around as the star rotates.

And from Earth, we see this as a pulse, a blip of increased brightness. This pulse can be detected in visible light, radio waves, and even X-rays. The spin of a neutron star is amazingly stable, making these pulses act like a very accurately timed cosmic clock. In 1967, no one could believe a natural object could do this. And this object was half jokingly given the name LGM1, Little Green Men One. Now, we know of over a thousand pulsars in just our galaxy alone, and we know they're the leftover cores of massive stars that exploded.

Some spin with periods many seconds or even minutes long. Some are in binary systems. Another normal star orbits them. If they're close enough together, the neutron star can rip material off the other star and feed on it. This increases the pulsar spin. And we know of a few that have incredibly rapid rotation rates. Some spin hundreds of times per second. These are called millisecond pulsars. And if they spun much faster, the centrifugal force would rip them apart despite their tremendous gravity. Even after a thousand years, a pulsar can still be a force to reckon with. There's a pulsar in the center of the Crab Nebula, the remains of a star that exploded to create that supernova remnant. A substantial fraction of the light emitted from the nebula is powered by the pulsar itself. Its fierce output energizes the nebula, causing it to glow brightly even after a millennium. I'm telling you, thinking about neutron stars makes the hair on the back of my neck stand up. And I haven't even mentioned magnetars yet. Neutron stars are more than just weird little balls of neutrons. They have a crust, probably a few centimeters thick, made of highly

compressed, but more or less normal matter squeezed into a kind of highly stiffened crystal state. The magnetic field of the star penetrates this crystallin crust and stretches out for quite a distance. In some neutron stars, the magnetic field is even stronger than usual and can be, get this, a quadrillion times stronger than the suns. These uber powerful neutron stars are given the name magnetars. They're relatively rare, maybe 10% of all neutron stars are born as magnetars, and they have short lifetimes. The magnetic field is so strong it acts like the brakes on a car, slowing the neutron star spin. That spin helps power the magnetic field. So, the field weakens as the star slows. But while they're around, magnetars are the most magnetic objects in the universe. And with great power comes great responsibility. If your responsibility is to be one of the scariest objects in the universe. Why?

In a neutron star, the crust and magnetic field are locked together. So, a change in one affects the other. The crust of the star is under incredible strain due to the intense gravity and rapid rotation. If the structure slips, it can snap, creating a star quake, like an earthquake, but just a wee bit stronger. In an earthquake, huge amounts of energy are released when the Earth's crust shifts and snaps, enough to destroy buildings and quite literally move mountains. But in a neutron star, this effect is multiplied hugely.

Remember, the crust is phenomenally dense and the gravity is enormous. If the crust strains and snaps, dropping just a single centimeter, the resulting release of energy is vast beyond imagining. This energy is released as a tremendous explosion in the crust, shaking it. This also shakes the magnetic field, which reacts poorly. When the sun's magnetic field throws a tantrum, we get a solar flare, which can be as powerful as billions of nuclear bombs. A magnetar flare dwarfs that into insignificance. It can be trillions of times stronger than a solar flare. In a fraction of a second, a magnetar can release as much energy as the sun gives off in a quarter of a million years. In 2004, astronomers were stunned when a huge [snorts] blast of X-rays slammed into orbiting satellites. One of these satellites, named Swift, actually had its detectors saturated with X-rays, even though it wasn't even pointing at the source at the time. The X-rays literally came right through the side of the satellite with such intensity that Swift, which was designed to detect

powerful X-ray sources, was momentarily blinded by them. The source of this X-ray burst was quickly determined to be a magnetar called SG1806US 20, and the effects were incredible. It actually compressed the Earth's magnetic field and partially ionized the Earth's upper atmosphere. Oh, did I mention that this magnetar is 50,000 lighty years away? That's halfway across the galaxy. That's incredible. At a distance of 500 quadrillion kilometers, its effects were felt more strongly than a powerful flare from the sun. The good news is that there are very few magnetars like this in the galaxy, probably fewer than a dozen. Also, catastrophic explosions like the one in 2004 are rare. If one had happened any other time in the past 40 years or so, we would have detected it. And frankly, it's really cool that we had astronomical satellites orbiting the Earth which could study it. We've come a long way in understanding neutron stars since they were first discovered. But there's much about them we don't

understand. Every time we find out more, we find out they're even weirder than we first thought. And yet, for all that, they're not the weirdest things in the sky. Not by a long shot. That place is held pretty securely by the other type of object created in a supernova, a black hole. Stay tuned. Today you learned that when a star between 8 and [music] 20 times the sun's mass explodes, the core collapses to form a neutron star. Neutron stars are incredibly dense, spin rapidly, and have very strong magnetic fields. Some of them we see as pulsars, [music] flashing in brightness as they spin.

Neutron stars with the strongest magnetic fields are called magnetars [music] and are capable of colossal bursts of energy that can be detected over vast distances. [music] As we've seen over the past few episodes, a lot of really epic stuff happens when a star dies. If the stars core is less than 1.4 times the mass of the sun, it becomes a white dwarf, a very hot ball of supercompressed matter about the size of the Earth. If the core is heftier, between 1.4 and 2.8 times the sun's mass, it collapses even further, becoming a neutron star that's only 20 km across. The neutron soup inside of it resists the collapse and prevents the core from shrinking anymore. But what if the mass is more than 2.8 times the sun's? If that happens, the gravity of the core can actually overcome the tremendous resistance of the neutrons and continue its collapse. What force can possibly stop it now? It turns out none. None more force. There is literally nothing in the universe that can stop the collapse. The core of the star is about

to go bye-bye. >> [music] >> Way back in episode 7, I talked about escape velocity and it's about to become a major player in the unfolding events of the collapsing core of a highmass star. In brief, it's the velocity at which you need to fling something off the surface of an object to get it to escape. For the Earth, the escape velocity is about 11 km/s. Get something moving that quickly and it's gone. It'll never fall back. The sun, which has much stronger gravity than Earth, has an escape velocity of over 600 km/s. A neutron star with its immense gravity can have an escape velocity of 150,000 km/s.

That's half the speed of light. Keep that in mind. And let's go back to the collapsing core of the star. As it shrinks, its gravity gets stronger and stronger. That means its escape velocity gets higher and higher. When it's neutron star size, the escape velocity is half the speed of light. But if it's more than 2.8 times the mass of the sun, the core will keep collapsing. When it size drops just a little bit more down to roughly 18 km, an amazing thing happens. The escape velocity at its surface is equal to the speed of light.

And well, that's a problem because in our universe, nothing can travel faster than the speed of light. Not a rock, not a rocket, not even light itself. Once the core of the star shrinks down smaller than that magic size, nothing can escape. No matter can come out. So it's like an infinitely deep hole and no light can come out. So it's black. We should come up with a snappy name for such an object. A black hole is the ultimate end state for the core of a highmass star. Whatever happens in a black hole stays in a black hole. That region of space, that surface around the black hole, where the escape velocity is the speed of light is called the event horizon. For that reason, any event that happens inside can't be known. It's beyond the horizon for us. Black holes mess with our concepts of space and time. The math and physics of black holes is incredibly complex. So much so that even after several decades of study, physicists still argue over a lot of their properties. This has led to a lot of misconceptions about them, too. All right, let's get this out of the way

right now. The sun cannot become a black hole. It takes a stellar core at least about three times the mass of the sun to overcome neutron degeneracy pressure. That means the original star must have something like 20 times the sun's mass or more. So, we're safe from that particular sci-fi scenario. Here's another misconception. A lot of people think of black holes as cosmic vacuum cleaners sucking in everything near them. But that's not really true. They have powerful gravity. Yeah, but only when you're very close to one. The power of a black hole comes from its mass, certainly. But just as important is its size, or really its lack of size. If you could turn the sun into a black hole, which you can't, but let's pretend you could, then the Earth would orbit it pretty much exactly as it does now. From 150 million km away, the Earth doesn't care if the Sun is big or tiny. We're so far away that it doesn't matter. It gets to be a big deal when you get close. Remember from episode 7 about gravity.

The strength of gravity you feel from an object depends on how massive it is and your distance from its center. The closest you can get to the sun is by touching it, being on its surface, about 700,000 km from its center. If you get any closer to its center, you're inside it. The material outside of your position is no longer pulling you down, and so the gravity you feel will actually decrease. But if the sun were crushed down to about 6 km across, it would be a black hole. You could get much closer than 700,000 km to it. And as you did, you'd feel a stronger and stronger pull as you approached it. So from far away, a black hole with, say, 10 times the sun's mass would pull on you just as hard as a normal star with that same mass. You can orbit a black hole, too, as long as you keep a safe distance between you and it. Orbiting a 10 solar mass black hole would be just like orbiting a 10 solar mass star, except not so hot and bright. Black holes are weird enough without the misconceptions. Black holes also come in different sizes. The kind of black holes

I've been talking about have a minimum mass of about three times the sun's and might get as high as a dozen or more times the sun's mass if the parent star was big enough. We call these stellar mass black holes. If it happens to gobble down more matter, it gets more massive and the event horizon grows as well. The black hole gets bigger. The idea that huge black holes could form in the centers of galaxies was first proposed in the 1970s. And it wasn't much later that the first one was found in the center of our own Milky Way galaxy. We've measured its mass at a whopping 4.3 million times the sun's mass. And now we think every major galaxy has one at its heart, too. And in fact, may be crucial in the formation of galaxies themselves. I'll discuss those more in a future episode. Here's a fun thought. What would happen if you fell into one? say a stellar black hole with 10 times the sun's mass, you die. But what happens in the few milliseconds before you leave the known universe forever is actually pretty interesting. As we've seen many times in our own

solar system, tides are important. They arise because gravity weakens with distance. So, a big object like a moon gets stretched by its planet's gravity. The far side of the moon is pulled less than the near side. A black hole has incredibly intense gravity. So, the tides it can inflict are serious indeed. They're so strong that if you fell into a stellar mass black hole feet first, the force of gravity on your feet can be millions of times stronger than the force on your head. Remember, even the meager tides of a planet can rip moons apart. When you multiply that force by a million, you're in trouble. As you fall in, your feet are pulled so much harder than your head that you stretch, pulled like taffy. You'd become a long, thin noodle, kilometers in length, but narrower than a hair wide. Astronomers call this, and no, I'm not kidding, spaghettification. This would happen pretty close to the black hole, just a few dozen kilometers out. If you fell in from a long distance, you'd be moving pretty near the speed of light by that point, and you'd only have a millisecond

or so before it killed you anyway. So, yay. Note that this is only for stellar mass black holes. Super massive black holes are far bigger, millions or billions of kilome across. Compared to that size, the distance between your head and feet is small, so the tides across you aren't nearly as severe. You'd fall in pretty much intact, if that makes you feel any better. But compared to either flavor of black hole, a star still has substantial size, and one that gets too close to any black hole can be disrupted via tides. In March 2011, astronomers witnessed just such an event. In a distant galaxy, a star apparently got too close to a black hole and was torn apart by the ferocious tides. As the star was disrupted, it flared into brightness, momentarily blasting out a trillion times the sun's energy. That's how we were able to see it, even though it was several billion lighty years away. But I've saved the weirdest thing for last. One of Albert Einstein's biggest ideas is that space

isn't just emptiness. It's an actual thing, like a fabric in which all matter and energy is embedded. What we perceive as gravity is really just a warping of this space. Like the way a bowling ball on top of a bed warps the shape of the mattress. The more mass of an object, the more it warps space. Not only that, but space and time are basically two parts of the same thing, what we now call spaceime. You can't affect one without affecting the other. Einstein calculated that when a massive object warps space, it also warps time. Someone deep inside the gravitational influence of an object perceives time as ticking more slowly than someone far away from that object. I know it's bizarre. We think of time as just flowing and everyone should see it move at the same rate. But the universe is under no obligation to obey our preconceptions.

Einstein was right. He was right a lot. This slowing of time is stronger the stronger the gravity of the object is. So your clock ticks a bit slower than someone far away from Earth, for example. The effect is tiny but real and we've actually measured it on Earth with extremely precise clocks. However, if you get near a black hole, the effect gets a lot stronger. In fact, black holes warp spaceime so much that at the event horizon, time essentially stops. You'd see your clock running normally and you'd just fall in, bloop, gone. But someone far away would see your clock ticking more slowly as you fell in. And this isn't a mechanical or perception effect. It's actually woven into the fabric of space. To someone outside looking down on you, your fall would literally take forever. But then they wouldn't be able to actually see you.

The light you emit would have to fight the intense gravity of the black hole to get out. And to do that, it would lose energy. This is very similar to the Doppler red shift I've talked about in earlier episodes, and it's called a gravitational red shift. When you're right at the event horizon, just when an outside observer would see your clock stop, they'd also see the light coming from you infinitely redshift. Your light would lose all its energy trying to leave the vicinity of the black hole and you'd be invisible. And from your viewpoint, buckle up because this is wow. You'd see the universe speed up and just as you hit the event horizon, all of time would pass. All of it. And all that light coming at you from the universe would be blueshifted, becoming such high energy that you'd be fried. But since you're about to fall into a black hole, you probably wouldn't care. See, like I said, wow, black holes are so strange with such fiercely complicated math and physics to explain them that scientists are still trying to figure out even basic things about them.

For example, some scientists argue that the event horizon as we understand it may not actually exist and that when you apply quantum mechanics to black hole physics, you find particles can slowly leak out. We're still new at this and struggling to understand what may be the most complex objects in the cosmos. Black holes, as bizarre and counterintuitive as they are, keep popping up from here on out as we poke our noses into more and bigger astronomical objects. While they may seem scary and weird, and let's be honest, they are. They have literally shaped most of the objects we see in the universe. Today, you learn that stellar mass black holes form when a very massive star dies and its core [music] collapses. The core has to be more than about 2.8s 8 times the sun's mass to form a black hole. Black holes come in different sizes, but for all [music] of them, the escape velocity is greater than the speed of light, so nothing can escape, not matter or light. They don't wander the universe, gobbling everything down around them. Their gravity is only really intense very close to them. Tides near a stellar mass black hole will

spaghettify you, and time slows down when you get near [music] a black hole. Not that this helps much if you're falling in. We have one star in the solar system, the sun. Sure, it has a lot of planets, moons, asteroids, and comets. It schles with it as it moves through space, but no other star is part of our family. The sun is alone. Based on that, you might naturally think that like the sun, stars are single, too. They sure look that way by eye. But when you point a telescope at the sky, you find that this is not the case. A lot of stars travel the universe with companions, and sometimes more than one.

I am for mankind. >> With so many stars in the sky, some appear close together just by coincidence, even though in space they're actually very far apart. We call these optical double stars. By the 18th century, astronomers were starting to recognize that many stars that appeared close together really were physically orbiting each other. We call these binary stars to distinguish them from the coincidentally close together double stars. Although the numbers are a little bit uncertain, something like a third to half of all stars in the sky are part of a binary or multiple star system. One such binary system is visible to the naked eye and has been known for thousands of years. You may have seen it yourself. The star marking the kink in the handle of the Big Dipper is actually two stars, one called Mysar and a fainer one called Alor. They're close enough together that you need decent eyesight to separate them. and they were actually used as an eye test in ancient times. Binary stars almost certainly formed

together near each other in the gas cloud that was their original stellar nursery. Instead of a single clump collapsing and forming a star like our sun, there are two such dense lumps and they both collect material until they become true stars. There are lots of different kinds of binary stars. If the two stars can be seen separately using a telescope, they're called a visual binary. This is kind of a fluid classification. As telescopes get better, stars that are closer together can be resolved. These kinds of stars are fairly common. The brightest star in the night sky, Sirius, is a visual binary. It's a luminous blue star about twice the mass of the sun, orbited by a smaller, much fainter white dwarf. It's funny, too. As I mentioned in an earlier episode, white dwarfs can be very hot and energetic and emit light at much higher energy than normal stars. When you observe Sirius with an X-ray telescope, the white dwarf is by far the brighter of the two. Visual binaries are important because if you observe them long enough, you might be able to see their orbital motion. If we can measure their distance from Earth, then the actual size and shape of their orbits can be determined. And in turn, using

the math and physics of gravity, this can be used to find the masses of the stars. In fact, the only way we know to get accurate measurements of stellar masses is when they're in binaries. And once we know the masses of the stars, as we saw in episode 26, we can learn everything else about them. how big they are, how brightly they shine, and even how long they live. It's no exaggeration to say that observing binary stars opened up the new scientific field of astrophysics, applying physics to astronomy, and that led to us understanding everything we do about the universe today. Not bad. Not all binaries are visual binaries, though. Some stars orbit so closely together that we can't split them, even with our biggest telescopes. So, how do we know they're binary? >> Spectroscopy. As the two stars orbit each other over time, one will appear to be heading toward us while the other circles away and vice versa as they switch sides. While we may not see that motion directly, if we take spectra of their light, breaking it up into individual narrow colors, we can see the Doppler shift in their spectra on their

merrygoround path. One under goes a red shift as it moves away and the other has a blue shift as it moves toward us. These kinds of stars are called spectroscopic binaries. Remember Msar and Alor, the Big Dipper eye test stars? I said they were a binary system, but I lied. Well, I understated. In even a small telescope, you can see that Mysar is actually a visual binary. But it turns out that both of those two stars making up Mysar are actually spectroscopic binaries, too. Missar is a binary binary star. Even better, Alor is a spectroscopic binary, too. Since Mysar and Alore orbit each other, it turns out they make up a sexuple star system. Six stars all gravitationally bound to one another. Obviously stars can be in bigger groups than binaries. There are triple star systems, quadruple and more. Polaris, the north star, is actually a pentuple system composed of five stars. It's possible lots of stars are born in multiple systems. However, it's pretty hard to get a stable system like that.

If the orbits aren't just right, some of the stars will tend to get ejected from the system. What we see today are the ones that coincidentally got things just right. Even then, they may not be stable in the long run. Was the sun born in such a multiple system? We don't really know. It's certainly possible. And one way to find out would be to look for stars that have a very similar elemental composition as the sun. But the sun was born billions of years ago. Plenty of time for any stars born with it to wander off. Even at relatively slow speeds, 4.5 billion years is a long time. And for all we know, they could be 50,000 lighty years away and completely invisible to us. If there are long- lost siblings to the sun out there, they may remain lost. Just like planets orbiting the sun, binary star orbits can be short or very long. Some stars separated by tens or hundreds of billions of kilome can take centuries to orbit each other, while some are so close they may only take days. One binary star, the most bizarre I know of, is called 4 U1820US30,

and it's composed of a neutron star and a white dwarf. Their gravity is so strong and they're so close together that they orbit each other in 685 seconds, 11.4 minutes, roughly the length of this episode. Like exoplanets, binary star orbits are tipped every which way to our line of sight from Earth. But for some of them, we see their orbits edge on or very nearly so. For these binaries, that means that every orbit, we see each of these stars pass in front of each other, blocking it from our view. We call these eclipsing binaries. Eclipsing binaries are interesting because as one star blocks another, the total light we see from the system dips just like in a solar eclipse when the moon blocks the sun. Over the course of one orbit, we see two such dips as the first star blocks the second and then half an orbit later when the second passes in front of the first. If the two stars are similar, say both like the sun, then the two dips look very similar. But if one star is much brighter than the other, then the two dips look very different. The brighter star dominates the total light we see. So when the fainer star goes behind the

brighter star, the light hardly drops at all. But when that fainter star blocks the brighter one, we see a bigger dip in the light. By carefully examining the sizes and shapes of the dips this way, a lot of interesting information can be gleaned from the system, including the sizes, masses, rotation rates, temperatures of the stars, the size and shape of the orbit, and even the distance to the system. Some stars, like humans, enjoy cuddling. They get so close together they become contact binaries. Literally two stars touching each other. These are very strange objects. The stars can be stretched out into teardrop shapes due to the mutual tidal effects. If they get very close together, they merge into a doublelobed stellar peanut shape like two stars cocooned in shared material. This can make things really weird for them.

Imagine two stars born at the same time, perhaps a few million km apart, tightly orbiting each other. One has, say, five times the mass of the sun. So, it's a hot blue star. And the other one just 1/2. So, it's a red dwarf. The red dwarf doesn't do much. It just slowly fuses hydrogen into helium, glowing feebly. The bigger star, though, goes through its nuclear fuel rapidly and becomes a red giant. It blows off a wind of matter and loses mass. Since the stars gravity depends on their masses, as the big star loses mass, the orbits get a little wonky, becoming more elliptical. But when the massive star swells, it gets so big the two become a contact binary. A lot of the material leaving the higher mass star gets dumped on the red dwarf, which starts to grow. Eventually, the big star loses most of its mass and becomes a white dwarf, while what used to be the lower mass star has grown and now might be more massive than the other star. It's a bit like Robin Hood taking from the rich and giving to the poor. If he gets too enthusiastic about it, then the poor become rich while the rich

become poor. When we look at that binary system, we see a white dwarf star that is clearly more evolved than a highmass one. the opposite of what we expect. This is called the Alol paradox after the contact binary star Alol in Perseus which shows this effect. Mass transfer between two stars can yield even more dramatic results. Imagine this same system a couple of billion years later. The high mass star has lost its outer layers and is a dense white dwarf. The other star eventually runs out of hydrogen fuel and swells into a red giant. This material then flows onto the white dwarf. White dwarfs have cruy strong gravity. If the hydrogen flowing onto its surface piles up enough, the gravity can squeeze it so hard it fuses into helium. If the flow rate is just right, it piles up on the white dwarf and then under goes fusion in a single colossal flash, erupting in a huge explosive flare. Some of these explosions can be incredibly violent, tens of thousands of times brighter than the sun. When this happens, a previously invisible star can suddenly flare into visibility in the sky. These have been seen historically and called stellar

nove for new star. I love the irony. These stars actually have to be old, near the ends of their lives to go nova. But the name stuck. The explosion can blow out the stream of matter falling from the other star. But when things settle down after a few weeks or months, the matter stream can fall back on the white dwarf and the whole cycle repeats. These are called recurrent nove. If the matter stream is slower, the material can fuse steadily, never piling up, so it never explodes. However, the mass of the white dwarf still increases. If it reaches a mass of around 1.4 four times that of the sun. It gets compressed by its own gravity so much that its temperature soarses upwards. It gets so hot that carbon fusion initiates. And that is a big problem. In a normal star, it would just expand due to all the extra energy being generated. But a white dwarf can't. It's ruled by electron degeneracy pressure. The extra energy just goes into fusing more carbon. And what you get is a runaway thermonuclear event. All the carbon everywhere inside the white dwarf fuses

all at once. All of it. Basically, a solar mass of carbon will instantly fuse, releasing all that energy all at once. It's like setting fire to a dynamite factory. The star explodes. You get a supernova. And it's a completely different process than what we saw when a highmass star explodes, but coincidentally, it releases about the same amount of energy. The star tears itself to vapor and gets so bright it can be seen literally most of the way across the universe. Ooh, this makes them very, very important indeed, as you'll see in a future episode. Today, you learn double stars are stars that appear to be near each other in the sky. But if they're gravitationally bound together, we call them binary stars. [music] Many stars are actually part of binary or multiple systems. If they're close enough together, they can actually touch, merging into one peanut-shaped star. In some close binaries, matter can flow from one star to the other, changing the way it ages. If one star is a white dwarf, this can cause periodic explosions and possibly even lead to blowing up the entire star. In the last

episode, I talked about stars that are orbiting one another. When it's two stars, it's called a binary. Three stars would be a trinary system, and so on. But what happens if you have 10 stars, a thousand, a million? What do you call it when stars cluster together? on the inside. [music] >> It's likely that some stars are born individually, but astronomers think that most stars are born in groups called clusters. Sometimes hundreds and sometimes hundreds of thousands at a time. There are two kinds of clusters.

Open or galactic clusters and globular clusters. Open clusters are loosely bound collections of dozens or thousands of stars, usually in an irregular shape. Some are big enough and close enough to us that they can be seen by the naked eye, faint fuzzy patches in the sky. Galileo was the first to discover or at least record the fact that they were actually composed of many stars so close together our unaded eyes couldn't resolve them. As we'll see in next week's episode, clusters are formed from gigantic clouds of gas and dust.

Depending on local conditions, like how massive they are, these clouds can form dozens, hundreds, or thousands of dense clumps, which then contract to form individual stars. These stars can be bound together by their mutual gravity, forming an open cluster. Unlike the solar system, where the sun dominates and provides a singular central source of gravity, holding the planets under its sway, in a cluster, all the stars contribute to the overall gravity. So, there's more general gravitational shmear. The stars all orbit the cluster center of mass, even though there may not be anything at the very center. Not only that, but the stars don't all orbit in a flat plane like our planets do.

Instead, their orbits are tilted in all different directions, more like the way long period comets are scattered around the sun. Typically, an open cluster is a couple of dozen light-years across with stars much closer together than they are in interstellar space. The nearest known star to the sun is over four light years away, while stars in an open cluster are typically a fraction of a lightyear apart. Some clusters are very young. Well, in an astronomical sense, they're a few million years old. Others are far, far older, billions of years in age. How do we know? By looking at the stars themselves. Remember, hot blue massive stars don't live long before exploding. If a cluster has those kinds of stars, it must be pretty youthful. If all it has are much lower mass red dwarfs, it must be much older, all the more massive stars having died off. In fact, clusters are pretty useful this way. If you assume all the stars in them were born at the same time, a reasonable starting guess, then the age can be found by looking at the most massive stars still on the main sequence, still fusing hydrogen into helium like the sun is. The most massive stars can be identified

by their spectra, which gives us their mass, and then that can be used to get the cluster's age. You might expect the distribution of cluster ages to be all over the place with some young, some old, and some middle-aged. But in fact, most clusters are young and very few are older than roughly 50 million years. That's because they evaporate. All the stars in the cluster interact with each other via gravity. That means they constantly tug on one another, changing each other's orbits. You may remember from our episode about gravity, that the kind of orbit an object has depends on the amount of energy it has. Give it more energy and the orbit gets bigger. Give it enough energy and it will achieve escape velocity and fly away. In clusters, the stars are always passing by each other and exchanging energy via their gravity. If a low mass star swings past a higher mass star, it can get a relatively big kick to its energy and get flung into a much higher orbit in the cluster, while the high mass star loses energy, dropping it down toward the center. Over time, these interactions tend to flick the lower mass stars out of the cluster

completely, and the cluster loses stars. It starts with the lowest mass stars, but as they leave, even higher mass stars can be lost this way. Eventually, only the most massive stars are left and those tend to blow up. Even then, the remaining neutron stars and black holes interact and fling each other away. This process is aided by collisions with gas clouds and the tidal forces of the collected stars in the galaxy itself. Given enough time, the cluster just disappears. The individual stars are left to orbit the galaxy on their own.

About a thousand open clusters are known, but their days are numbered. Eventually, [music] over millions of years, they'll go away. their stars scattered across tens of thousands of light years merging with the population of other stars in space. So better observe them while you can. One of the most famous and beautiful open clusters in the sky is the Pleaides about 500 lighty years from Earth. They appear as a small tightly packed collection of six or seven stars to the eye and a lot of people call them the seven sisters. Through binoculars, several dozen stars can be seen. Most of them look blue, but that's what we in the science business call a selection effect or selection bias. The blue stars are the brightest, so we see them more easily. Through a telescope, redder stars are apparent. Long exposures of the Pletes reveal a surprise. The stars are embedded in a dust cloud, which glows blue from reflected and scattered starlight. When I was younger, it was thought that this was the leftover stuff from which the stars formed. But now we know it's a coincidence. By sheer happen stance, the cluster is colliding with an unrelated cloud of dust. It's like driving a car through a dust cloud

thrown up by a truck that went past you earlier. By another coincidence, the Pletes are located in the sky next to the Hiades, the only other cluster with stars resolvable by the naked eye. The Hiades are much closer and appear bigger. In fact, they form the distinctive Vshape of the horns of Taurus, the bull. As someone who enjoys taking out a telescope and observing the sky, I'm a fan of open clusters. A few are favorite targets whenever I'm outside. I love looking at the Pletes with binoculars. An M35 at the foot of Gemini is another must-see for me in the winter through my telescope. But as much fun as they are to find, and as beautiful as they are to see, they can't hold a candle to globular clusters. These are clusters of not thousands, but hundreds of thousands of stars. They form a roughly spherical shape, hence the term globular, and generally have a well- definfined core with stars scattered around them in a halo that fades away with distance. Most globulars are bigger than open clusters with diameters perhaps a 100red lightyears across, though size is a bit hard to measure since they don't really have an

edge. Like open clusters, the stars in them orbit every which way like bees around a beehive. Unlike open clusters, though, globulars are old, very old. Most are over 10 billion years old. We think they were among the first objects to form after the universe itself did. We know this because in most globulars, the most massive stars are less massive than the sun. They've been around so long that even stars like the sun have had enough time to become red giants and die. And that takes a long, long time. The stars in globular clusters tend to have less heavy elements in them, too, which you expect if they're old. Massive stars explode and scatter their heavy elements into the universe, and that takes time. It took billions of years for stars like the sun to form, the ones with more heavy elements in them. Old stars formed before the universe was seated with those elements, so they have more lighter elements in them than later generation stars like the sun. Despite being larger than open clusters, globulars have more stars and are denser, which is one reason they live

longer than open clusters. Another is that they tend to spend most of their lives outside the galaxy on long looping orbits, and there's nothing out there to interfere with their solitary existence. Globulars are great laboratories for stellar astrophysicists. All the stars in them are born at the same time, and so they're the same age. They're also the same distance from Earth. So, if you see one star in a globular that's twice as bright as another, it really is twice as luminous. That's handy if you're comparing stars for other characteristics. Because globulars are so old, they contain a lot of dead stars. We see quite a few white dwarfs as well as neutron stars and black holes in them. If they have stellar companions, they strip off their material, which falls in and emits X-rays. What normal stars are left are low mass red stars, so globulars tend to look red. Weirdly, there are some blue stars in globulars called blue stragglers, and they were a mystery at first since they shouldn't be there in such old objects. But astronomers figured out they're actually stars that have physically collided and merged to

form a single higher mass star. Such collisions are extremely rare in space, but the stars are so tightly packed in globulars that collisions are more common. We know of about 150 globular clusters orbiting the Milky Way galaxy, our own local collection of stars that we'll dive into in more detail in an episode very soon. But other galaxies have more, many hundreds or even thousands. Globulars are among my very favorite objects to observe with a small telescope. They're bright, compact, and their fuzziness, due to so many stars packed so closely together, gives them an illusion of activity. Again, like bees swarming a hive. They are incredibly beautiful. And that's the view from tens of thousands of light years away. Imagine what the sky would look like from a planet orbiting a star inside the cluster. The sky would be filled with stars, some so close and luminous, there could be dozens brighter than Venus appears in our sky. At night, you might be able to read by starlight.

Sadly though, this fantastic site is probably more science fiction than science. Because globular stars lack heavy metals, it's unlikely they would have formed planets like Earth. And the stars are so closely packed that nearby encounters might eject any planets that did form. Such a scene would be amazing, but unfortunately, it's probably one that exists only in our imagination. Today, you learn that open clusters contain hundreds or thousands of stars [music] held together by gravity. They're young and evaporate over time. Their stars let loose to roam space freely. Globular clusters are larger, have hundreds of thousands of stars, and are more spherical. They're very old, a significant fraction of the age of the universe itself.

>> [music] >> And that means their stars have less heavy elements in them, are redder, and probably don't have planets, though [music] we're not really sure. Astronomers have it pretty easy when we talk to the public. I may be biased, but I think astronomy is the most beautiful of all sciences. Sure, other fields of science have lots of eye candy, but all I have to do is pull out a shot of Saturn, and I win because Saturn, it's all gorgeous. Planets, moons, stars, clusters, but of all of them, you just can't beat a nebula. Why? Because nebula for mankind.

>> Nebula is Latin for cloud and for once in astronomy we have a name that actually describes the object accurately. Nebula are clouds of gas and dust in [music] space. I've already talked about them a bit. For example, stars form from nebula. Our sun did about 4.6 billion years ago. When a medium-sized star dies, it blows off winds of gas, then lights them up as the white dwarf core of the star is revealed, creating a planetary nebula. Also, when a highmass star explodes, it catastrophically vaporizes itself, becoming a violently expanding cloud of gas. Nebula are literally part of the births, lives, and deaths of stars. So, besides being beautiful, they're also pretty versatile. There are a lot of ways of categorizing nebula. One way is by how we see them. For example, if a cloud of gas is blasted by light from a nearby massive star, the gas in it becomes excited. The electrons in its atoms jump up to a higher energy level. When the electrons drop back down, they emit light. The gas glows, and we call this an emission nebula. The color of an

emission nebula depends on the gas in it and how hot it is. Hydrogen, for example, glows most strongly in the red, and we see that color in most emission nebula. Oxygen tends to glow green, but to a lesser extent, it gives off blue light, too. Other elements span the spectrum in colors they give off. And these colors aren't limited to visible light. Hydrogen can emit infrared and even radio light. And if it's energized enough, it'll emit in the ultraviolet, too. That's true for many elements. Although most emission nebula look substantial, they're actually incredibly tenuous. A typical density in a gas nebula is only a few thousand atoms per cubic centimeter. Mind you, in the air you breathe, there are about 10 the 19 atoms per cubic centimeter, a thousand trillion times denser than a typical nebula. Really, a nebula is barely more than a vacuum. The reason they look so cloudy is that they're big. Really, really big. A decentsized nebula is several light years in diameter. And that's a lot of centimeters. That much gas adds up. And so some nebula can be

pretty bright. While emission nebula glow due to their own light, reflection nebula are bright because, can you guess? They reflect the light of nearby bright massive stars. In this case though, the nebula isn't made of gas, but is instead mostly dust. And I don't mean like the hair and skin flake dust bunnies you find under your couch either. When astronomers talk about dust, they mean tiny grains, a micron across. Just so you know, a human hair is 100 times wider than that. These tiny grains contain things like silicates, aluminum oxide, and calcium. And in many cases, this dust is laced with complex molecules called polyyclic aromatic hydrocarbons. While I love that fancy name for them, you might know them better as soot. Yep, when you light a match, you're pretty much making some of the same stuff that lurks between the stars. Dust doesn't emit visible light, but it can affect the visible light from stars if they're inside the dust cloud or nearby. Turns out dust is very good at scattering light. That means that when light hits it, the light gets sent

off in some other direction. This scattering is highly wavelength dependent. So blue light is scattered very strongly while red light can go right through. We saw this in the last episode. The dust surrounding the Pletes star cluster is a reflection nebula. The light from the stars in the Pletes is scattered by the nearby dust and the blue light gets sent in every direction, including toward us. The red light doesn't scatter nearly as well, so we don't see it. it never gets sent toward us. Thick dust is also very good at absorbing visible light. If a star is embedded in enough dust, the light from it is dimmed considerably. If the cloud is dense enough or big enough, the dust can completely extinguish the light seen from a star. At the same time, if the dust is at the right density, the blue light from a star inside a dust cloud gets scattered while the red light can get through. This effect redens starlight and in some dust clouds, it provides a striking view. Stars outside the cloud look normal enough, but closer in they get redder and redder and then fade out entirely. The result is a fuzzy red-edged hole in space. Pretty cool.

You can see that effect in Barnard 68, a small dust cloud just half a lightyear in size. These are also sometimes called molecular clouds. They're cold enough that atoms can stick together to form molecules. Their cores can be hundreds of degrees below 0 C. Some dust clouds like this are relatively small, but others can get downright huge. We call these giant molecular clouds because why not? These can be incredibly massive with thousands or hundreds of thousands of times the mass of the sun and stretch for hundreds of light years. And that brings us to one of the most glorious objects in the sky, the Orion Nebula.

This is an emission nebula located just below Orion's belt. It's actually a naked eye object visible in modestly dark skies. It looks like a star by eye, but even binoculars reveal it to be fuzzy, and through a telescope or with long exposure images, you get unmitigated majesty. The Orion Nebula is a star forming factory. A bunch of stars have been born in it. Some of them are very massive and incredibly luminous. The entire nebula is lit by four stars located in its heart, collectively called the trapezium. These are four brutes, huge, brilliant stars that are each far more massive than the sun.

Their light is so fierce it illuminates the entire nebula which is about 20 lighty years across. And here's a funny thing about Orion. What you're seeing is not just a gas cloud in space. It's actually just a bubble sitting on the edge of a much, much larger molecular cloud hundreds of light years across. That cloud is cold and dark and so we don't see it by eye. The trapezium stars formed inside that cloud very near the edge. When they turned on, fusing hydrogen into helium, they started blasting out a mind-numbing amount of ultraviolet light, which began eating away at the gas and dust. Eventually, they blew a hole in the side of the cloud like a weak spot in a bicycle tire blowing out. What we see as the magnificent Orion Nebula is just a dimple, a cavity in the side of the cloud, filled with gas, heated to glowing by the stars. There are still stars forming there today, too. I mean, literally, right now, we can see it happening. In episode 9, I talked about the solar system and how it starts off as a flattened disc of gas and dust.

When we look at the Orion Nebula with Hubble, we see those discs. They're called protolanetary discs and they're so dense that they absorb almost all the light from the stars forming inside them. So, they're dark and we see them in silhouette against the brighter gas of the nebula. Unless you look in the infrared, that kind of light can pierce the dark disc. And when we use infrared telescopes, we can see the protostarss forming in the centers of those discs. Take a good look. Those are baby stars, literally stars that are forming right this very minute. They're still hot due to their contraction, but in a few million years, they'll ignite fusion in their cores and become real stars.

They'll blow away the remaining material around them, revealing themselves, and perhaps any planets orbiting them as well. In fact, once stars start forming inside a nebula, its days are numbered. The Eagle Nebula is another star factory with active star birth going on inside of it. Some of these are massive luminous stars and give off so much ultraviolet light it erodess away at the surrounding nebula in a process called photo evaporation. However, dense knots of material forming new stars can resist that erosion better and protect the material behind them in essence shadowing it. This results in long fingers of material we see in silhouette against the hotter gas like sand bars in a stream. observing their infrared light. We can also see the stars embedded inside them. There are several of these giant towers in the Eagle Nebula, three of which have been called the Pillars of Creation. Stars are forming at their tips. Eventually, though, the light from the massive stars will wind, zapping away at the structures, dissolving them. There's also some very hot gas in the nebula that might be the result of a star that

has already exploded. If so, then the pillars really don't have long to live. In a few thousand years, they won't be eroded away. they'll be blasted away. In a lot of nebula, there's no sharp edge. They just kind of fade away. Sometimes that's because the gas thins out, so there's not enough stuff there to get lit and see. Other times, it's because there's just one or maybe a few stars lighting up the whole cloud, and at some distance from them, the starlight fades and can't illuminate the gas anymore. But sometimes, nebula do have sharp edges. That usually happens when a gas cloud is expanding, like in a planetary nebula or supernova. The gas slams into the much thinner gas that is strewn between the stars, what we call the interstellar medium. The expanding gas piles up like snow in a snow plow, getting denser and glowing more brightly. Gas inside a nebula can be in turmoil, too. Winds from stars compress the gas. Shock waves form when stars explode and when they're born. These can create lovely sheets, tendrils, and filaments in nebula as well. All of these factors can come together to create great beauty. Not too far from

the Orion Nebula in the sky is another dark nebula superposed on a bright emission nebula. By coincidence, the dense dark material is shaped like a gigantic chest piece and it's called the Horsehead Nebula. It's being eroded by a star called Sigma Orion off the top of the frame here. And that's also making the gas behind it glow in that sharp ridge. One of my favorite nebula in the sky is Barnard's Loop, a huge arc of material that's formed either by the expanding gas from supernova or the winds of all the massive stars being born in the Orion complex. It's also the outer edge of a huge bubble surrounding a substantial amount of real estate in the constellation Orion. In this image, you can see both the Orion and the Horsehead Nebula. The loop is so big you could fit 25 full moons across it. One more thing, I've been talking about bright and dark nebula, but that's an old-fashioned way of thinking of them.

I've also mentioned that infrared light can get through them, but remember from episode 24 that the kind of light an object gives off depends on its temperature. Clouds of dust that look dark to the human eye are actually glowing if you observe them in the far infrared, well outside the colors our eyes can detect. But we have telescopes that can see at these much longer wavelengths. In Orion, there's a reflection nebula called M78. Between M78 and Earth are long filaments of very cold and dark dust, blocking the light from the reflection nebula behind and looking like dark rivers running through it. But when you use a telescope that can see light with a wavelength of a millimeter or so, that dust glows brightly, threading through M78 like ribbons of fire. Like so much else in life, what you see really depends on how you see it. If there's a life lesson there, feel free to take it. Today, you learned that nebula are clouds of gas and dust in space. They can glow on their own or reflect light from nearby stars. [music] When they glow, it's usually predominantly red from hydrogen and green from oxygen. And when they

reflect and scatter light is from [music] massive hot stars, so they look blue. Stars are born in some nebula and create new ones as they die. Some nebula are small and dense. Others can be dozens [music] or hundreds of light years across. Also, they're incredibly beautiful. Over the past few weeks, I've talked about our sun, the planets and other objects in our solar system, stars, both big and small, brown dwarfs, exoplanets, supernova, and clouds of gas and dust. What do all these things have in common? Well, they we all live in the same town. It's a big town, and it's called the Milky Way galaxy. Let's cut to the chase. The Milky Way galaxy is a vast, flat, discshaped collection of gas, dust, and stars. a sprawling megalopous that's a mind-cussing 100,000 light-years across and several thousand light-years thick. The sun is but one of hundreds of billions of stars in it and were located halfway out from the center in the suburbs. The disc has two major spiral arms in it and two minor ones.

These come together more or less in the middle of the galaxy. Located in the center of the disc is a bar, an elongated, roughly cylindrical collection of old red stars. I know this is a lot to take in. I'll go over it piece by piece. And how we know all this, like with so many other such deep astronomical discoveries, begins simply enough. Going outside and looking up. [music] If you happen to be in a dark spot on a moonless night, you'll see a faint fuzzy glow stretching across the sky. It's brighter in some spots than others. And if the constellations of Sagittarius and Scorpius are visible, you'll notice the stream broadens there, forming a bulge apparently bigger than your outstretched hand. This glows resemblance to a pathway or river is strong. And many ancient cultures saw it that way as well. Because it glows a dull white, the ancient Greeks called it Galaxius or Milky. Over the millennia, that name

stuck and now we call it the Milky Way, or redundantly, the Milky Way galaxy. But what is that glow? It was thought to be nebulous, cloudy, until one day, a guy by the name of Galileo pointed his newly crafted telescope at it. He was astonished to see that it was actually made up of stars, thousands of them. They were so close together and so faint that our naked eyes couldn't see them individually. Instead, they blended together to create that glow. Clearly, there were far more stars in the sky than the 6,000 or so visible to our unaded eyes. But how many more? And what overall shape did they form? Astronomers reasoned that counting stars in different directions might reveal the shape. If the galaxy is longer in one direction than others, you should see more stars in that direction. Over the centuries, maps were made this way, and they generally showed the galaxy to be ovalshaped or a flattened disc with the sun near the center. This star counting method is a good idea, but it's fatally flawed. It doesn't account for interstellar dust. As we saw in our episode about nebula, dust absorbs visible light from stars, hiding them

from our view. It's like being in a smoky room where you can't see the walls. It looks like the room is smaller than it really is, and you look like you're in the center, even if you're near one of the walls. Globular clusters were a good clue that there was more going on. These spherical balls of stars orbit the galaxy well outside the main body of the galaxy itself. There are more of them on one side of the sky than the other, which you wouldn't expect if they were evenly distributed in space. The simplest explanation was that they actually were evenly distributed in space, but it was we who were offc center. Measurements put the sun about 25,000 light-years from the galactic center, which was in the direction of Sagittarius. Hey, that's where the glow of the Milky Way in the sky gets brighter and broadens out, too.

Coincidence? So, at this point, things were coming together. The Milky Way must be a flattened disc with a bulge in the middle. The sun is in that disc far from the center. From inside it, we see it as a broad line in the sky. And when we look toward the center, towards Sagittarius in the sky, we see that bulge as a big blob. The disc is thinner than it is broad. So, we see more stars looking into the disc than when we do looking up or down out of it. But what about the disc itself? Does it have structure in it, or is it more like a smooth flat plate? That's hard to determine using visible light. Dust severely restricts our view. Looking into the disc, we can only see about a thousand lightyears before dust blocks everything behind it. But with the advent of radio astronomy, we figured that out, too. Star forming gas clouds emit a very narrow wavelength range of radio waves, and this could be used to measure their Doppler shift, how quickly they were moving toward or away from us.

Making a few other basic assumptions, their rough distance could be found as well. An amazing result was found. The locations of these nebula indicated that the disc of the Milky Way was divided into spiral arms. Huge sweeping curving arms that were tens of thousands of light years along their lengths. Interestingly, all the young massive bright stars appeared to be in these spiral arms too with none in the apparent gaps between the arms. But that makes sense. Like all stars, these massive ones form in giant nebula. But unlike most stars, they don't live very long before exploding as supernova. If most nebula are in the arms, then these massive stars will be, too. They don't live long enough to get very far from their nurseries before they explode.

Here's where things get a bit weird. You might want to think of the arms as structures, huge collections of stars and nebula moving together around the galaxy's center. But that can't be right. If that were the case, they'd wind up like string on a spool since the stars and nebula closer to the galactic center orbit faster. But we know the spiral arms have been around a while and aren't wound up. That's because they aren't actual structures. They're traffic jams. Think about a highway with heavy traffic on it. For some reason, one car slows down a little. The car behind it slows and the car behind that one hits the brakes, too. It's like a wave that moves backwards through traffic, slowing everyone down. Where they slow down, they tend to clump up, too. So, you see more cars and trucks in the jam. The funny thing about traffic jams is they tend to persist for a long time, even as cars move out of them. The car at the front might slow for a moment, then accelerate away, but the cars behind it are still moving slowly, and more cars are piling up in the rear.

For every car that leaves the jam at the front, another enters from the back. Even though the actual individual cars come and go, the jam persists. Spiral arms are the same sort of phenomenon. The initial wave, the jam, may be started by some sort of disturbance in the disc, perhaps an overdense region created when a couple of giant molecular clouds smash together. Once created, this effect propagates outward like a ripple in a pond, and get sheared into a spiral wave pattern by the rotation of objects in the disc. Stars and nebula enter the wave, pile up a bit, move through it, and then resume their merry way once they leave it. Our sun takes about 250 million years to orbit the galaxy, which means it's made about 20 orbits since it was born. It's moved in and out of spiral arms many times over its long life. That's why spiral arms don't wind up. The arms are waves made of objects in them, but the objects themselves are transient. However, there's more to it. A nebula entering a spiral arm might hit another one already in there, a cosmic fender bender. But

when that happens, the nebula can collapse, forming stars. Some of them are massive, luminous, blue, and hot. These stars are very easy to spot at great distances, making the galaxy's spiral arms look blue. They also light up the gas around them, accentuating the spiral arms. Spiral arms are where the majority of star birth occurs in the galaxy, which helps maintain the spiral pattern. They're the ultimate crowdsourced projects. After all that, it's actually hard to say how many spiral arms the Milky Way has. It's been thought to have two for a long time, but a recent study of the locations of star clusters seems to indicate there are actually four. Milky Wayologists are still arguing about it. But either way, there are also short spurs that stream off of the major arms as well. Our sun is located just outside one of these smaller arms called the Orion arm. Mapping the galaxy's spiral arms is ongoing work and it's kind of a mess. It's like being inside a hurricane and trying to figure out its structure, but we're doing pretty well so far. In the center of the galaxy is the bulge. Recent observations indicate this is

actually a cylindrical bar of redder stars that's pointed nearly directly at us, so it looks more circular to us. This region is very old. All the star birth in there happened long ago and ceased when the gas ran out. Unlike the spiral arms where stars are still actively being born, all the blue stars in the bar are long dead, exploded, leaving behind billions of lower mass redder stars. The bar is probably about 20,000 light-years long. This odd structure is due to the weird gravity of the galactic disc. Unlike our solar system, where the central sun dominates the gravity, the gravity of the galaxy is spread out over a huge area. This distribution of stars and gravity means weird structures can pop up and self-maintain like spiral arms and central bars. Unlike the arms though, the bar rotates as a single unit. Stars near the edge make one orbit in the same time as the stars closer in. In the very center of the galaxy sits a very massive black hole, one with 4 million times the

mass of the sun. This might seem huge, but remember the galaxy has hundreds of billions of stars in it. The central black hole is actually just a teeny tiny part of the galaxy by mass, but we'll learn more about it in a future episode and see that it may have played a very big role in our galaxy's formation and evolution. The last component of the Milky Way is its halo, a vast spherical cloud of stars surrounding it out to great distance, more than a 100,000 lightyear. We don't know much about the halo, but we know the stars in it are old. No star formation occurs there, so any stars we do see would be old. It's possible some stars in the halo form there, while others were flung out into the halo after collisions with other stars and clusters. Speaking of which, most of the globular clusters orbiting the Milky Way are in the halo. There are more than 150 in total. So, there you have it. Our local galactic town. But like any denison of a small town, it's natural to wonder if this is all there is. What else is out there? And at the beginning of the 20th century, astronomers started to ask the same thing. Is the Milky Way the galaxy or a

galaxy? Oh, we're about to take a big step here with Crash Course Astronomy. A really big step. Better strap in. Starting with the next episode, the universe is about to get a lot bigger. Today, you learn the Milky Way is a disc galaxy, a collection of dust, gas, and hundreds of billions of stars. With the sun located about halfway out from the center, the disc has grand spiral patterns in it formed by the traffic jams of stars and nebula where stars [music] are born. The central region is shaped like a bar and is mostly old red stars. There's also a halo surrounding us of old stars. We live in a pretty cool neighborhood, the Milky Way galaxy. We're out in the suburb, sure, but it's still an interesting place, buzzing with activity. Stars, nebula, stellar clusters of various sorts, the occasional supernova. It's a happening place. In the earliest part of the 20th century, astronomers were just starting to figure this all out. But there were a handful of objects that were puzzling, dotting the sky here and there were faint fuzzies displaying a variety of

shapes. Some were round, some elongated, and some even seemed to have spiral arms. Even with big telescopes, they looked smoky. So, they were simply called nebula. Their existence was puzzling, though. What were they? How did they form? Were they big, small, near, far? Eventually, astronomers had uncovered the key to these objects. And in one fell swoop, our universe got a lot bigger. A lot. [music] In 1920, there were two competing ideas about the universe. One was that our Milky Way was it and that everything we saw was in it. The other was that the spiral nebula seen in the sky were also like our Milky Way, island universes in their own right. Two astronomers debated this controversy in that year. Harlo Chappé argued that the Milky Way was all there is while Haber Curtis was of the opinion that we were one of many galaxies. It wasn't a debate as such,

more of a presentation of ideas. And there was no clear winner. Both sides had fragmentaryary data and we now know some shaky observational evidence that turned out not to be correct. For example, Chappé noted that one of the spiral nebula had been seen to rotate, so it must be small. It turns out that was just wrong. Dead wrong. On the other hand, Curtis noted that if galaxies were as big as Shappley claimed, hundreds of thousands of light years across, then other galaxies must be impossibly far away. But hello, galaxies really are that big. And they truly are mind-numbingly distant. The observation that finally unlocked this mystery was made just a few years later when Edwin Hubble and Milton Humson observed the Great Spiral Nebula M31 in Andromeda using what was at the time the largest telescope in the world. They found dozens of pulsating stars in it. Literally stars that change their brightness in a regular periodic fashion. These are called sephiid variables and they were critically important because it was known that the time it took them to pulse was directly related to their luminosity, how much

energy they emitted. That means if you can measure their period, you can determine how far away they are simply by measuring their apparent brightness. The distance they found to M31 was 900,000 light-years, clearly outside even the largest estimates of the size of the Milky Way. They also resolved swarms of faint stars, cinching the fact that the Great Andromeda Nebula was actually the Andromeda galaxy. At that moment, our understanding of the universe swelled in size, and we've never looked back. We've learned a lot about galaxies in the ensuing century or so. Each is a collection of at least hundreds of millions or even trillions of stars and most contain some amount of gas and dust. They range in size from tens of thousands to hundreds of thousands of lightyears across and they come in a variety of shapes. We use these overall shapes to classify them. Broadly speaking, there are four major types of galaxies: elliptical, spiral, peculiar, and irregular. Spiral galaxies we've seen. We live in one. These are characterized by broad flat rotating

discs of stars gas and dust, a central bulge of older, redder stars, sometimes with a long cylindrical or rectangular bar of stars as well, and a huge extended halo of older stars. Some spirals have large central bulges and some have much smaller ones. Most have bars of stars extending across the central part of the galaxy, our Milky Way does. The spirals come in a variety of flavors as well. Grand Design Spirals have magnificent, wellorganized spiral arms that extend from the very center out to the visible edge of the galaxy.

Others have choppy or patchy arms. These are called fauulent spirals resembling toughs of cotton. Some spirals have wide flung arms while others are tightly wound. Spirals take on wildly different appearances depending on their angle to us. Some we see face on and these may be the most magnificent objects in the entire sky. Huge and sprawling, their structure is obvious and easy to see. Star forming nebula are laid out like beads on a string colored pink by the characteristic glow of warm hydrogen. Young massive luminous stars blaze blue tracing the spiral's form. Filigres of dust clouds align with the arms as well and the central buler bar glows an eerie reddish yellow. The star formation there long since ceased. The bluer stars all exploded, leaving only the redder stars behind. When spirals are more tilted to our line of sight, some of this structure is hidden. And when they're edge on, we see them as the flat discs they truly are. Dust clouds are tightly restricted to the midplanes of spiral galaxies, and we sometimes see them bicting the galaxy, like a racing stripe right down the middle. The reason for this is the same reason our solar system

is flat. Galaxies probably formed from huge clouds of gas billions of years ago. As a cloud collapses, small eddies in the gas would get amplified and would create an overall spin. This naturally leads to a flattening of the cloud. And by the time the stars in the galaxy were starting to turn on, the overall structure of a flat disc was in place. Elliptical galaxies are well, elliptical, kind of. Some are nearly spherical, gigantic cotton balls of billions of stars. Others are more elongated, shaped like cigars or American footballs. They tend to have no overall structure as spirals do.

Ellipticals are puffy, and they range in size dramatically. Some are what are called dwarf ellipticals just a few thousand lighty years across to monsters that hugely outmuscle our own milky way. Besides their shape, ellipticals are characterized by a lack of gas and dust in them and also are populated with older stars. Apparently, star formation in ellipticals came and went eons ago, and all the young massive stars have long since exploded, leaving less massive, redder stars. This makes them similar to the stars in the central bulges of spiral galaxies. It's not exactly clear how ellipticals form.

Current [music] thinking is that they are the products of, and this is mindblowing, galactic collisions. Yes, you heard me. Entire galaxies collide. When galaxies collide, it's a train wreck on a cosmic scale. Galaxies are huge structures, and with hundreds of billions of stars in them, their gravity is pretty strong. If two galaxies get close enough together, they can draw each other in and collide. A galactic collision is a weird event. Even though the collision speeds can be hundreds of kilometers per second, the event plays out over hundreds of millions of years. Remember, we're talking about distances of tens of thousands of lightyear here. In the early stages of the collision, title effects can be strong. Stars on the side of the galaxy near the other one get pulled [music] toward it more strongly than the stars farther away.

So, the galaxies can get stretched and long tendrils of stars and gas [music] get drawn out. Generally, collisions aren't head-on, but more of a sideswipe. So there's [music] some sideways motion. When that happens, the tidal streamer can become curved, a long [music] graceful arc. Colliding galaxies in this stage form all sorts of bizarre and spectacular shapes. Sometimes the galaxies separate and then fall back together again. When that happens, [music] the main bodies collide. But this isn't like two cars crashing together. Stars are very, very small compared to the space between them. So, even though hundreds of billions of stars can be involved, the odds are good that no two stars will ever physically hit each other. Space is weird. Gas clouds though are huge and they do collide. They slam into each other, collapse, and form stars at a furious rate. Colliding galaxies can glow pink and blue as stars are born and light up the hydrogen clouds around them.

Sometimes, the two galaxies will collide at high enough velocity that they pass right through each other. But even then, in most cases, they'll slow, stop, and then recolide. Eventually, they merge. They're huge energies of motion absorbed by the orbiting stars, puffing them up into vast, sweeping orbits. The results, so we think, are elliptical galaxies. Not all collisions result in galaxies becoming ellipticals. If a big spiral collides with a much smaller galaxy, it can tear apart the interloper and literally absorb it into itself. So, galaxies are cannibals. We think most large galaxies grew to their current bulk by consuming smaller galaxies, including our own Milky Way. In fact, we have proof. We're currently in the process of eating several smaller galaxies right now. Tarzan 5 is a small knot of stars that may be the leftover core of a galaxy cannibalized by our own. And two huge looping streams of stars circling our galaxy are parts of the Sagittarius and Canis Majorus dwarf galaxies being torn apart by the Milky Way. This brings us to the third type of

galaxies, peculiarss. These are not shapeless really, but have a shape that's weird. Peculiar galaxies are essentially all due to collisions. Colliding galaxies clearly have structure, but can be all sorts of odd distorted shapes. Sometimes a small galaxy will plunge right through the heart of a much larger galaxy. The gravity of the smaller galaxy draws in stars and then flings them away in the bigger one, like ripples in a pond. The result is a ring galaxy, sometimes with a culprit intruder seen nearby. The famous cartwheel galaxy is a fine example of this. Another is Hoag's object. Though to be honest, it's not completely clear if this bizarre galaxy is a product of a collision or some other process. It's peculiar either way.

Finally, we come to galaxies that truly are shapeless, called irregular galaxies. These tend to be small and chaotically shaped. Bigger ones may be victims of collisions, their shapes distorted so much that the structure was lost. Smaller ones may simply be too tiny to collect themselves together into a recognizable shape. Many galaxies have companion satellite galaxies. The Milky Way has a couple of dozen such satellites. The two biggest are the large and small melanic clouds visible from the southern hemisphere. Both are irregular in shape, though the large melanic cloud is barely coherent enough to qualify as a disrupted though small barge spiral. Both are riddled with gas and dust. The large cloud sports the biggest and most active star forming gas cloud in any nearby galaxy, the Tarantula Nebula. It's producing so many stars that astronomers think it may be in the process of forming a globular cluster. There are other types of galaxies as well. Some, called active galaxies, pour out far more energy than normal ones. And galaxies aren't just

scattered evenly across space. They tend to be found in clusters, some containing thousands of galaxies. We'll get to all that in the next episode. We now know that the universe is far, far larger than just our Milky Way galaxy. It's a recurring theme, isn't it? We thought the Earth was the center of creation and then the sun and then the galaxy. But every time we investigate these situations earnestly and engage in honest debate, we find that we're smaller and farther removed from the action than we [music] thought or than we thought we deserved. Astronomy is really, really good at putting us in our [music] place. But it's also really good at showing us just how grand and all inspiring that place is. [music] Today, you learn that the Milky Way is a galaxy, one of many, containing gas, dust, [music] and billions of stars or more. Galaxies come in four main shapes: elliptical, spiral, peculiar, and irregular. Galaxies can collide and grow in size by eating each other. Hey astronomers, Phil Plate here. In our last episode, I talked about galaxies,

vast collections of gas, dust, and upwards of hundreds of billions of stars. We live in one, the Milky Way, a gigantic disc galaxy with sprawling spiral arms. Other galaxies are elliptical or irregular or peculiar. But those are classifications based on shape. We also classify galaxies on their behavior and sometimes even on their location and mass. To understand why, we have to take a step back and look at the environments in which galaxies sometimes find themselves. And if you thought galaxies were big and powerful, well, I'm about to crush your brain again.

[music] In the 1960s, a peculiar object was found called 3C273. Through optical telescopes, it looked like an unassuming blue star, but through a radio telescope, it was seen to be a blaze with light, a luminous powerhouse. Stars didn't blast out that much radio radiation. So, this was baffling. The mystery deepened when spectra of 3C273 were taken. It wasn't a star. It was an entire galaxy. And not just any galaxy, but one very, very far away, well over two billion lightyears. Far from being some dim thing, 3C273 revealed itself to be the most luminous object in the universe ever seen at that time. It blasts out over 4 trillion times the energy the sun does. And yet, it appears star-like, a mere dot in the sky. Because of this, it was dubbed a quasi stellar radio source, which is pretty underwhelming for the most powerful energy source in the entire cosmos. Happily, the name was shortened

to quazar, which you'll admit is way cooler. Once 3C273 became known, lots more such objects were found. With the advent of X-ray observatories launched into space, even more energetic point sources were found, which is amazing. X-rays are a very high energy flavor of light, and it takes a lot of power to make them. Eventually, galaxies like these were even found to be blasting out gamma rays, the very highest energy kind of light. Clearly, these were no regular galaxies. Astronomers gave them the generic name active galaxies and classified them into various subcategories depending on how they emitted their light and what kind of spectre they had. But what could power these immensely energetic galaxies? It turns out to create that kind of energy, you need to have an object with a lot of gravity. And what kind of object has a lot of gravity? In the 1980s, astronomers were getting suspicious that all large galaxies had very massive black holes in their cores.

In fact, one of the reasons the Hubble Space Telescope was built and launched was to explore this idea and characterize, that is, find out as much as it could about these black holes. Over time, we found this idea is absolutely correct. Every big galaxy we see appears to have a huge black hole in its heart. Even the smallest is a monster with millions of times the sun's mass and some tip the cosmic scale at billions of solar masses. We now think that these super massive black holes form at the same time galaxies do. As the material coaleses to create a galaxy, some falls to the center and feeds the black hole there. It grows as its host galaxy does. But I can hear you thinking, "Hey, Phil, don't black holes suck down everything, even light itself?

How could they power active galaxies? the brightest objects in the universe. Ah, you can't escape from a black hole once you fall all the way in. Just outside the black holes event horizon, things can still get out. If a black hole is sitting all by its lonesome out in space, it's well black. But if matter like gas, dust, or even whole stars falls into the black hole, it can be shredded by the fierce gravity. This material forms a flat disc called an accretion disc. the matter swirling madly at ferocious speeds before falling in like water down a bathtub drain. Stuff closer to the black hole orbits faster than stuff farther out. This means material in the disc rubs together and heats up just like rubbing your hands on a cold day warms them up via friction. But around a black hole, the orbital speeds are near the speed of light. Try rubbing your hands together at a couple of hundred thousand kilometers/s and see how much heat you make. So friction and other forces heat the material falling in to millions of degrees. So hot that it blasts out light

across the electromagnetic spectrum. And that's what powers active galaxies. The black hole is the energy source, but the matter falling into it is the actual light bulb. Active galaxies are so bright they can be seen clear across the universe. Not only that, but some active galaxies have jets. magnetic fields coupled with the incredible rotation of the accretion discs can launch twin beams of matter and energy directly away from the black hole along the poles of the disc. These beams pack a huge [snorts] wallup traveling for hundreds of thousands of light years. Eventually, they slow down as they ram through the thin material between galaxies. But when they do, they puff up. They look like huge cotton swabs which glow in radio waves. Active galaxies can look pretty different from each other. And we now think that's due to our viewing angle on their accretion disc. When we see it edge on, the thick dust in the disc blocks the intense highest energy light. But we do see lots of infrared as the radiation from the disc heats up clouds of dust around it. If the accretion disc

is tipped a bit to our line of sight, we see more optical and high energy light from it. And if the poles are aimed right at us, all that ridiculously energetic X and gammaray light can be seen. The Milky Way has a super massive black hole in its heart, too, with a mass of over 4 million times the suns. That might sound huge, but remember, the galaxy has hundreds of billions of stars in it. The black hole is only a teeny tiny fraction of the total mass of the Milky Way. Our black hole is quiescent, that is not actively feeding. So, we're not an active galaxy. Every now and again, we'll see a flare from it as it swallows down a gas cloud or something like that, but nowhere near what's needed to switch it fully on. Happily, we appear to be safe from any tantrums it might throw. But that may not always be the case. One way to flip such a black hole from milk toast to monster is through galactic collisions. When two galaxies collide, a lot of gas can be dumped into their centers where it can be gobbled down and heated up. We do see a lot of evidence that active galaxies are disturbed as if they recently collided. So, could that happen to us?

Yes. Yes, it can. In fact, it will, but not for a few billion more years. To understand that, we have to take a small step back. Well, actually a huge step back, a few million lightyear, and take a look at where galaxies live. Our Milky Way isn't alone. It's part of a small knot of galaxies we call in long, boring astronomical nomenclature tradition the local group. It consists of a few dozen galaxies, most of which are small and dim, so faint that we're still discovering them. Two galaxies completely overpower the group, the Milky Way and the Andromeda galaxy. The local group is elongated, almost dumbbell-shaped, with the Milky Way on one side and Andromeda on the other. In the past, the local group probably had lots more galaxies, but over the eons, the two big galaxies ate them all, growing huge. Andromeda is bigger than we are and has more stars, but honestly, we're both pretty big as galaxies go, and someday we'll be bigger. The Andromeda galaxy is about 2.5 million

lighty years away. Close enough that it can be seen by the naked eye on dark nights. The most distant object easily seen without aid. Spectra taken of Andromeda reveal an interesting fact. It's headed right for us. Its spectrum is blueshifted, meaning it's approaching us and it's doing so at quite a clip about 100 kilometers/s. That's fast, but 2.5 million lightyears is a long way. The collision is inevitable, but it won't happen for several billion years. When it does, both galaxies will stretch out due to galactic tides, forming long curving streamers of stars. They may pass by each other during the first pass, but over the next few hundred million years, they'll slow fall back toward each other and merge. They'll then form one much larger galaxy, probably an elliptical, which astronomers have called Milometa.

I know that's awful, but if you can come up with a better name, let us know. Anyway, although this won't happen for billions of years, that's still long before the sun dies. The Earth may still be around when the galaxies collide. It's not clear what will happen to us. The sun may continue to lazily orbit the core of the new galaxy, or it may move farther in toward the center or farther out in the galactic suburbs. And here's another fun fact. Andromeda has a gigantic black hole in its core, too, which has 40 million solar masses, 10 times the mass of ours. When the galaxies merge, the two monsters will probably go into orbit around one another. Not only that, but any gas and dust left over from star formation during the collision may fall toward the center of Milometa, where the two black holes will gobble them down and may turn the galaxy into an active one. Hopefully, any death rays launched from that will miss Earth. But that won't happen for like 4 billion years anyway.

I'm not too concerned over the fate of the Earth at that point. I feel that right now is a good time to give you a heads up. We're about to take a very, very big step. Up to this point in the series, we've talked about some pretty big distances. Millions or billions of kilometers to the planets, trillions of kilome to the stars, and then jumping to thousands of light years, quadrillions or quintilions of kilome when talking about the galaxy itself. But those distances are as nothing when you start talking about intergalactic trips. We're about to venture out into the greater universe and things are about to get very large. When we step outside our Milky Way, we find that a few galaxies have clumped together to form the local group. But as we look farther out into the universe, we see that galaxies tend to clump together on larger scales as well. Many are in small groups like ours, but sometimes they aggregate into much larger galaxy clusters. A typical galaxy cluster is a few tens of millions of lightyears across and can contain thousands of galaxies. The nearest one

to us is the Virgo cluster located about 50 million lighty years away in the direction of the constellation Virgo. It has well over a thousand galaxies in it, maybe twice that much. It may have as many as a quadrillion stars in it. Like star clusters, galaxies and galaxy clusters are bound to the cluster by their own mutual gravity and move through the cluster on long orbits that can take billions of years to complete. Thousands of clusters are known and they contain every kind of galaxy imaginable. spirals, ellipticals, irregulars, peculiars, active galaxies. In many clusters, a huge elliptical galaxy sits right at the very center. This is probably the result of collisions between smaller galaxies. When they smack into each other, their velocities through the cluster tend to cancel out, like two cars hitting head-on and stopping, so they fall to the center. As more mass falls to the center, the galaxy there grows huge. As mind-boggling as this all is, we're not done. Surveys of the sky have revealed that not only do galaxies clump together in clusters, but clusters themselves fall into even bigger groups called

superclusters. A supercluster usually has several dozen clusters making it up and are hundreds of millions of light years across. Our local group is near the Virgo cluster and both are part of the Virgo supercluster. Recent observations indicate the Virgo supercluster is actually only an appendage of the even larger Lania supercluster, which may have 100,000 galaxies in it, stretching across 500 million lightyear. This new result is a bit controversial. I mean, it's hard to know exactly how big such a structure is, especially when we're inside it, but it gives you an idea of the vast sizes and distances we're talking about here. Superclusters themselves aren't just randomly distributed through the universe, either. They appear to fall along tremendously long interconnected and intersecting filaments making the universe appear almost foamy on the biggest scales like a sponge. In between the filaments are vast regions relatively empty of galaxies called voids. This cosmic large scale structure, its size, shape, distribution

of matter and more holds clues to some of the biggest questions we can ask. What is the universe made of? How did it start? What is its eventual fate? These are questions we'll get to in future episodes very soon. And I promise you, they'll stretch your mind like nothing you've ever encountered before. But before we wrap up, there's one more thing I want you to see. When you look at all these pictures of galaxies, of clusters, of superclusters, a question pops up. How many galaxies are there? Can we count them all? To help answer that question, back in the 1990s, astronomers used the Hubble Space Telescope. They pointed it toward the emptiest part of the sky they could, a spot with little or no stars, nebula, or galaxies in it. They then let it stare, simply collecting light from whatever it could see, letting light accumulate until incredibly faint objects could be detected. And what did it find? Wonder.

Pure simple wonder. Oh yeah, and thousands of galaxies. This is the Hubble Deep Field. Mind you, the area of sky you see here is roughly the same as the apparent size of a grain of sand held in your palm with your arm outstretched. And yet, in that tiny section of sky, there are thousands of galaxies. Essentially, everything you see in that image is a galaxy, a huge collection of gas, dust, and billions of stars. The deep field was repeated in different parts of the sky, and the result was always the same. Crowds of galaxies jostling for position, crammed together, even in a tiny slice of the heavens. You can count all the galaxies in these deep fields and then use them to extrapolate to the entire sky, giving you the total number of galaxies in the universe. And what do you get? Well, give or take 100 billion galaxies. 100 billion and each with billions of stars. The universe is mind crushingly huge.

And yet here we are a part of it, learning more about it all the time. It's easy to think the universe is too big to comprehend and makes us seem tiny and insignificant in comparison. To me, the opposite is true. It's our curiosity about this enormous cosmos that makes us significant. We yearn to learn more, to seek out knowledge that doesn't make us small, it makes us vast. Today, you learn that active galaxies pour out lots of energy due to their central super massive black holes gobbling [music] down matter. Galaxies tend not to be loners, but instead exist in smaller groups and larger clusters. [music] Our Milky Way is part of the local group and will one day collide with the Andromeda galaxy. Clusters of galaxies also clump together to form superclusters, the [music] largest structures in the universe. In total, there are hundreds of billions of galaxies in the [music] universe. Sometimes in science, the story of how we learn something is just as cool as what we learned. In the case of gammaray bursts, it's kind of hard to beat the awesomeness of what they are.

But of all the plot lines in astronomy, their origin story comes the closest. It begins quite literally in the grip of cold war paranoia and ends well, it doesn't end. What true story ever does? But it does lead us to discovering the single most violent events occurring in the universe. Events which paradoxically and ironically are almost entirely hidden from our view. >> [music] >> After World War II, the Allies that were the United States of America and the United Soviet Socialist Republic had gone their separate ways. They had fought together against a common enemy, but that war was done and a newer, colder one forged. The US and USSR became sworn enemies themselves, each determined to bring the downfall of the other. Both sides had nuclear weapons, so this downfall was not as impossible as it might seem. It was a terrifying

likelihood taken very seriously by everyone involved. Both sides were testing nukes at every available opportunity, pushing them for ever greater explosive yield. At the same time, both factions were becoming more adept at space travel, using satellites to spy on each other, and both were looking at the idea of orbiting platforms from which to launch nuclear weapons. You could lob bombs on the enemy within minutes instead of needing the better part of an hour using ballistic missiles. Fear of this, as much as anything else, drove the writing of the Outer Space Testban Treaty in 1963, forbidding the testing or use of nuclear weapons in space. Among the signitories were the Soviet Union and the United States. Of course, neither side trusted the other. Fearful the Soviets might try to test anyway, perhaps blowing up nukes on the far side of the moon where they couldn't be detected, the US launched a series of satellites called VA. Nuclear detonations produce a flash of gamma rays, the highest energy form of light.

The VA satellites were designed to detect that high energy pulse. Two scientists, Roy Olsen and Ray Cleveraddle, were assigned the task of analyzing the data. They laboriously combed through the observations, checking them for anything that looked like a nuke. Signal after signal turned out to be false. But finally in 1969, they found their first hit, a flash of gamma rays seen by several of the satellites on July 2nd, 1967. But there was one problem. Whatever caused the gammaray event didn't look like a nuclear blast. The amount of gamma radiation and how it fades with time are very distinctive for a nuclear weapon. And the July 2nd event looked completely different than that. There was a strong sharp peak of emission lasting less than a second, followed by a longer, weaker pulse lasting for several more seconds.

A quick look at solar flare data revealed no activity that day that could generate gamma rays either. Weird. Over time, more and more of these mysterious bursts of gamma rays were found. As analysis techniques got better, it was found that they were not coming from the surface of the Earth, nor from nearby space, that is Earth orbit. Whatever these bursts were, they were originating randomly in the sky and were happening in deep space. In 1973, Olsen and Clubbasaddle went public, publishing a paper with their results. Astronomers were intrigued. What could cause these gammaray bursts?

Generating gamma rays is hard and takes incredibly violent events, exploding stars, massive solar flares, and the like. But these bursts weren't obviously associated with any of these events. Making it worse, gammaray bursts, let's call them GRBs for short, okay? GRBs fade rapidly, lasting mere seconds or minutes, making it impossible to follow up with optical telescopes. It took weeks or months after the event to get a position in the sky for them. And even then, the uncertainties were huge. At the time, gammaray telescopes had very fuzzy vision and couldn't pinpoint directions well at all. That meant thousands of stars, galaxies, and other objects nearby were candidate progenitors of the detected bursts. It didn't narrow things down at all. It's like telling someone you dropped a quarter and you want help finding it.

When they ask you where you dropped it, you reply Wyoming. As more of these objects were found, it was seen that they really were occurring on random points in the sky. And that itself was a problem. If they were coming from, say, comet impacts on neutron stars, which was one possible hypothesis, then we should see more bursts along the plane of the Milky Way than above it. Pretty much the only place you find neutron stars is in the plane of the galaxy where all the massive star formation takes place. If GRBs were from neutron stars, then that's where we'd see them. But we see them all over the sky. That meant GRBs were either very nearby, no more than a few hundred lightyears, or that they were coming from incredibly far away. So far that even nearby galaxies weren't affecting the distribution. We didn't see a surplus toward the nearby Virgo galaxy cluster, for example. So, they'd have to be coming from even more distant galaxies. clear across the universe. Neither explanation made sense since astronomers couldn't think of anything that could generate bursts that were close by and obviously the energies involved in creating a burst of gamma rays from billions of light-years away were

impossibly huge. It was the single most enduring mystery in astronomy for decades. The only hope was to have a faster response time so that any fading afterglow from an event might be caught before it became invisible. In 1997, that hope became reality. The Dutch Italian satellite Beepo Saxs had launched the year before, designed in part to look for transient flashes of high energy light and nail down their positions. In 97, it detected a gammaray burst and was able to get a reasonably decent location for it in the sky.

Within hours, groundbased telescopes pinpointed the position and for the first time saw the fading afterglow of a GRB. Astronomers were stunned. The burst was clearly and obviously sitting right on top of a faint galaxy. Another different GRB was detected just months later, also in a faint galaxy. When the distance to that galaxy was found, astronomers were shocked again. It was a truly staggering 6 billion lighty years away. The mystery was over, but it was replaced by a bigger one. These things were happening incredibly far away. But that meant they must be unbelievably powerful. What could cause such a catastrophic explosion? When you need raw power, a good place to look is a black hole. Those are created when the cores of massive stars collapse and the stars explode. But there's still a problem. Given their distance and brightness, even a supernova couldn't power a GRB. Think about that for a second. The most violent known events in the universe at the time were inadequate

to explain the ferocity of a gammaray burst. Unless astronomers came up with an idea. What if the energy blasting outward from a supernova were focused somehow? In a supernova, the energy gets flung out in all directions, expanding as a sphere. If instead that energy could be collected and sent out as a beam, that could explain the bursts. We now understand this to indeed be the case. When the core of a very massive star collapses, forming a black hole, the material just outside the core falls down, forming an incredibly hot swirling maelstrom called an accretion disc. The magnetic field of that material and from the black hole coil around wound up by the rapidly spinning disc pointing up and down out of the disc and away from the black hole. The details still aren't entirely clear, but this launches twin beams of matter and energy up and away from the black hole. The amount of energy in the beams is mind crushing, equal to the total energy of the supernova event itself. They scream away from the black hole at very nearly the speed of light, burning through the

star, blasting away across space. These death rays are so phenomenally bright that we can detect them from billions of light years away. The supernova explosion is no small thing either. The star is so massive it explodes with more energy than a normal supernova. They're so powerful that astronomers call them hypernova. Cool. And you don't always need fancy equipment to see them either. On March 19th, 2008, a GRB erupted into view. And its distance was quickly determined to be 7.5 billion lightyears from Earth. Despite that ridiculous distance, it got so bright that if you had happened to be looking at that part of the sky, you would have seen it with your naked eye.

It's thought that in this case, the beam was aimed almost precisely at us, which is why it got so bright. Good thing it was so far away. And that explains gammaray bursts. Well, one kind of burst at least. It turns out there are two kinds. When you look at the duration of all the bursts detected, they divide pretty well into two groups. ones that last longer than two seconds and come from hypernova and ones that are much more rapid. Sometimes these short bursts last literally for milliseconds, way too fast to be from core collapse supernova.

Something else must be behind them. But what else could be as soulcrushingly energetic as the explosion of a hypernova. Turns out it's two neutron stars crashing together and exploding. Imagine two massive stars born together as a binary star. Eventually, one goes supernova, as does the other, leaving two neutron stars orbiting each other. They'd stay in orbit like this forever if it weren't for a subtle aspect of gravity predicted by Einstein's theory of relativity. Massive objects revolving around each other very slowly lose orbital energy by radiating away gravitational waves, essentially ripples in the fabric of space itself. I know it's weird. Relativity is like that. But think of it as a slow leak in the orbits, very gradually, dropping the neutron stars together. Over billions of years, the two stars draw ever closer, getting so close they spin madly around each other. Finally, they merge in a flash. Literally, if their combined mass is more than 2.8 times that of the sun, they'll collapse to form a black hole. What happens next is as bizarre as it is

awesome. For a very brief moment, the system becomes a black hole orbited by ultra dense debris from the merger. A huge amount of neutroneum, neutron star stuff. This then mimics what happens in a hypernova. It becomes an accretion disc heated to ridiculous temperatures, blasting out those beams of matter and energy. Because the material is more compact, the gamay flash is much shorter. In case you're wondering, yes, this is precisely what my nightmares are made of. Which brings me to this week's focus on if GRBs are so explosive we can see them from halfway across the universe. What would happen if one were nearby? Well, not good things. I already talked about the dangers from a nearby supernova. And the dangers from GRBs are about the same. However, because the energy is beamed, GRBs are dangerous from much farther away. A supernova has to be only a few hundred lighty years away to hurt us. But a GRB can be over 7,000 lighty years away and do the same amount of damage.

But there's an upside to those beams. Because they're so narrow, we can only see a burst if the beam is aimed right at us. That significantly lowers the chances of getting hit by a nearby one. As it happens, there are two stars that could one day explode as gammaray bursts that are within the danger zone. Eticarina and WR104. The good news is that both are at the edge of that distance limit, so they probably can't hurt us. Even better, it doesn't look like either of them is aimed at us. As far as we know, we're safe from hypernova induced GRBs. We don't know of any about to merge neutron stars either.

It's possible they'd be dark and difficult to detect, but they're so rare that it's incredibly unlikely that any are nearby. Because of this, I'm not really worried about them. Over the years, more space observatories have been launched to detect bursts. Probably the most important observatory is NASA's Swift, designed to detect the flash of gamma rays from a burst, then swing rapidly into action to point its ultraviolet and optical telescopes at the area, precisely locating the burst. It then sends the coordinates down to Earth so that more telescopes on the ground can join in on the fun. As of 2015, Swift has detected over 900 GRBs. The rapid response time is critically important in getting follow-up data of the bursts. And since the launch of Swift, our understanding of these phenomena has grown by leaps and bounds.

Now, with our fleet of satellites scanning the skies, we see a GRB pretty much every day. And remember, we only see them when they're aimed at us. That means we miss most of them. So, the actual rate of GRBs is much higher in the universe. There may be hundreds happening every day somewhere in the cosmos. Gamay bursts are truly one of nature's most incredible events, the most violent and energetic explosions the universe is capable of. Everything about them is amazing, from their discovery to what actually powers them and what they create. In fact, when you think about it, here's the most astonishing thing about them. Every time we see one, we're witnessing a black hole being born. Gamay bursts are the birth cries of black holes. Today you learn that gamay bursts were discovered during the cold war when both the US and USSR [music] were worried about the other group detonating nuclear weapons in space. Bursts come in two rough varieties, long and short. Long ones are from hypernova, massive stars exploding, sending out twin beams of matter and

energy. Short ones are from merging neutron stars. Both kinds are so energetic they're visible for billions of light years. And both are also the birth announcements of black holes. A lot of people have noted that astronomy is a humbling enterprise to pursue. After all, every time we make a new discovery, we find ourselves further removed from importance. The Earth is but one planet among many, orbiting a sun that is one star among hundreds of billions, out in the suburbs of a galaxy that is one among hundreds of billions more. It's easy to feel pretty small when you see all of that magnificence out there. And we astronomers keep making it worse because now we know that what we can see isn't even everything there is. Normal matter, the stuff that makes up you and me and all we observe in the universe, that's only a small fraction of what's actually out there. It's time we talk about some very, very dark matters. [music] brought by a fish for mankind.

[music] In the 1960s and 1970s, astronomer Vera Rubin was observing spiral galaxies. She was interested in how they rotate because you can learn a lot about a galaxy that way. Think about the solar system. Back in the 1600s, Johannes Kepler figured out the farther a planet is from the sun, the slower it orbits. Isaac Newton put numbers to that, calculating the strength of the sun's gravity, which means we could in turn get the sun's mass. Same with galaxies.

If you can measure how they rotate, how rapidly gas clouds move in their orbits near the edge of the galaxy, for example, you can calculate the mass of the entire galaxy. Galaxies are so big that you can't physically see the nebula move, but you can measure their Doppler shift, which gives you their velocity. What Reuben expected to see was that the farther out from the center of the galaxy the gas cloud was, the slower it would be moving, just like more distant planets from the sun move more slowly in their orbits. What she got though was the opposite. For many galaxies, the farther out from the center you went, the faster the clouds were moving. Even at best, the velocities flattened out with distance when they should have declined. That meant the gravity of the galaxies was constant throughout the disc, not dropping from the center as you'd expect. But that's bizarre. Images of the galaxies showed that the number of stars and other massive objects clearly got lower the farther from the center you went. There simply isn't enough mass far out from the center to account for the rapid rotation rates or not enough mass from things we can see.

The only explanation is that there must be dark material contributing to the gravity, something besides stars, gas, and dust. Not only that, the galaxy must be embedded in a halo of this material to get the shapes of the rotation graphs right. and there must be a lot of it. Reuben found that there must be five or six times as much of this invisible material than the visible matter in galaxies. Back in the 1930s, astronomer Fritz Swiki had drawn a similar conclusion measuring the speeds of galaxies in galaxy clusters. The member galaxies were moving too quickly to stay in the cluster. At the measured speeds, they should have been flung off. Therefore, he concluded there must be far more gravity in the clusters than just from the visible material. It turns out Zwicki's observations had way too much uncertainty in them to make any solid claims. He hugely overestimated the amount of invisible material.

Reubin's observations were far, far better and more accurate. However, the terms Wiki used to dub this mysterious material stuck, and we still use it, dark matter. Over the years, more observations have only confirmed Reubin's measurements. We see similar behavior in elliptical galaxies, for example. Ironically, better measurements made of galaxy cluster member velocities show they do in fact move too quickly and clusters must have dark matter in them, too. Ziki was right for the wrong reason, and in the end, Reuben is credited for making the discovery. Of course, the idea that so much of the material in the universe must be dark was met with skepticism by astronomers.

Everything gives off some kind of light, but more observations just kept supporting the existence of dark matter. So, what is dark matter? That was the big question. Astronomers were methodical. They listed every single thing they could think of that dark matter could possibly be. Cold gas, dust, dead stars, rogue planets, everything. Even weird subatomic particles that were predicted to exist in quantum mechanics theories, but never seen before. Then they thought of ways they could detect these objects. Cold gas would emit radio waves, for example. But everything they tried came up empty.

One by one, they crossed objects off the list. And eventually, everything made of normal matter, atoms and molecules, protons, electrons, and neutrons was eliminated. All that was left on the list was that truly bizarre stuff, those screwy subatomic particles no one had ever seen before. One such particle is called an axion. They've never been detected, but their properties match what we see of dark matter. Axons have mass, so if you have a huge cloud of them, they'll have enough gravity to affect galaxies. They don't tend to emit much light, so even a huge cloud of them would be dark. And they have another weird property. They don't interact with normal matter terribly well. An axion would pass right through you like you weren't there. If dark matter were made of axons, then clouds of it can be enveloping clusters of galaxies and we'd never see them. If that's the case, how could we ever know if they're there or not? It turns out there is a way. But before I talk about that, we have to go over something pretty weird. Actually, several somethings weird. [music] As I mentioned in our black hole episode, one of Albert Einstein's big ideas was that

space wasn't just emptiness between stars. In a sense, it was an actual thing with all of matter and energy embedded in it. Although you have to be careful not to take the analogy too literally. In many ways, it acts like a fabric with everything stuck to it. This is more than just a theoretical construct, it has real implications. For one, what we perceive as gravity, the force pulling two objects together was actually just a bending of this fabric of space, a warp. It's like a bowling ball sitting on a soft mattress. The surface of the mattress bends, and if you roll a marble past it, the path of the marble will curve. This is true for light, too. It's like having a bend in the road. Cars follow the bend as they move, and trucks do, too. Everything does. With light, it doesn't bend nearly as much as matter does, but it does curve if it moves through space distorted by gravity. The more massive an object is, the more gravity it has, the more it warps space and the more it can warp the path of a light beam. You know what else bends light? A lens. So we call this effect gravitational lensing. Now picture a cluster of

galaxies. It has a lot of mass in a relatively small space. Well, in cosmic terms, if there's a galaxy on the other side of the cluster from us, much farther away, the light that more distant galaxy sends out gets bent on its way to us. The image of the galaxy can get smeared out, distorted, forming fantastic and weird shapes. Einstein's equations tell us that the amount of bending depends on the mass of the cluster. So, we can in theory measure the mass of the cluster by the distortion of objects behind it. Not only that, but it gives us a map of where that mass is. Astronomers use this method on a cluster of galaxies located about 3.5 billion light-years away called the bullet cluster. It's a very special object. It's actually not just a cluster, but a collision of two clusters. That's right. Two huge groups of galaxies are physically colliding and may eventually merge to form one huger cluster. When galaxies collide, they tend to pass through each other like ghosts. But in clusters, between the galaxies, there are vast amounts of gas.

When clusters collide, the gas in the two clusters does indeed smack into each other and gets incredibly hot. So hot, in fact, the gas will emit X-rays. This provides an interesting opportunity. Optical light images show the two clusters next to each other. They've already done one pass. In fact, the galaxies move through each other as expected. The gas in the clusters can't do that, though, so you'd expect most of it to be between the galaxies, having slowed down as the clouds collided with each other more or less headon. Using the Chandra X-ray Observatory, astronomers could map out where that hot gas was. And as expected, it lies mostly between the galaxies, having slowed down after the collision. You can even see how the collision has shaped the gas, forming a bow shock in one cluster, like the waves of water created by a rapidly moving boat. But there's more. Even though the bullet cluster is very far away, there are actually hundreds of galaxies even farther away that can be seen in the optical images. The gravity of the matter in the bullet cluster distorted those background galaxy images subtly. And by very carefully measuring

that distortion, a map of all the mass in the bullet cluster was made, including dark matter. If dark matter is made of axons, then you'd expect it to mostly be surrounding the subclusters themselves because like the galaxies, the clouds of dark matter axons would pass right through each other. And when you do make the map, that's exactly what you see. The background galaxies show there's a lot of matter shown here in violet centered on the two clusters, but it's clearly not the hot gas seen by Chandra and is giving off no light. It looks very much like dark matter. Since the bullet cluster observations were made, several other clusters have been observed showing the same sort of behavior. Attempts have been made to explain these clusters without using dark matter, but in the end, the simplest explanation looks to be the best one. The stuff we see isn't all the stuff there is. To be honest, we still don't know what dark matter is. Axons are one possibility, but others exist.

Lots of experiments have been set up to try to detect the various flavors of subatomic particles. But the very nature of dark matter, it doesn't give off light and doesn't interact well with normal matter, makes it really hard to find. That's why it took so long to even know it existed in the first place. But even though it's incredibly elusive, it turns out that dark matter has had a profound effect on the universe. As we'll see in upcoming episodes, we're getting a pretty good idea of how the universe got its start and how it's evolved over the eons. We think smaller objects formed first, clumping together into larger and larger structures. So stars formed first, then galaxies, then clusters. It turns out that larger structures would have had a hard time forming in the early universe as energy was blasted out by the newborn stars and galaxies. Bigger stuff couldn't aggregate due to all that heat. That is without dark matter. When you include dark matter in the physics, the structures we see in the universe can form. How about that? Something like 85% of the matter in the universe is stuff we can't see, can barely detect, and is

made of something we know not what, but the largest structures in the cosmos owe their existence to it. We humans can get a little arrogant thinking we occupy a special place in the universe. In a sense, we do because most of the universe is cold, empty space, and we live in a relatively warm and dense part of it. But the stuff that makes us up, the protons, electrons, and neutrons of normal matter, that's in a serious minority when it comes to all the matter there is. In a way, Obi-Wan Kenobi was right. There may not be an actual force, but there is dark matter. It surrounds us and penetrates us. It binds the galaxy together. Today, you learn that the kind of matter we see, what we call normal matter, is only one kind of matter. There's also dark matter which we [music] cannot directly see and which interacts with normal matter only through gravity. It affects how galaxies rotate, how galaxies move in clusters and [music] how large structures form in the universe. It can be detected in many ways, [music] one of which is by seeing how its mass affects the path of light coming from distant galaxies as it passes [music] through dark matter in

galaxy clusters. What is the nature of the universe? How's that for a question? For a long time, we humans had no idea what was going on in the universe. To help, we made up stories to either help us explain what we saw or to make us feel better about what we didn't understand. But then, science came along and we started to understand more. We could test our ideas. And as we got more confident in the process, our ideas grew. The field of cosmology was born, the study of the cosmos itself. And now, after centuries of speculation and just so stories, we're starting to get a grasp on the biggest ideas there are.

What is the nature of the universe? Let's find out. [music] By the turn of the 20th century, scientists knew the Earth was old. Darwin's theory of evolution strongly implied the Earth was at least millions of years old. And Lord Kelvin, a hugely respected physicist and engineer, confirmed the Earth was ancient, given that it must have cooled from an initially molten state. That takes a while, at least a million years. How old exactly? No one knew. As for the universe itself, it logically must be as old or older than Earth. A popular model for the universe was that it was static.

It is and always has been as we see it now and in general hasn't changed. Stars may be born and they may die, but overall things pretty much stayed in balance. The universe always existed, always will, always had galaxies in it, and so on. There are variations on this idea, but that's it in a nutshell, and it's what many astronomers believed. This is important. When we try to understand observations in astronomy, we fit them into a framework of understanding, things we think we already know. When something doesn't fit, it's a problem. Maybe the observation is wrong, or maybe we're misinterpreting it, or maybe the framework is wrong. That's a big step to undertake and needs proper contemplation and skepticism. Science is a tapestry, and when you yank at one thread, the whole thing may need reweaving.

Sometimes, rarely, but sometimes, you have to yank that thread. The thread that got pulled in this picture was first uncovered in 1912. That was when astronomer Vesto Slifer, who has the uncontested coolest name for an astronomer ever, started taking spectra of the so-called spiral nebula, hoping to get some insight on their characteristics. Remember, this was before we understood what galaxies actually were. It took him several years of observations. But by 1917, he had observed 25 of them, and he found something astonishing. When he examined their spectra, he saw that almost all of them were highly redshifted. In other words, it looked like most of these objects were rushing away from us at high speed, millions of kilometers per hour. What could that mean? At this point, two different lines of work began to converge. One was by a Belgian theoretical physicist named George Lmetra. In the 1920s, he had been studying Albert Einstein's work, the equations dealing with the behavior of the universe as a whole. Einstein had concluded that the universe was static, unchanging. But Lamemetra disagreed. He

found that an expanding or contracting universe fit the equations better. And given the red shifts observed by Sliper, he proposed the universe itself was getting bigger, which is why the galaxies appeared to be moving away from us. Another brilliant physicist, Alexander Freriedman, had also reached the same conclusion. At the same time, astronomers were trying to determine the distances to the nebula, now understood to be galaxies in their own right. As I mentioned in our first episode about galaxies, Edwin Hubble and his assistant Milton Humson were at the forefront of this. They observed variable stars in the Andromeda galaxy that allowed them to get the distance to the galaxy. They then observed some of the same galaxies Sliper did and measured their distances.

When they compared distances to the red shift Sliper observed, they found that the farther away the galaxy was, the faster it was moving away from us. Let me repeat that because it's kind of important. The farther away a galaxy was, the faster it appeared to recede from us. Some other astronomers had also found similar results. But the work of Hubble and Humson clinched it. We now know it to be true for every distant galaxy we observe. They are all redshifted, all heading away from us. And this ties into what Lmetra had concluded. The universe is expanding. Wait, what? The universe is getting bigger. How can that be? What does that even mean? There are lots of different ways of looking at this. Lamemetra himself suggested imagining the cosmic clock running backwards. Right now, as time inexurably marches on, all the galaxies in the sky are getting farther and farther away from us. But that means that in the past, they were closer together. Run the clock back far enough and they get closer and closer together until at some point in the past everything in the entire universe was crammed together into an uber dense

thing. That is a really, really weird thought. It's hard to imagine everything in the whole cosmos, every star, nebula, galaxy, every atom, electron, and proton all squeezed together into one infinitely dense blob. But that's what the observations are telling us. Lamentra called this a primeval atom, or more colorfully, the cosmic egg. Fair enough. But this has implications. If you squeeze all the energy everywhere into one place, that place is going to be hot. When the universe was a tiny dot, it would have been unimaginably hellishly hot. Then for some reason it suddenly expanded violently and started cooling. This sounds an awful lot like an explosion bang involving the entire universe which is big. What else would you call this but the big bang? In fact, the term became popular when astronomer Fred Hy used it on a radio show and later in a widely read magazine article. Ironically, he meant it somewhat disparagingly since he didn't think the big bang model was correct. To his and

many other astronomers chagrin, the name stuck. I like it. It's not perfectly accurate, but it's succinct. Again, this is all pretty strange, and astronomers had a hard time accepting it. After all, it went against everything they thought was true at the time. In science, though, a hypothesis needs to make testable predictions before it can be taken seriously. What predictions could the Big Bang model of the universe make that we can observe today? The speed of light is fast, 300,000 km/s, or about a billion kmh. Like I said, fast, but not infinitely fast. The sun is 150 million km away. It takes light about 8 minutes to reach the earth. So in a sense, you're seeing the sun as it was 8 minutes ago. The nearest star system to us is Alpha Centuri, 4.3 lighty years away. So we see it as it was 4.3 years ago. The Andromeda galaxy is about 2.5 million lighty years away. The light we see from it now left that galaxy when Oralopythecus walked the Earth. The farther away something is, the farther in the past we see it. This is called the look back time, and it's a crucial

tool for cosmology. By observing very distant objects, we can see the universe when it was young. You might think that we could see all the way back to the moment of the Big Bang. But there's a problem. At some point back in time, the universe was so hot and dense that it was the same temperature as the surface of a star. It would have been very luminous, but also opaque. As it expanded, it cooled and became transparent. If we look back far enough, that moment in time when it cleared up is as far back as we can see. What does that moment look like? By looking at the physics of the Big Bang, the math that describes how matter, energy, space, and time behave, astronomers could predict when this moment happened in the lifetime of the universe, a few hundred,000 years after the bang itself. Using the idea of look back time, they could predict how far away it would be from us and therefore calculate its red shift. Remember, red shift stretches the wavelength of light. The light the universe emitted at the time would have been like a star in the visible part of the electromagnetic spectrum. But the light that reaches us billions of years

later now should be redshifted into microwave wavelengths. In 1965, a pair of radio astronomers announced they had found a signal in their radio telescope that was like a background noise coming from everywhere in the sky. They tried everything they could to explain it, including scraping out the bird poop inside their radio telescope in case that might be causing it. But the only thing that made sense was that this was indeed the redshifted light from the early universe. They had discovered the glow of the fireball left over from the birth of the cosmos. Later in the 1990s, satellite observations further refined the measurements of this cosmic microwave background, and now it's essentially confirmed. This glow was successfully predicted by the big bang model and now we see it in exquisite detail. Its discovery was a huge step in cosmology. The red shift of distant galaxies and the cosmic background are not the only confirmations we have that the big bang model is correct. For example, the model also makes predictions about the elements we see in the universe. At first, when the universe was dense and hot, only

subatomic particles could exist. But as the universe cooled for a brief time, they could fuse and form heavier elements. The Big Bang model predicts certain abundances of elements, ratios of them compared to hydrogen, and that's just what we see in the universe at large. Also, the sizes and shapes of the largest structures in the cosmos are in line with what a big bang model predicts. There's lots of other observational evidence as well. Pretty much every modern astronomer on Earth understands that the Big Bang model of how the universe got its start is the correct one. But what does it mean? I mean, physically, it's a very common misconception that the Big Bang was an explosion in space with everything rushing away from some point. But that's not what's really happening. Remember, I've talked about space being a thing in which matter and energy exist. Space can be warped or bent by mass, creating what we think of as gravity. When we talk about the universe expanding, we mean space itself is expanding, and when it does, it carries galaxies along with it.

In a sense, it's like having a rubber ruler. When you pull on it, it gets longer and the distance between the tick marks gets wider. When the ruler doubles in length, the tick marks that started out a millimeter apart are now 2 mm apart. But tick marks that were 10 cm apart are now 20 cm apart. In other words, the farther away a tick mark is, the faster it appears to move away. Sound familiar? That's just what galaxy red shifts are telling us. It also means that really the galaxies aren't actually doing any moving. It's that space between them is expanding. This may seem like a nitpicky semantic point, but it's physically true. The galaxies are for all intents and purposes standing still.

The space in between them is where all the action is. And it gets even weirder. This is true no matter where you are in the universe. From any galaxy, it looks like all the others are rushing away from you. Look back at that ruler. No matter what tick mark you start with, when the ruler stretches from that spot, it looks like the tick marks are all moving away from you. This is what Einstein's equation showed and what Lmetra saw in them. Space is expanding. But that means the Big Bang wasn't an explosion in some pre-existing space. It was the initial exploding expansion of space itself. The universe isn't expanding into anything because it's all there is. There's nothing outside the universe for it to expand into. This also means the universe has no center, no point of origin. Imagine the ruler is now a circle and the diameter is expanding. No tick mark is the actual center. Yet, no matter where you are on the ruler, every tick mark appears to move away from you. In a similar way, every spot in the universe appears like the center, which means none is. No

place in the universe is more special than any place else. We're all in this together. It can be hard to grasp and I'll admit we all have some difficulty with these concepts. But the math bears them out and so do essentially all the observations we make of the distant universe. And in all this weirdness, don't lose sight of the big picture. The universe had a beginning and we can see evidence of it. Not only that, but by measuring how quickly it's expanding, we can use math to run the clock backwards and determine the age of the universe. Currently, the best measurement we have of the age of the universe is 13.82 82 billion years. Or perhaps I should say 13.82 billion years. That's an amazing number. It's a long, long time. Three times older than the Earth. But what gets me is that we can figure it out at all.

Pretty smart, us apes. Today you learned that distant galaxies show a red shift in their spectra, which means they're moving away from us. The universe is expanding. This means it used [music] to be hot and dense, then it started expanding and cooling. This model of the universe's early behavior is called the big bang and it was confirmed when the background radiation, the glow of the fireball, was detected in the 1960s. Other lines of evidence support it as well. Using this information, we have measured that the universe is nearly 14 billion years old. The previous episode of Crash Course Astronomy was a bit of a brain stretcher. We saw that the universe is expanding, space is expanding, and it's carrying galaxies along with it. That means it was denser in the past and at some point 13.82 billion years ago to be fairly precise all of space, time, matter, and energy was compressed into a single infinitely dense point. Astronomers call this the singularity, which is as good a name as any. Something caused this singularity to suddenly let loose, expanding

violently, cooling, and forming the universe we see today. Coming to grips with this idea took a while for astronomers. But nowadays, the current working model for how the universe started is with a big bang. All the galaxies we see are moving away from each other as space expands between them. That is on large scales. Remember the ruler analogy where on small scales the expansion is small and on bigger scales the expansion is faster. That's why distant galaxies appear to be rushing away from us faster. On small scales the expansion is small enough that gravity can overcome it. The Andromeda galaxy, for example, is about 2.5 million light-years away. That means it should be moving away from us at about 50 or so kilometers/s. But because of our mutual gravity, it's moving toward us. Its motion locally through space is more than enough to overcome the [clears throat] expansion of space between us. It's like running up the down escalator. Run fast enough and you can make it to the top. But every galaxy has gravity and there are a lot of galaxies in the universe. That adds up and should affect the expansion rate.

It's a lot like the idea of escape velocity. Throw a rock hard enough and even though gravity will slow it down, it will escape. But if you don't throw it fast enough, it'll slow, stop, reverse course, and fall back down. Astronomers fully expected to see this effect on the expansion of space. If you looked on the very largest of scales, you'd expect to see the universe slowing down, the gravity of the matter in the universe itself putting the brakes on the expansion. And with the discovery of dark matter, that meant the universe should be slowing down even more than we first thought. But when astronomers went looking for evidence of this, what they got instead was probably the single biggest shock in the history of astronomy.

[music] >> In the 1990s, two teams of astronomers were using the world's biggest telescopes to peer as deeply as they could into the universe. They were looking for incredibly distant supernova, and not just any kind, but special ones called type 1a. I talked about these before. They occur when a white dwarf increases in mass until electron degeneracy pressure can no longer sustain it against its own gravity. It collapses, under goes a catastrophic wave of thermonuclear fusion, and explodes. The beauty of these types of supernova is that they all occur when the mass of the white dwarf gets to about 1.4 times the mass of the sun. That's the magic number where pressure overcomes gravity and they go kabooy. That makes them good standard candles, objects whose intrinsic brightness, whose luminosity is known. Knowing that plus measuring how bright they appear to be in a telescope, lets you calculate their distances. Then those can be compared to the supernova red shifts, which is a different way of getting their distance. This then tells you how fast the universe is expanding on really big scales. But the results they got didn't make sense. Time and again, the supernova were all fainter than they

expected. It was as if the predictions based on the red shifts were underestimating the distances to the exploding stars. The astronomers did everything they could to see if maybe they had made a mistake somewhere, including making a literal list of things that could make stars look fainter. Intergalactic dust, different chemical compositions for the stars that blew up, all kinds of things. But in the end, both teams independently came to the same conclusion. The supernova were farther away than expected. And that meant something truly shocking. The expansion of the universe was accelerating. Now remember, that's nuts. We were expecting the expansion of space to be slowing down due to the gravity of all the matter in the universe, but instead it was speeding up. It's hard to overstate how shocking this is. It's like tossing a rock in the air and instead of it slowing down and falling back down into your hand, it shot upwards faster and faster, defying Earth's gravity. Of course, scientists were skeptical. Many still are. But in the end, several other independent measurements have verified this result.

The universe is not only expanding, but that expansion is getting faster every day. What could possibly cause such a thing? To be flatly honest, we don't know. Well, not exactly. But whatever it is acts like an energy susing space, pushing on the expansion, and we can't see it, so it's invisible. We already have dark matter. So naturally, this got tagged dark energy. It seems to be a property of space itself. A tiny bit of energy in every single cubic centimeter of the universe. The amount per cc is incredibly small, but there are a lot of cubic centimeters in the universe. It adds up and we can add it up. Now that we have measurements of this, we can take an inventory of the universe. We can total all of the matter and energy there is, making a sort of budget of stuff in the cosmos. When we do, it looks like this. 95% of the universe is made of stuff we can't directly see. Normal matter is outnumbered 20 to1.

Maybe we should rethink calling it normal. So if 2/3 of the cosmic budget is made up of dark energy, it must have some pretty big effects, right? Yeah. Like changing the eventual fate of the entire universe, a big question, one of the biggest is, will the universe expand forever? Well, astronomers have a framework to answer this question. We call it the geometry of the universe. Matter has gravity, and gravity bends space. So, is there enough matter in the universe to stop the expansion? The geometry of the universe mathematically describes its overall curvature, the shape of space on the largest scales of all. To be clear, this concept is important to cosmologists, but it can be weird and confusing to someone who is just learning about all this. Still, a lot of astronomy classes teach it, so I'm going to go over it very briefly.

And if you want more information, we have links in the doobly-doo. The idea behind the geometry of the universe is that if there's enough matter, the cosmic expansion will slow, stop, and then everything will recolapse. A sort of big bang in reverse. If there's not enough matter, then the universe will expand forever. And in between the two, there's just enough matter that the expansion slows, but never quite reaches zero until an infinite amount of time in the future. Conceptually, it's a lot like escape velocity. It was once thought that the geometry of the universe tied to the amount of matter in it determined its destiny. But dark energy threw a monkey in the wrench for that. And geometry alone doesn't determine the eventual fate of the cosmos. We think there's enough dark energy in space to ensure the expansion will continue forever despite the geometry. Dark energy is just too powerful and will always drive the expansion of the universe ever faster.

So for now, the answer, as far as we can see, is yes. The universe will expand forever. Okay, there's one more brainmelty thing we need to talk about. For this part, you might want to sit down. Space itself is expanding. As light travels from one galaxy to the next, it fights that expansion, losing energy, just like you use up energy climbing a staircase. When light loses energy, its wavelength gets longer. That's what cosmological red shift is. The more distant the galaxy, the faster it recedes, and the more energy light loses as it travels to us. But wait, at some distance from us, space would be expanding so quickly that a galaxy in that part of the universe would be moving away from us at the speed of light. Anything farther away would be swept away from us faster than light. Now, before you start complaining, yes, this is possible. The speed of light is the ultimate speed limit in the universe. If you're traveling through space, but space itself is exempt from that rule. It can expand at whatever speed it wants. The matter in it,

galaxies, stars, and such, is swept along with it. So, they're not traveling so much through space as with it. When you solve the equations to calculate distance and red shift, the distance a galaxy would have to be from us to be moving away at the speed of light is about 13.8 billion lightyears. Here's the fun thing. We can still see galaxies that far away. We can even see them farther than that. How? It's because that distance is how far the galaxy is from us now. When it emitted that light, the universe was much younger, smaller, and the galaxy closer to us. It would have been about 4.5 billion light-years away at the time. And the light took over 9 billion years to get here. Back then, the space between us and the galaxy wasn't expanding as rapidly, so the light could keep pace. Now, after all this time, the space between us and that galaxy means we're moving away from each other at light speed. But back then, we were close enough to see each other. For that same reason, we can see galaxies that are moving away from us faster than light because when they emitted that light, there was less space separating us. The most distant galaxies

we see are now about 45 billion lightyears from us. We call this the radius of the observable universe. It's essentially the cosmic horizon. Mind you, the actual universe may be far larger than this. Who knows, maybe even infinite. But we can only see galaxies that are by now 45 billion lightyears away. That's our horizon. If space were simply expanding, the size of the observable universe would expand as well. But we have dark energy, and space expands more rapidly every day. That means a truly distant galaxy's light is fighting more and more expansion all the time. It's like climbing an ever steepening staircase. Compared to a constant expansion, a galaxy in an accelerating universe has to be closer for us to see it. It's losing energy faster. Worse, that fight gets harder every day. A galaxy just at the cosmic horizon, right on the edge of the observable universe now, might be visible, but in the future, the space between us and it will have expanded even more. The light can't beat that expansion and it'll never reach us. The

galaxy over time disappears. This has a weird and unnerving effect. The observable universe is getting smaller. The cosmic horizon is approaching us. Eventually, it'll be so close that every galaxy in the sky except our own will lie beyond it. At that point, our own galactic gravity may overcome the expansion of space. So, we'll remain intact. But the sky beyond will be black. The edge of the universe hanging over us just a few hundred,000 lightyears away. instead of tens of billions. It's the ultimate irony. The universe itself is expanding, but we see less of it every day. So, we better study it while we can. We may only have a few trillion more years left to figure this out. Today, you learn that the majority of the universe is made up of dark energy, a currently mysterious entity that pervades space. We don't know exactly what it is, but we can understand what it does. It accelerates the expansion of space. We think this means the universe will expand [music] forever, even as our view of it shrinks as space expands faster all the time.

Hey folks, Phil Plate here, and for the past few episodes, I've been going over what we know about the structure, history, and evolution of the universe and how we know it. Now, it's time to put that into action. We can use this knowledge of physics, math, and astronomy to figure out what the universe was like in the past, going all the way back to literally the first moment after it was born. So, here you go. A brief history of the universe. In the beginning, there was nothing. Then there was everything. Oh, you want more?

[music] >> It may seem a little weird to suppose that we can understand how the universe got its start, but it's much like any other field of science. We have clues, observations based on what we see going on. Now, knowing the rules of physics, we can then run the clock backwards and see what things were like farther and farther into the past. For example, as I've talked about in the past couple of episodes, the universe is expanding. That means in the past it was denser, more crowded, and hotter. At some point, it was hotter than the surface of a star, hotter than the core of a star, hotter than the heart of a supernova.

And as we push the timer back even farther, we find temperatures and densities that make a supernova look chilly and positively rarified. A lot of what we know about the early universe comes from experiments done in giant particle colliders. When the cosmos was very young and very hot, particles were whizzing around at high speeds and slamming into each other, creating other subatomic particles in the process. That's exactly what colliders do. The higher energy we can give our colliders, the faster we can whack particles together, and the earlier the phase of the universe we can investigate. That's one of the main reasons why we keep making them bigger and more powerful to test our ideas of what the young cosmos was like. So, let's wind the clock back.

Looking around us, peering into the universe both near and far, what can we say about the beginning of everything? When the universe got its start, it was unfathomably hot and dense. It was totally different than it is today. Because when you pump more energy into something, the way it behaves, even its fundamental physical nature, changes. If you take a snowball and heat it up, it'll melt. We call that a phase change or a change of state. Heat it more and it vaporizes, changing into a gas. It's still water, still composed of water molecules, but it looks and acts pretty differently, right? When you heat something up, what you're doing is giving it more energy. In a solid, this means the atoms wiggle around more and more until they break free of their restrictive bonds with each other, and the solid melts. The atoms are still bound by other forces, but if you heat them more, they break free of those, too, and the liquid becomes a gas. Heat them more, give them more energy, and the atoms whiz around faster and faster. heat them to millions or billions of degrees and the atoms themselves fall apart. They collide so violently they

can overcome the hugely strong forces holding their nuclei together and you get a soup of subatomic particles, electrons, neutrons, and protons. Heat them more and even protons and neutrons will collide hard enough to shatter into their constituent subatomic particles, which are called quarks. And as far as we know, quarks and electrons are basic particles, so they can't be subdivided anymore. Maybe you can see where I'm going here. As we wind the clock backwards, the universe gets denser and hotter. At some point in the past, it was so hot that atoms wouldn't have been able to hold on to their electrons. A little farther back, and it was so hot that nuclei couldn't stay together, and the universe was a small, ultra- dense ball of energy mixed with neutrons, protons, and electrons. Go a wee bit farther back, and even that changes.

Neutrons and protons couldn't form because the instant they did, they'd whack into each other hard enough to fall apart. The universe was a sea of electrons and quarks. The cosmos was a bizarre, unfamiliar place back then. Even the basic forces we see today, gravity, electromagnetism, and the two nuclear forces responsible for holding atomic nuclei together, as well as letting them disintegrate in radioactive decay, were all squeezed together into one unified super force. Like the snowball melting and vaporizing, each of these moments in the history of the universe was like a phase change. The very nature of reality was changing its laws and behavior different. At some point, we go so far back, so close to that first moment in time that our laws of physics, well, they don't break down so much as say, "Here be dragons." We just don't understand the rules well enough to be able to say anything about that first razor thin slice of time. How far back are we talking here? If we call the instant of the big bang time zero, then our physics cannot describe what

happens in the first 10 to the minus 43 seconds. Now, let me just say only semi sarcastically that that's not so bad. The universe is 13.82 billion years old. So, being able to go back to that very first 110 millionth of a trillionth of a trillionth of a trillionth of a second is a massive triumph of physics. What happened after that fraction of a second is better understood. The universe expanded and cooled. The four forces went their separate ways and the first basic subatomic particles were able to hold themselves together. This all happened in the very first second of the universe's existence. 3 minutes later, yes, 3 minutes, the universe cooled enough that these subatomic particles could start to stick together. For the next 17 minutes, the universe did something remarkable. It made atoms. It was still ridiculously hot, like the core of a star. But it's at those temperatures that nuclear fusion can occur. For a few minutes, the particles smashed together, forming dutarium, an isotope of hydrogen, helium, and just a smattering of lithium. A little bit of

burillium was made as well, but it was radioactive and rapidly decayed into lithium. Then at t plus 20 minutes, the universe cooled enough that fusion stopped. When it did, there was three times as much hydrogen as helium in the universe. This primordial ratio is still pretty much true today. When we measure the sun's elemental abundance, we see it's roughly 75% hydrogen, 25% helium. At this point, the universe is still hotter than a star's surface, but it's also still expanding and cooling. As it does, structures start to form as the gravity of matter can overcome the tremendous heat. These will become the galaxies we see today. This is important and a bit weird, so I'll get back to it in a minute. The next big event happened when the universe was at the ripe old age of about 380,000 years. Up to this point, electrons couldn't bond with the atomic nuclei zipping around. Every time they did, it was so hot that random photons would blow them off again. The universe was ionized. But then, after 380 millennia, it had cooled enough that the electrons could combine with protons

and helium nuclei, becoming stable, neutral atoms for the very first time. We call this moment recombination. This was an important event. Free electrons are really, really good at absorbing photons, absorbing light. When the universe was still ionized prior to recombination, it was opaque. A photon couldn't get very far before an electron sucked it up. But after recombination, the photons were free to fly. The universe became transparent. Why is this important? Because the light emitted at this time is what we see as the cosmic microwave background today. Those neutral atoms emitted light. They were as hot as a red dwarf star. Those photons have been traveling ever since, fighting the expansion of the universe, redshifting into the microwave part of the spectrum and seen today all over the sky. That background glow predicted by the Big Bang model has been on its journey to Earth for almost 13.8 billion years. This light is incredibly important because it tells us what the universe was like not long after it formed. For example, that light looks

almost exactly the same everywhere you look in the sky. It looks smooth. That tells us that matter was very evenly distributed everywhere in the universe at that time and also that all the matter had the same temperature. If there had been one spot that was denser, lumpier, then it would have been hotter and we'd see that in the background radiation as a patch of bluer light. That's pretty weird. When you look at the background radiation from opposite sides of the sky, you're seeing it coming from opposite ends of the universe. Even back then, those regions of the universe were separated by vast distances and had plenty of time to go their separate ways, change in different ways. They should look pretty different, but they don't. As telescopes got better, very tiny variations in the light were found, but they were really teeny, only a factor of 1 in 100,000. In other words, one part of the sky may look like it has a temperature of 2.7250 Kelvin, but another spot is at 2.752501.

The universe had lumps, but they were far, far smaller than expected. Something must have happened in the universe to force it to be this smooth, even hundreds of thousands of years after the Big Bang. This led theoretical physicist Alan Guth to propose a dramatic addition to the Big Bang model. At some point in the very early universe, the expansion suddenly accelerated vastly. For the tiniest fraction of a second, space inflated hugely, far faster than the normal expansion, increasing in size by something like a 100 trillion trillion times. We call this super expansion inflation. It sounds a little arbitrary, but it actually has quite a bit of physical foundation. Now, in a sense, it's like one of the phase changes of the universe that happened in that first fraction of a second dumped huge amounts of energy into the fabric of spacetime, causing it to swell enormously.

Inflation explains why the universe was so smooth at the time of recombination. Space expanded so rapidly that any lumps in it were smoothed out, like pulling on a bed sheet to flatten out the wrinkles. Inflation explains several other problems in cosmology as well. And although the details are still being hammered out, the basic idea is almost, pardon the expression, universally agreed upon by astronomers. The fluctuations we see in the background glow now were actually incredibly small perturbations in the fabric of space at the time of inflation which got stretched by inflation to macroscopic size. These denser spots were seeds eventually growing even more their gravity attracting flows of dark matter. Normal matter collected there too condensing eventually forming the first stars about 400 million years after the big bang. Eventually, those teeny little bumps from the beginning of the universe [music] became galaxies and clusters of galaxies now tens of billions of light years away. Our own galaxy, our own piece of the universe, started the same

way as a quantum fluctuation in space 13.8 billion years ago. Now look at us. How's that for an origin story? There are still many unanswered questions in our understanding of cosmology. What's dark energy? What was the role of dark matter in the early universe? Where did the universe come from in the first place? Are there more universes out there hidden away where we can't see them? If time and space started in the Big Bang, does it even make sense to ask what came before it? Or is that like asking what's north of the North Pole?

We don't know the answers to these questions. And trust me, there are thousands more just like them. But here's the fun part. We might yet be able to answer them. After all, even asking if the universe had a beginning, let alone what happened between then and now, was nuts just a century or two ago. Now we have a decent handle on it, and our grip is getting better all the time. Science, asking and answering the biggest questions of them all. I love this stuff. Today, you learn that the timeline of the universe's history can be mapped using modern-day physics and astronomical observations. It started with a big bang when the universe was incredibly dense and hot. It expanded and cooled, going through multiple stages where different kinds of matter could form. It underwent a phenomenally rapid moment of expansion called inflation, which smoothed out much of the lumpiness in the matter. Normal matter formed atoms between 3 and [music] 20 minutes after the bang, and the lumps left over from inflation formed the galaxies and larger structures we see today. Compared to human experience, the universe has been

around a long time, nearly 14 billion years. That's a soul crushingly long time. That's older than Earth, older than the sun, older than, well, everything pretty much by definition. And yet, when you think about time itself, how long is 14 billion years? Really, it seems like a long time to us now. But time keeps on slipping into the future, and every day the universe is a little bit older. I mean, sure, duh, of course it is. But have you ever really thought about what that means? Someday the universe will be 20 billion years old, 30, then 50 billion, 100 billion, a trillion, and even then the clock still ticks. Those numbers sound abstract, but those days will inevitably come. Time runs long. What will the universe look like in that far, far future? Different.

It'll look different. We're about to go on a long journey. Literally the longest possible. We're going to the end of the universe. >> [music] >> First, a bit of a content warning. The stuff I'm going to show you today is incredibly interesting, incredibly cool, and well, maybe a wee bit distressing to some people. But I want to say that it's not all darkness and dissolution. Well, to be honest, it's mostly darkness and dissolution, but there is some light almost literally at the end of the long, long tunnel. I'm going to have to use some pretty big numbers in this episode. And by big, I mean big.

>> Probably bigger than any numbers you've come across before. I'll have to use scientific notation, which is a shorthand way of expressing numbers using powers of 10. For example, the number 100 is equal to 10 * 10. So, we say it's 10^ squared or 10 ^ of 2. 1,00 is 10 x 10 x 10 x 10 x 10 x 10. So it's 10 cubed or 10^ the 3 power. This might seem silly for smaller numbers, but it gets useful for much larger ones. A million is 10 the 6th. A billion is 10 the 9th and so on. Mind you, every time the power, the exponent goes up by 1.

The actual number is 10 times bigger. So 10 18th isn't twice as big as 10 the 9th. It's a billion times bigger. At different times in the life of the universe, different objects dominate. For example, in the current universe now, you can make the case that stars are the dominant objects. They produce the most amount of energy. Before stars, something else was dispersed plasma. I suppose generally speaking, astronomers Fred Adams and Greg Laughlin looked at what will happen to the universe on very long time scales and divided it into five broad epics, then wrote about it in their book, The Five Ages of the Universe, which is a pretty good read. Their divisions aren't official in any way, but I think they did a good job given what we know. So, let's use them.

First came the primordial era, which we already went over in the last episode. It's basically everything from the moment the universe big banged up to when the first stars formed about 400 million years later. The second era is the one we live in now, where stars rule the night. This is the Stelliferous era. We're about 13.4 billion years into it. But how long will it last? Or to phrase it another way, when will the last star die? The lower the mass of a star, the longer it will live, steadily fusing hydrogen into helium at a very slow rate. Models indicate the lowest mass red dwarf can do this for about a trillion years. That is a long time. A red dwarf that formed when the universe was young would only now have used up about 1% of its hydrogen and has about 99% of its life ahead of it. In human terms, they're still infants. Right now, galaxies are merrily churning away, converting nebular gas into stars. But eventually that gas will run out. Estimates vary, but star formation in most galaxies will start to peter out in a few billion years. Fewer stars will be

born and the ones already born will start to die off. Galaxy collisions and other events may trigger star formation after that. So maybe they'll extend things a bit. But even if it lasts another 50 billion years or 100, it hardly matters. When the longest star sticks around for a trillion years, why quibble about a few billion here and there? As this happens, galaxies will change color and fade. Most spiral galaxies have discs that are a vibrant blue in color. The massive hot luminous stars dominating their emission. But as these stars die and the fainter lower mass stars take over, the galaxy will reen and dim. A few billion years after the nebular gas runs out, the only stars left will be those longived red dwarfs. In a trillion years, this is what awaits us. It may take longer. I've seen some calculations that show the very lowest mass stars might last 10 times that long. Whatever. I'm not going to worry too much about factors of 10 here or there. As the universe ages, those are essentially statistical fluctuations. But when that does happen, when the last star dies, it means that the only objects left in the universe generating

appreciable energy will be the corpses of stars, white dwarfs, neutron stars, and black holes. And to be fair, brown dwarfs as well. Those objects intermediate in mass between stars and planets. With the exception of black holes, those objects are supported by various forms of degeneracy pressure. That means in a few trillion years, it'll be the end of the Stelliferous era and the start of the degenerate era. The universe will be dark. Dark to humanize at least, assuming we have eyes in a trillion years, or that were even around. In the infrared, things are a bit brighter. Many of those objects will be glowing at those wavelengths since they'll be warm. Well, lukewarm. Neutron stars and white dwarfs are born very hot and fade over time. How long that takes depends on how massive they are and other factors, but it's safe to assume that they'll have cooled to room temperature at best in a few trillion years. And the deeper we get into the degenerate era, the cooler they'll be.

They'll have their moments, though, because over trillions of years, binary white dwarf orbits will decay, and the stars will merge and explode as supernova. Binary neutron stars will merge and form gamay bursts, too. Explosions so bright they'll outshine a thousand galaxies in this far-flung future. briefly. These are short-lived events and soon thereafter the universe will return to darkness. Interestingly, brown dwarfs are a better bet as an energy source. Binary brown dwarfs could merge to form a relatively normal if low mass star that could shine for hundreds of billions of years. But again, time is long. After a quadrillion years, this too shall pass. Stars just ain't what they used to be. In fact, in the degenerate era, neither is the universe.

In our dark energy episode, I mentioned that as the universe expands, our view of it will shrink. At the same time, all the galaxies in the local group will collide and merge, forming one big elliptical galaxy. By deep into the degenerate era, all we'll be able to see is our own bloated, dark galaxy. The rest of the universe will be forever cut off from us. Not that there'll be all that much to see anyway. And I hate to say this, but even that won't last. Our models of how subatomic particles behave predicts that over very long periods of time, protons, the positively charged particles in the nuclei of atoms will decay. The half-life of such an event is at least 10 the 34 years and it's almost certainly longer. But the degenerate era is longer yet. As protons decay one by one, matter itself will disintegrate. White dwarfs, neutron stars, brown dwarfs, planets, all of them will dissolve as their constituent protons decay. There's an upside kind of. Adam and Laughlin calculate that proton decay, which releases a tiny bit of energy, will cause white dwarfs to

radiate heat with about the energy of 400 W. Mind you, the microwave oven in my kitchen generates more power, and it's a lot smaller than a white dwarf. But comparatively, in 10 the 38 years, this will look like a bright star does to us. Now, by 10 the 40 or so years from now, even degenerate stars will be gone. The only big objects that will be left are black holes. Thus begins the black hole era. Black holes don't generate a lot of energy unless they happen to suck down a large amount of material that can be torn apart and turned into an accretion disc. Otherwise, basically they just sit there. However, there is a way black holes can make energy. They evaporate. I know most people think that black holes can only eat stuff and once it's in there, that's where it is forever. No, but that's a pesky word, forever. When you're talking 10 to the 50th, 10 to the 60th, 10 to the 90th years, what does forever mean? Back in the 1970s, physicist Stephven Hawking worked out the math of black holes combined with quantum mechanics and discovered that

under some circumstances, they can emit particles. This is a very weak effect and has extraordinarily complicated physical mechanisms behind it. Check the doobly-doo for links that explain it. But the end result is that black holes can very slowly leak mass. And the more massive they are, the slower the leak. How slow? A black hole with three times the sun's mass, the minimum size for one created in a supernova, will take about 10 to the 68 years to evaporate. That's a ridiculously long time. But the universe can wait. Even super massive black holes in galactic centers evaporate. It takes them, get this, 10^ the 92 years. 10 to the 92nd power. That number is so huge, so colossally ridiculous that I can't even come up with an analogy for it. It's a one followed by 92 zeros. There aren't even that many subatomic particles in the entire universe. See what I mean? Ridiculous. That's the length of time we're talking here. But it'll happen eventually. As black holes lose mass,

they emit particles and energy faster. So each time one evaporates completely, it'll emit a flare of light, like a small bomb going off. During the black hole era, that'll be the only source of energy in the universe. Eventually, they'll all go away, and that'll be it. There won't be anything else in the universe except subatomic particles and photons, and they'll all be so low energy, they might as well not exist. That'll happen pretty much by 10 92 or 93 years from now. At this point, the universe, it's safe to say, is dead.

Kaput, done. We have entered the ominously named dark era. It'll stretch to infinity if time even has any meaning by then. That's awful. I mean, seriously, writing about this and talking about it is hard because it's not fun to think about this stuff. I mean, it is kind of, but when you internalize it, it's bleak. There is another idea that would prevent this potentially eternal darkness from happening, but you won't like it much. Dark energy is pumping up the universal expansion, causing it to accelerate. We don't know much about dark energy. We don't even know what it is. We do know that the cosmic horizon, how far out we can see into the universe, is shrinking as the expansion accelerates. One idea among many about dark energy is that its influence will get stronger and stronger. As it does, the horizon will shrink ever faster, closing in on us more and more rapidly. But that's not some illusion. It's a physical limit to the universe, a stretching of spaceime.

No force, not gravity or electromagnetism or anything can cross it. It will be as if that part of the universe across the horizon is ripped away from us, stretched beyond breaking. Over time, according to this hypothesis, the cosmic horizon will eventually shrink until it's smaller than our galaxy, smaller than the nearest stars, smaller than the solar system, smaller than our planet, and it'll keep shrinking until it's smaller than a subatomic particle. When that happens, it'll be as if the universe is torn apart at the most fundamental quantum level. Astronomers call this the big rib. We have no idea if it will actually happen or not, but if it does, it'll be many billions of years from now, long after the sun dies, but long before the dark era. I'm not particularly comforted by this idea, but the good news is that this idea isn't really much more than conjecture. As we learn more about dark energy, we'll learn more about its eventual influence on the fate of the cosmos. Yay. So, is there any reprieve? Maybe. It's possible, though by no means proven, that our universe is one of many universes, a multiverse, if you will.

Those other universes may be ticking along just fine long after ours is wound down. Of course, we can't get to them, but still good on them. And there's another idea. It's a little far-fetched, but not completely outside the realm of physics as we know it. We think of the vacuum of space as being devoid of energy, empty. But there's an idea in quantum mechanics that this might be an illusion. There might yet be a lower energy state we don't see. Think of it like a staircase. You've been standing on what you think is the bottom step, but then you find out you're actually one step up from that. Our universe may be narrowly balanced on this second to lowest step. It'll stay there, but if something bumps it, down it goes. It's possible that somewhere out in the dead, dark universe, for whatever reason, after a gazillion years, some small bit of space will be quantumly jostled and drop down to that next lower state, the true vacuum. What happens when it does? It'll bump the regions around it and they'll drop and so on and so on. Here's the weird part. Well, it's all weird,

but here's the really weird part. Inside this region, the laws of physics get rewritten. How? We don't know. As far as we know, we can't know what happens in there. But in a sense, it'll erase space and time inside it. Like poof, gone. Everything changes at some fundamental level we can't even understand. This wave of destruction expands at the speed of light, engulfing all of what remains of the universe. What's left behind in its wake is something new, something different. We literally can't know. This idea actually gives me hope. Think of it as a cosmic reboot. The universe has led a long, long life and lingered in unimaginably slow death. This gives it a new lease, a chance to start over again.

Maybe this is similar to how our universe came into being in the first place as a quantum fluctuation in some other universe butdding off from there to create everything we know. Maybe this has happened before and will happen again over and again an infinite number of times. I have to say I like this idea. If it's true, and we don't know, it's just conjecture at this point. But if it is true, then it's not the death of our one universe. It's the opportunity for the birth of an infinite number more. And that is [snorts] perhaps the single most hopeful thing I know. Today you learn that our universe's days [music] are numbered. Stars will die out after a few trillion years. Protons will decay and matter will dissolve after a thousand trillion trillion trillion years. [music] Black holes will evaporate after 10 the 92 years. And then all will be dark. But there's hope that a new universe will be born from it. [music] There's always hope. Hey everyone, Phil Plate here with Crash Course Astronomy for one last time. We've come a long way together. you and I. From the banging beginning of the universe to its far,

far future, from subatomic scales to superclusters of galaxies, we've watched the sky, mapped how it looks to us from the ground, and then sailed out into the black to investigate its denisens up close. And one of my favorite parts, we've seen how human beings, people like you and me, have worked over the centuries to figure all this stuff out. We're pretty clever for a bunch of apes who only recently figured out how to walk upright. But after all the topics we've covered here, all the space we've explored, there's still one more thing I want to talk about that didn't quite fit in anywhere else in the series, it's yet another fascination of mine. And judging from talking to so many people over the years, it's something important to a lot of us, which doesn't surprise me. I think asking this question is a deeply human thing to do. Perhaps ironically, part of what makes us who we are. And it's simply put too, are we alone?

caused by a wish for mankind. [music] When people ask me what field of astronomy is the most exciting right now, I don't even hesitate. Exoplanets. I talked about these back in episode 27. Alien worlds, planets orbiting other stars. The first were discovered in 1992, and now we know of thousands with many thousands more awaiting confirmation. Most of these exoplanets are giants, far more massive than Earth, and likely to have very thick atmospheres like Neptune or Jupiter.

They also orbit so close to their host stars that they're hotter than Venus. But as our techniques got better, we started finding smaller planets farther out. And now we have a short list of planets that could very well be Earthlike in many ways, close to our size, density, temperature, surface gravity. But that's all we know. With our current abilities, these planets are on the thin bleeding edge of what we can observe. Do they have atmospheres? If they do, do they have a copious amount of oxygen in their atmospheres? That would be a critical observation. In Earthlike atmospheres, oxygen molecules are extremely reactive chemically and won't last long without being replenished. The easiest way we know to do that is biology. It's not the only way, so it's no guarantee. There are some other processes that can produce and maintain oxygen in a planet's atmosphere, but seeing it would be a good sign. And there are other fragile molecules that if detected would strengthen the idea of biological activity. Detecting the presence of O2 in an exoplanet's atmosphere is just tantalizingly out of reach right now. We

can almost do it. We may not have to wait too much longer, though. The James Web Space Telescope, the successor to Hubble, is a gigantic infrared space telescope currently scheduled to launch in late 2018. It should be able to tease apart the light from some of the nearer and brighter of these distant worlds and look for the signature of oxygen as well as other biologically produced gases. Still, finding them in an Earth-sized planet may be beyond James Web, too. NASA has studies looking into space telescopes specifically designed to look for alien Earths, but that's a ways off yet. But think of it. When I was a kid, there were nine planets. Okay, eight. Now we have thousands, and they're not at all what we expected. The diversity of worlds is stunning. With some hot, some cold, some big, some little. Some in solar systems so compact they can fit five planets inside the orbit of Mercury, and some with planets far-flung. Nature is more clever than we are. Sometimes I think that if something's possible to do, nature's done it somewhere. But what's more amazing to me is that we now think that most most stars in the galaxy have

planets. Given that multiple planets orbiting each star is common, that means there are likely more planets than stars in our galaxy. And there are over 200 billion stars in our galaxy. I also want to point out that we see traces of life on Earth going back nearly 4 billion years. There was certainly life before that, but didn't leave evidence we can find so many eons later. That timing is interesting. This means life existed on Earth not too long after it cooled enough to have liquid water on its surface. That in turn strongly implies getting life started on planets is easy. The ancient Earth was a tough place to live. Yet life took hold within a few hundred million years after it formed.

Put this all together and what do you get? If the galaxy is filled with planets and life gets its start so easily, the galaxy may be filled with life. We need more evidence, more observations. But that's the way the numbers lean. Mind you, I'm not talking about intelligent life, just life itself. But still, that's just about the coolest thing I can imagine. So, are they out there? This is a question I am always asked when I give public talks. Do you believe in UFOs and aliens? My answer is always the same. No and yes.

Given what I just said, it seems inevitable that life exists out there in the universe. The conditions for life to arise are easy to make and there are a lot of chances for it out there. On the other hand, for most of Earth's history, the most advanced life was basically algae. Complex life, life capable of intelligent thought, is a relatively recent development. So, alien life, sure, but aliens sophisticated enough that they can come here to pose for blurry photos, abduct people, and slice up cows, I'm not buying it. The evidence just isn't there. And now that so many people walk around with cameras, we see even fewer reports of mysterious lights in the sky than we used to. Or at least most of these lights seen are explainable. wedding lanterns, balloons, Venus meteors, the moon, satellites, and so on. I'll note, astronomers are watching the skies literally all the time. If UFOs were real, we'd be reporting the vast majority of them. That doesn't mean aliens aren't out there, though. Space is big. Remember folks, that's why we call it space. And

getting here is a huge pain in the butt, or whatever aliens use to excrete waste matter. It takes a vast amount of energy to accelerate a spaceship to the speeds needed to travel to other planets. Even then, that's slow compared to the speed of light. Not only that, light is easy to make. You don't need a lot of super sophisticated tech to make radio waves, and it's easy to embed a lot of information in them. After all, that's how radios and Wi-Fi work. So, why go to all the trouble of meeting other life forms in person when you can chat via radio? This is the premise behind SETI, the search for extraterrestrial intelligence, that is. This is based on a simple idea. If aliens are out there, it's easier to stay home and talk via radio than through the interstellar version of the Pony Express. There are a lot of assumptions built into this idea, of course. One is that, well, aliens exist. The next is that they're much more technologically advanced than we are. But that makes sense. Any aliens that don't have tech can't communicate with us, so they don't play into this.

And we know how quickly our technology advances. We're just starting out and already have some pretty cool gizmos. Give us a hundred years, a thousand, and where will we be? The Earth is over 4.5 billion years old, but we've detected exoplanets far older than that. Life there might be that much more advanced than we are. They might communicate with each other using nutrinos or ripples in spaceime or quantum entanglement or some method we can't even imagine. But if they're interested in finding life in the galaxy that's just starting to get their technological legs under them, radio waves are the way to go. Even primitive societies like ours use radio waves, and they can travel for thousands of light years. Aliens could broadcast radio waves across the galaxy or target likely stars or wait for us to make enough noise and for those waves to travel outward at the speed of light long enough to reach a listening post and then aim a transmitter at us. So, the folks at SETI scan the skies at various radio wavelengths, hoping to hear a signal. So far, nothing yet, but they don't despair. My friend, the astronomer Seth Shastak, is a SETI

scientist. He's been doing this for years and recently made a bold but logical claim. Seditech is advancing rapidly and they're getting so good at listening that they'll be able to scan billions of stars. He predicts that if anyone out there is broadcasting, we'll pick up the signal in the next 20 years. If we don't, then given the statistics, it may very well mean that we're alone in the galaxy or aliens might exist, but they're not broadcasting. Or of course, that perhaps there's another flaw in the chain of logic that we've missed. Either way, if they detect a signal or they don't, the resulting conclusion is at least interesting and at most terribly profound. And it's only our first logical step in attempting to look for life out there. I love astronomy. Maybe you figured that out over these past 46 episodes. I love it for so many reasons.

The beauty, the grand nature of it, the incredible vistas, the depth of the wonder and science of it. When I was little, the subtle profoundity of the questions related to astronomy were perhaps beyond me. But even then, the questions themselves provoked a sense of wonder, of awe, so much so that even as a little kid, I could be stirred by them. And now, as an adult, I grasp these questions better. How did we come to be? What was it like in those first few fractions of a second after the Big Bang? Is ours the only cosmos out there?

Or are we part of a much larger multiverse riding along with other universes like bubbles in a sea of multi-dimensional foam? What is the nature of stars? How do planets form? Are there other Earths? Are we alone? These are some of the grandest questions we can ask and in one shape or another have been asked by humans for thousands of years. These are also questions that were once only in the purview of religion and philosophy. But now we have observations to pursue them, evidence to back up our conditional answers. Because of astronomy, because of science, we now stand on the edge of understanding, our cleverness and brilliance having taken us to the point where these questions finally have an answer we can almost touch. And the best part, as we investigate, as we peak around the next corner, as we climb over the next hill, we find those answers. And they reveal a host of more questions. The exploration never ends. There's always something else to discover. And that that is why I

love astronomy. And so it is not out of arrogance, not out of false smuggness, but out of pride and understanding, hard won by a legion of scientists who came before me, who came before all of us, that I can now say, "Let's [music] go explore the universe." Thank you for watching. And if I may indulge myself, [music] my personal thanks to Derek Mueller of Veritasium for his hand in getting me set up with this dream gig. [music] And of course to Hank Green, who wants to make the world a better place and is doing a fine job at it. And to all of you [music] who have watched Crash Course Astronomy. Don't forget to be awesome. This episode was written by me, Phil Plate. The script was edited by Blake Pastino and our [music] consultant is Dr. Michelle Ther. It was directed by Nicholas Jenkins, edited by Nicole Sweeney. [music] The sound designer is Michael Oronda and the graphics team is Thought Cafe.