Welcome back everyone. So, the last lecture we talked about the route that people humanity has gone through in understanding the sun. Going from the sun being a god to the sun being a giant ball of gas that was producing energy. And, as we said, there was this great mystery that emerged which was, how does the sun get its energy. Where's the power come from? Lord Kelvin figured out that if the sun was just a contracting ball of gas, that the energy you'd get out of just shrinking and releasing heat was, no way was there enough energy there to account for the song, the sun's longevity. We know that the Earth is about 5 billion years old. The sun has to be 5 billion years because the Earth was born around the same time as the sun, little bit after actually. So it wasn't until the 1920s when the development of a subject called quantum physics, quantum mechanics, where people began to really understand the fundamental nature of matter. That the question of how the sun was powered could be answered. So the basic idea with in our understanding of matter, is that every element, be it Hydrogen, or Uranium, is made up of, there's a nucleus composed of protons and neutrons, and around that nucleus are whizzing a bunch of electrons. So every atom is made up of a nucleus, and a a bunch of orbiting electrons. Now in order to understand how energy is produced, people had to come up with some mechanism by which these atoms could be the, be the source of, of energy release. And the first thing we have to understand about energy release is that, when it comes to stars. Is the famous relationship E equals mc squared, where energy and matter Einstein showed, were equivalent. And the whole idea of nuclear reactions, reactions between the nuclei of elements leading to energy generation is the possibility of converting some of the mass In the nucleus, into energy. So there are two kinds of nuclear reactions that can yield energy. Now when I say nuclear reactions, you should remember chemical reactions, right. We're all used to the fact that you know, you can put chemical elements together, and you can get reactions between them, and you'll generate all kinds of wonderful things. Like plastic, or you know, or fossil fuels. Nuclear reactions are the same idea, but you're basically working with nuclei. You're working with these bound collections of protons and neutrons. Hydrogen for example, is just, the nuclei is just one proton. Helium is two protons and two neutrons. Protons have positive charge, neutrons have no charge. And electrons, of course, have, a negative charge. Alright, so there's two kinds of nuclear reactions. The first is what is called fission. And fission is really what you'll, is going on in the nuclear power plants we have around the world. And that's where you're taking a very heavy element. Something like uranium 236. 236 means that there are 236 protons and, neutrons all together in the, in the nucleus. And you break them apart. They, they naturally, or they very easily those nuclei want to break apart. And when they break apart, they'll pre-create two daughter nuclei and release some energy in the form of light. So that's fission. Fusion is a different process where you take very light nuclei, like two hydrogen nuclei, and you jam them together. And you're able to make something heavier, and in the process the heavier thing is actually lighter than the sum of the things that went into it. So in both fission and fusion you're always going from something you know you're, you're you're always getting a little bit, you're losing a little bit of mass in the reaction and gaining a little bit of energy. So the energy that powers stars is always fusion. It is fusion. You're always taking light elements and jamming their nuclei together. Going through a series of transformations. And making other elements, heavier elements. And then producing, some, some energy in the process. So just to review, we have protons and neutrons, at the center of a nucleus. We have electrons orbiting the nucleus. And in these chemical, I'm sorry, these nuclear reactions that occur, we're also going to produce other things. As I said, the energy is going to be in the form mainly of light. We're also going to produce, some, particles called neutrinos. Which are very light, rather ghostly particles, that, hardly ever interact with other matter but in these nuclear reactions you may produce neutrinos and they will also carry away some of the energy in the terms of motion. You also can produce antimatter. Antimatter is a remarkable it's a remarkable fact that there's antimatter in the universe. And so an electron its antimatter particle is a positron it is something that looks exactly like an electron but it has a positive rather than a negative charge. It is antimatter in the fact that if you bring an electron and the positron together they annihilate each other in a burst of light. Okay, so that's the fundamental idea behind what powers the sun. It is nuclear fusion. And one important thing to notice here, here's this plot of binding energy per nucleon. The idea of how much energy is available to you for jamming things together and what you see is this is a binding, this curve is the curve of binding energy. And it goes with the number of nucleons in the nucleus, and what you see is the curve rises and rises and rises and that's because for, you know, a heavy, or for these nuclei, as you put them together you'll actually get a little bit of energy out, but you'll notice at iron, it turns over. And ir, and what that tells us is that iron is the last element that we can fuse together, that we can, take lighter elements and build into iron and still get energy out. Anything above that, you'll notice the curve goes down, and that's telling you from this point onward, you have to put energy in if you want to make that element. Of course, what you can do because that curve is going down, is you can tickle those elements and get them to break apart. Then you'll get a little bit of energy out via fission, okay. So basically fusion is going on everywhere up until building iron. And then after that, you're not going to be able to really use fusion anymore, to power your stars. So what's going on in the sun? Well, the sun has a very specific chain of reactions that go from taking hydrogen nuclei, which is just a proton. To building a stable helium nuclei, which is two protons and two neutrons. And what you're, what I'm showing you here is this diagram of the p-p Chain. And what it normally it always has to happen at high temperature. And that can be explained very easily because, really if you want to get two protons together, you have to overcome the electromagnetic force. And, because they have the same charge. You know, if you try and take two things that have the same charge and try and put them together, they repel. So, what you have to do, is you have to have them moving fast enough. Meaning the gas is hot enough, for those two charges to come close enough. That you can now get what is called the strong nuclear force which is another of the four fundamental forces to come into play which only operates at very small distances to bind them together. So just to review there we have the electromagnetic force which is to do with electric charges, and we have the strong nuclear force which binds nuclei together inside a nucleus. There's also two other forces we might as well mention. There's the weak nuclear force. Which, as you can guess is, you know, is not as strong as the strong nuclear force. And also governs things that can happen in terms of nuclear reactions. And then finally, there is gravity, which we're all familiar. So just four basic forces. That is all the forces there are in the universe. So, to continue with the p-p chain, what you can see from this is that you take two hydrogen nuclei and you bring them together. And what happens is, is you generate a a neutrino and a photon. And in the process what you get is you change, you can bind these two hydrogen nuclei together but only at a price and that's converting one of the protons into a neutron. So we see in these nuclear reactions you can actually change the character of the nuclei and so this chain is really, you know continually bringing new nuclei, new Hydrogen nuclei together. Eventually you form what is called Helium-3 which are two protons and one neutron, and then you bring those two together and eventually you'll create, a Helium-4 nucleus, which is, again, two nuc-, two protons and two neutrons. And so, it was, once this chain was understood, which was again sort of, you know, took a while, you know, 1930s really before people understood when the, the nuclear reactions, what nuclear reactions were possible for powering the sun. Then we came to understand how the sun could take Hydrogen gas at its core and turn it into energy. Now the one thing we want to remember about this whole process is essentially you're taking Hydrogen and turning it into Helium and releasing energy in the process. And then at the center of the star you're left with Helium, right? You're left with ash. You're burning Hydrogen into Helium and you're going to be leaving ash. You know, quote/unquote ash Helium at the center of the star. We're going to see as we go on the problems that can lead to. One thing to also note is that when people, so there's a certain number of neutrinos you expect to come out of this whole set of reactions. If the, you know the burning at the center of the sun goes on that and for years we didn't see as many neutrinos as we expected. Neutrinos are very hard to observe. We had to build giant containers of you know, certain kinds of liquids to be able to catch the interaction between a neutrino and a, an atom of normal matter. And and it was very difficult, and it wasn't until 2002 that actually, and we'll talk about this later, how the neutrino problem was solved. One last thing is just to emphasize this idea of that the fusion can only happen at the core of a star. You need very high temperatures as we talked about, but also very high density, because you need lots and lots of reactions to be able to create the energy which supports the sun against its own weight. And also, you know, prays to the light that we see today. So you need high densities and high temperatures. And of course that's at the center of the sun because there's all the weight of the material above the sun weighing down on it. Just like when you dive into a swimming pool and you go to the bottom, you feel the weight of the swimming pool above you in terms of the pressure on your ears.
