Let's talk about Cosmic Chemistry. The business of stars is nucleosynthesis, the fusion of different atomic nuclei under conditions of enormous temperature and pressure to produce new elements. Essentially, everything in the material world except for hydrogen and helium, which problem primordial in the Big Bang, were created in this way. Welcome back to the Star Cook. Thank you, thank you. Welcome back, I'm Michael and I'm here with my very special guest, Mr. Bill Nye. Thank you, thank you. Michael, it's so good to be back on the show and you are looking fabulous. Thank you. You too, you too. Thank you Michael. So tell me Bill, what are we going to do today? We have here the ingredients to make virtually anything. These are different kinds of atoms. That's right, these are all the different elements, all the different kinds of atoms in the universe. Now, atoms come from stars. Now, take for example our planet. Our planet has lot of iron and silicon along with many other elements, and our star has a lot of hydrogen and helium. Michael, that's our hydrogen, almost lost it there. It's okay everybody, it's okay. Now Michael, you and I are 65 percent water. Right. Now, what's water? H2O. That's right, H2O. I have some water here, which I prepared earlier. Of course. The water is two parts H and one part O, H2O, water. So with this here, we can make virtually anything. Well, actually no, because in order to make all these atoms, you need the force of exploding stars and billions of years, but brownies on the other hand only take about 35 minutes. And even brownies are made of atoms. That's right. Oh Bill, this looks fabulous. People, it looks fabulous, doesn't it? Thank you, thank you. Here Michael, you please have the first one. Oh, thank you. We'll be right back. Thank you. Now people say, why make observations of stars, why look into space? Are you kidding me? It give you a perspective of where you are in the universe, of what we do all day. Are you trying to make some kind of joke? I mean, we are made of the same material as stars. Stars, you, me, the camera, the television you're watching, everything, made of the same stuff that stars are made of. So you have to study stars in order to understand what we're made of, for crying out loud. Wow. Low mass stars like the sun and a little more massive than the sun, can go one step beyond helium to make carbon. This is a critical step in the fusion process because, of course, biology would not be possible without carbon, and it requires two extraordinary tricks, coincidences of nuclear physics if you like. You might imagine, as with fusion of helium from hydrogen, that you could do this in two simple steps, by taking a helium nucleus, fusing it with another one to make beryllium, and then adding a third to make a carbon nucleus, but it doesn't work this way because beryllium, for reasons of nuclear physics, has an incredibly short decay time. It's radioactive and disintegrates into trillionth of a second or less. That means that inside a star, when two helium nuclei fuse, the result, the beryllium nucleus disappears almost as quickly as it's assembled, disappearing like sand through your fingers, and the star cannot make carbon that way. The way the star makes carbon is by a much rarer triple collision, which instantly creates carbon from three helium nuclei arriving in the same place at the same time. It turns out that this process would be vanishingly rare, leading to essentially no carbon in the universe, if it weren't for a nuclear resonance state first noticed by Willie Fowler, 60 or more years ago. This nuclear resonance state increases the probability that the triple fusion will take place, and means that we actually have carbon in the universe, but the scarcity of the triple collision, nonetheless, means that carbon is hundreds of times rarer than hydrogen and helium, the most abundant elements. In general, as we move up the fusion chain, higher and higher temperatures and pressures and densities are required to generate heavier and heavier elements. To make hydrogen into helium requires overcoming the repulsion of two hydrogen nuclei protons, but to make carbon from helium nuclei means we have to force helium nuclei with double the electrical charge, and therefore four times the repulsion to get close together, requiring correspondingly higher temperatures. Carbon is a critical ingredient in the story. And as we know and in this video from Linda Williams tells us, carbon is a girl's best friend. [MUSIC] If we consider the full evolutionary path of a low mass star like the sun, and this of course includes the story of our sun, it goes through several phases. The long main sequence phase,10 billion years long for the Sun. Then the star re-configures to a new state, starting a new energy source, releasing a large envelope, and having a collapsing core, this is the red giant phase. In our sun when this happens, the outer envelope will expand past the orbit of the earth, perhaps to Mars, engulfing the biosphere and frying it. If life still exists on earth billions of years from now, this will probably be the end point. Thereafter, the sun goes through an unstable phase ejecting much of its mass into space as a planetary nebula, while the remaining core shrinks. With no new fusion process possible, the core gradually cools as an amber forever. This final state for the sun is a white dwarf. Since most stars are like the Sun or even lower in mass, this is the fate of most stars. So most of the stellar remnants in the universe are expected to be white dwarfs. If we look at this picture, such as taken with the Hubble Space Telescope of a planetary nebula, we can see both parts of this fate which awaits the sun. The sloughing off of layers of gas out into space where they're heated up and glow brightly, and at the center, a cooling ember, the white dwarf. For a stars, mass is destiny. For stars, the mass of the sun up to about 1.5 times the mass of the sun. The battle between gravity and radiation is as always won by gravity, and what's left behind is a compact core called a white dwarf, a cooling ember of a star. Only certain heavy elements can be created in stars like the mass of the sun up to about carbon. Heavier elements depend on more gravity and larger mass.
