WATCH: What Did Stars Give Us?

DAVID CHRISTIAN: Here we are at Lakeside High School in beautiful Seattle. Now look around you on this beautiful campus, okay? I'm going to make you a bet. This contains a lot more than hydrogen and helium; in fact, we can be pretty sure it contains a fair bit of carbon, oxygen, nitrogen, probably a fair bit of phosphorus, sulfur, and trace elements of all the other things in the periodic table. This is the periodic table; you can see all the elements we were just talking about. So here's the problem: There's hydrogen, there's helium. In a Universe that had only hydrogen and helium, what could you make? Well, you certainly couldn't make all the stuff out there, you couldn't make a planet, you couldn't make a laptop, and you couldn't make my friend Raul, nor could you make my living friends over here. So this is a real problem. Where did all those other elements come from? And the answer is, they came from stars. So far our stories have been about a Universe that's cooling down. And that cooling down was really important because it allowed matter and energy to separate from each other, and it created the forms of matter that we've seen so far. But now we need to talk about ways in which the Universe began to heat up. And that is something that happened inside stars. It was that heating-up process that allowed stars to cook all the other elements that we've seen around us. And that's why stars are, sort of, the stars of this part of the story. The nearest star to us is our Sun. At the surface the Sun is 5,800 degrees Celsius, but at its center, it's 15 million degrees. Think about that. Water boils at about 100 degrees Celsius; that's about 373 degrees above absolute zero, the coldest temperature there is. So the center of the Sun is about 40,000 times hotter than boiling water. At these enormous temperatures protons have a huge amount of energy, and as we saw in the last unit, they smash together really violently and eventually they fuse to form helium nuclei. Now that's pretty hard, but here's the problem: There's carbon up there; it's got six protons in the center. You can see the six above the carbon. So to get carbon, we need to smash six protons together, and for that, you need much higher temperatures, like 200 million degrees. And now let's go on to iron. Where's iron? There's iron, 26 protons. So now you need to smash 26 protons together to get iron, and to do that you need temperatures as high as three billion degrees. So, where in our young Universe are you going to find temperatures of three billion degrees? The answer is: inside dying stars. That's right, dying stars. And here's why. Remember that most stars spend most of their life, about 90 percent of their life over billions of years, fusing protons, hydrogen nuclei into helium nuclei. But think, what happens when they run out of fuel? Well, what happens is that the furnace at the center of the star stops supporting the star; gravity takes over and collapses the whole thing. Now that collapse is really violent, and it creates high temperatures at the center, but how high depends on how large the star is, how much stuff there is, how powerful gravity is. Now think of small stars. A small star doesn't have much pressure at the center, it burns hydrogen slowly over billions of years at low temperatures, and it lives a very long, slow life. And when it dies, eventually it runs out of fuel, it will just slowly fade away, like a dying campfire. Nothing very interesting happens. Larger stars are much more interesting. They create high temperature at their cores; they burn hydrogen much more violently; and when they run out of hydrogen and collapse, they generate much higher temperatures, up to 200 million degrees. Now you may remember that's the temperature at which you can fuse six protons to form carbon. So they start burning helium to form carbon. Now when stars run out of helium, things start moving faster and faster. If a star runs out of helium, it will start fusing carbon into neon at close to one billion degrees. And then, in a whole series of collapses and new fusion processes that get faster and faster and faster, it fuses neon into oxygen, then oxygen into silicon. And finally, at three billion degrees, it fuses silicon into iron, and that's as far as the process can go. I'd like to read to you Cesare Emiliani's wonderful description of the final few million years in the life of a dying, huge star: "A star 25 times more massive than the Sun "will exhaust the hydrogen in its core in a few million years, "will burn helium for half a million years, "and, as the core continues to contract "and the temperature continues to rise, "will burn carbon for 600 years, "oxygen for six months, and silicon for one day." By this time, the center of the star is like a sort of layer cake, with all these different elements. And eventually, when it fills up with iron, it can't go any further. It will collapse; it will scatter it's outer layers into space, and so they'll spread around the star into nearby space all the elements that it's just created. Well, that's great. Now we've seen how to generate all the elements in the periodic table up to iron, but what about all of these? Where do they come from? Well, the answer is the rest of these elements are produced not in dying stars but in exploding stars. That's right, exploding stars. Now when a really large star fills up with iron at its center, it eventually collapses, and it explodes, generating staggering temperatures. These explosions are called supernovae, and they are amongst the most spectacular things you can see in the whole of astronomy. In just a few seconds all the elements in the periodic table are manufactured in that supernova explosion. It shines so brightly, it generates such high temperatures, that for a few weeks a supernova can outshine an entire galaxy. In fact, many of the "new stars" that we hear about in history, such the Star over Bethlehem, may well have been supernovae. So now, where the dead star is, where the supernova was, we have a huge cloud of sort of dust and particles containing every single element in the periodic table, and it's drifting out into space. But let's put all of this in perspective. We began this unit, remember, in a Universe that had just helium and hydrogen, nothing else. Now, at the end of this unit, we've got all of the elements in the periodic table. But the truth is that even after billions of supernovae and billions of years, helium and hydrogen make up 98 percent of the atoms in the Universe. All the rest makes up just two percent. So you may be thinking, What's the big deal about that? Well, that two percent is actually a huge deal. Without it, you couldn't make my friend Raul over here, you couldn't make me, you couldn't make you. So it really makes a difference. And that's why in this course we call the creation of new chemical elements the third great threshold of complexity. So, now we've got to the time where we need to ask some questions: What are the main features of this threshold? And is it really important? And also, you should be asking, what were the Goldilocks conditions for this threshold? And now, here's another group of questions: Would it matter if we hadn't crossed that threshold? If the Universe had never contained really large stars? And finally, you should be thinking about evidence. I don't think I've given a single piece of evidence during this talk. Why should you believe me? I'm a historian, not a scientist. Think about it. So we've covered a lot of territory in this unit. Now it's your job to dig deeper. Okay.