WATCH: How Were Stars Formed?

DAVID CHRISTIAN: We've all looked up at the stars at night and wondered about them. But, could you imagine what it would feel like if you looked up at the stars and you saw nothing? No stars at all? Well, that's what it was like for about 200 million years after the Big Bang. As the Universe expanded, it got colder and colder and darker and darker and, frankly, less and less like a place that might produce things like you and me. Astronomers call this part of the Universe's history the Dark Ages. During the Dark Ages, you had a lot of atoms flowing through space. You had... about 75 percent of them were hydrogen, with one proton; about 25 percent, most of the rest, were helium, with two protons, and there was a tiny sprinkling of beryllium, of lithium-- lithium's got three, beryllium's got four protons-- and, finally, boron. There was also stuff that astronomers call dark matter, quite frankly, because they don't understand what it is. But it doesn't seem to play much of a role in the story, so we're going to ignore it. The whole Universe was really very, very simple. We know this because of studies of the cosmic background radiation that was released, you remember, about 380,000 years after the Big Bang. What that shows is that matter was distributed extremely evenly through the Universe. Everywhere you looked, you seemed to have the same temperature, the same density, the same types of atoms. Really, everything was uniform. And that's a real problem. Because it seems as if the Universe was just too simple, too uniform for anything interesting to happen. How could you produce you and me from such a Universe? Well, we actually know how this happened, and the key players in all of this are stars. So what we're going to do in this unit is we're going to focus on how the first stars appeared. We'll see throughout this course that more complex things seem to appear when you have just the right Goldilocks conditions for their appearance. Not too hot. Not too cold. Not too big. Not too small. Not too close together. Not too far apart. You get the idea. So what were the perfect Goldilocks conditions for creating just a bit more complexity in the early Universe? Well, it turns out that those conditions were scattered all through the Universe. The crucial things you needed were: first, lots of matter; secondly, gravity; and third, tiny differences in the distribution of that matter. And they were all there. Recent studies of the cosmic background radiation, using special satellites such as the WMAP satellite, have shown that, in fact, there were tiny differences in the temperature of the cosmic background radiation. Some regions, for example, were just a thousandth of a degree hotter than other regions. Now, this was just enough for gravity to get to work. And what gravity could do was to magnify those differences and turn them into something much more interesting. And so this is what happened: gravity began to get to work on those differences, and eventually it created stars, something entirely new. So let's see how this works. Gravity, you'll remember, is one of the four fundamental forces, and it's the star of this part of the story. As Newton showed, gravity is more powerful where there is more stuff and when things are closer together. To give an example, the gravitational pull of the Earth is extremely powerful on you, but if you move away out into space, it suddenly gets much, much weaker. So now let's move back to the early Universe and think how this force might have worked. Remember, there are some areas that are just slightly hotter and slightly denser than others. In those areas, gravity was just slightly more powerful. So what it did was it clumped those areas together. As they clumped together, they got denser, so the power of gravity increased and they began to clump even further together. Gravity increases, so the whole thing is clumping a bit like a runaway train. Now, this gets faster and faster and faster. And now what happens is at the center of each of those clouds of atoms, atoms begin to bang into each other really violently, and they begin to heat up, particularly at the center, where there are the most atoms. Now notice something. So far our story has been about a Universe that's cooling down. Suddenly, we're talking about an area of the Universe that's beginning to heat up for the first time. Eventually, the temperature reaches about 3,000 degrees. Now, that temperature should sound familiar. It's the temperature at which atoms can't hold together anymore, because protons can't hold on to electrons. So what happens is you recreate the sort of plasma that existed before the creation of the cosmic background radiation. Now, the temperature in the cloud keeps rising until eventually, it reaches 10 million degrees. And something spectacular happens at that temperature. Protons start banging together so violently that they overcome the repulsion of their positive charges, and they fuse together, and are now held together by the "strong nuclear force." As that happens, there is a huge release of energy as some of their matter is turned into pure energy. This is very similar to what happens in an H-bomb. So now, at the center of the cloud, we have a sort of furnace that's pushing back against the force of gravity and that stabilizes the whole thing. And now what's happened is a star has lit up. And that star is going to shine for millions or billions of years. We've now crossed our second major threshold of complexity in this course. From about 200 million years after the Big Bang, the Universe starts filling up with stars-- billions and billions and billions of them. And the Universe is now a much more interesting place. Instead of the sort of uniform mush that we saw before the appearance of the first stars, we now have a Universe that's filled with stars. It's not just that it's more interesting to look at, stars are much more important than that. Our Universe is filled with these sort of glowing batteries that emanate light and heat. It's a much more interesting place. In fact, astronomers can see stars still forming today; it's a process that's still going on. They find them in star nurseries. They're some of the most beautiful places you can see in the heavens. And, in fact, it's worth going onto the Hubble website or looking through a telescope at some of these star nurseries because they are amongst the most beautiful sights you can see in the sky. Stars increased the complexity of the Universe in another way. They gave it new types of structure at many different scales-- from the level of the stars themselves to galaxies, to superclusters. So let me try and describe these structures one by one. Let's begin with the stars. Stars themselves have a very clear structure. At the center, you've got protons that are at an extremely high temperature, as we've seen, and they're fusing to form helium nuclei. Just around the center, around the core, you have a sort of store of protons ready to be fused eventually when they sink down into the center. Now, photons of energy and light from the center slowly work their way through the plasma, taking sometimes thousands of years, until eventually they reach the surface and then they flash out into space. So stars have a lot of structure, but stars themselves are gathered together by gravity into a much larger structure. We call these galaxies. Our Milky Way is our galaxy. It contains perhaps 100 billion, some say 200 billion, stars. It's absolutely huge. And there may be 100 billion galaxies in the entire Universe. But structures exist at even larger scales too. Gravity gathers galaxies together into what are called clusters. Our local group is a cluster like that. It contains about 30 galaxies, including Andromeda and the Magellanic Clouds, both of which you can see with the naked eye. Gravity can even hold clusters together to form what are called superclusters. These scatter through the Universe in huge webs and sort of chains. But beyond that, gravity is too weak to hold superclusters together. And it's beyond the level of superclusters that you begin to see finally what Hubble saw. You begin to see whole superclusters moving apart, and there, at that scale, you can see the expansion of the Universe. Now, let's summarize. We'll see throughout this course that complexity builds on complexity. Now we've got stars, and stars are going to be the key to later forms of complexity. Most of the Universe was then, and still is, cold, dark, empty, and from our perspective, very, very boring indeed. But with stars, you have something like campfires in Antarctica: lights that light up a cold Universe. And we'll see that from now on, the Goldilocks conditions for further complexity are to be found, not throughout the whole Universe, but in galaxies, and above all, around the stars, those cold campfires. That's where our story is going to go now.