How Were Stars Formed?

0:01(heart pounding music) Male narrator: We've all looked up at the stars at night and wondered about them but can 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.

0:26As 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.

0:49You had about 75% of them were hydrogen with one proton, about 25% most of the rest were helium with two protons. Those are tiny sprinkling of beryllium, of lithium. Lithium's got three, beryllium's got four protons and finally boron. There's also stuff that astronomers call dark matter quite frankly because they don't understand what it is.

1:14But 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.

1:29What that shows is that matter was distributed extremely evenly through the universe. Everywhere you look you seem 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.

1:49How could you produce you and me from such a universe? Well we actually know how this happened and the key players in all of these are stars. 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.

2:12Not to hot, not to cold. Not to big, not to small. Not to close together, not to far apart. You get the idea. What were the perfect Goldilocks conditions for creating just a bit more complexity than the early universe. Well it turns out that those conditions were scattered all through the universe.

2:33The 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.

2:58Some regions, for example were just a thousand [certain] degree hotter than other regions, now this was just enough for gravity to get to work. What gravity could do was to magnify those differences and turn them into something much more interesting. So this is what happened, gravity began to get to work on those differences and eventually it created stars, something entirely new.

3:23Let's see how this works. Gravity you'll remember is one of the four fundamental forces and it's the star at this part of the story. As [you've been] showed gravity is more powerful where there's more stuff and when things are closing together. Give it 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.

3:48Now 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.

4:05As 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 run away train and this gets faster and faster and faster. 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.

4:36Now 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.

4:56It's the temperature which atoms can't hold together anymore because protons can't hold on to electrons. What happens is you recreate the sort of plasma that existed before the creation of the cosmic background radiation. Now the temperature on the cloud keeps rising until eventually it reaches 10 million degrees.

5:18Something 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 our now held together by the strong nuclear force. As that happens, there's a huge release of energy some of that matter is turned into pure energy.

5:38This is very similar to what happens in an h-bomb. Now the center of the cloud, we have a sort of furnace that's pushing back against the force of gravity and it stabilizes the whole thing. Now what's happened is a star has lit up. That star is going to shine for millions or billions of years.

5:59We'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. The universe is now a much more interesting place.

6:16Instead of the sort of uniform mash 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 if filled with this sort of glowing batteries that emanate light and heat, it's a much more interesting place.

6:37In 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. In fact, it's worth going on to the hubble website or looking through a telescope at some of these star nurseries because they're amongst most beautiful sites you can see in the sky.

6:59Stars increase 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 super clusters. We'll be trying to discuss these structures one by one. Let's begin with the stars.

7:16Stars themselves have a very clear structure. At the center you've got protons that are at 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.

7:37Now 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. Stars have a lot of structure, but stars themselves are gathered together by gravity into much larger structure.

7:56We call these galaxies. Our milky way is our galaxy, it contains perhaps a 100 billion, some say 200 billion stars, it's absolutely huge. There may be a 100 billion galaxies in the entire universe. But structures exist at even larger scales too. Gravity gathers galaxies together into what are called clusters.

8:18Our 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 super clusters. This scatter through the universe in huge webs in sort of chains but beyond that gravity is too weak to hold super cluster together.

8:43It's beyond the level of super clusters that you begin to see finally what Hubble saw. You'll be able to see whole super clusters moving apart and there at that scale, you can see the expansion of the universe. Let's summarize. We'll see throughout this course that complexity builds on complexity.

9:04Now 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 starts you have something like campfires in Antarctica.

9:26Lights that light up a cold universe. We'll see that from now on the Goldilocks conditions for further complexity and 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.

9:44(heart pounding music)