 Greetings and welcome to the Introduction to Astronomy. In this video, we are going to talk about the Sun and very specifically energy generation within the Sun, so how does the Sun produce the massive amounts of energy that it is able to put out into space? So let's take a look and start off with some of the early ideas, and even just a hundred years ago, this was something hard to understand. How was the Sun putting out all of this energy? We knew that it could not be burning in the traditional sense that we have here on Earth. When we talk about chemical reactions or chemical burning as we burn wood or burn coal, even if the Sun was a gigantic lump of coal, that would only last a few thousand years, and already at that point we knew that the Earth was much older than that. We also know that there is what we call the conservation of energy, and that energy cannot be created or destroyed, but it can change its form. So we can convert energy from one form to another, and that can be a way to convert, for example, gravitational energy into heat energy. Now various ways of doing that, we do it through burning things. Burning is one way of changing forms of energy from a chemical potential energy into a heat energy, for example, when we are burning coal. But there are other forms of energy that we can use as well, and let's take a look at some of those other ideas that we had here. And one was the idea of gravitational contraction. And this was looked at in the mid-1800s by two scientists, Kelvin and Helmholtz, and you may recognize Kelvin from our temperature scale that we use. And they suggested that the Sun could convert gravitational energy into heat. And this is what we use right now. This is our understanding of why Saturn has excess heat. And essentially what we mean is as something tracks down and gets smaller, it has potential energy because of its size, because material is out here around the edges, and as we bring that material in closer to the center, it contracts and it converts potential energy, energy of position, into kinetic energy, energy of motion as it moves down, and then through friction that would convert energy into heat, and that would give us heat, explaining how we could get heat from the Sun. Now it wouldn't have to be very large, it's only needs to shrink by about 40 meters per year to account for the amount of energy that we see, and at that rate the change in size of the Sun would be unnoticeable. We would not notice the Sun changing by a mere 40 meters every year, that would be an incredibly small change compared to the entire size of the Sun. And this would be sufficient to power the Sun for 100 million years just by this gravitational contraction. So that was great, at the time it was larger than the estimated age of the Earth. We did not know how old the Earth was at the time. Now we know that the Earth is four and a half billion years old, and this is not sufficient. So while gravitational contraction did likely play a part in the early history of the Sun when the Sun was forming, and in the planets when they collapsed, this is a method of energy, it is not how the Sun produces its energy today. In order to get our understanding of how it produces energy today, we can look at what Albert Einstein gave us in 1905. And that was, he found that matter and energy are just different forms of the same thing, and we have his famous equation E equals mc squared. And what that tells us is that we can convert matter to energy and we can convert energy into matter, that they can convert from one form to another, and they are related by just this constant called the speed of light. And the speed of light is a very big number. So what that means is if we take a very small amount of mass, we multiply that small number by a very big number squared, that gives us a very big change in the amount of energy. So what it tells us is that a very small amount of mass can be converted into a very large amount of energy. So if we take an example here of just a paper clip, if we could take a paper clip, convert it completely to energy, that would be the equivalent of 15,000 barrels of oil. Of course, we can't convert things that efficiently. We have no way to convert a paper clip completely to energy. But just to give you the concept of how much energy is contained just within the mass of an object. So to try to understand this, let's take a look at what some of the elementary particles are. Atoms, we know, are made up of three things. They are made up of protons, neutrons, and electrons. This is what we see on the left side of the diagram here. We have negatively charged electrons here. We have positively charged protons and neutral neutrons. But each of these has an antiparticle that we see on the left. So this is normal matter here on the left-hand side. And on the right, we see antimatter, which includes anti-electrons, anti-protons, and anti-neutrons. Anti-electrons are also known as positrons. They are exactly like an electron, the only difference being the electrical charge. They have the same mass. Other characteristics are the same. But when matter and antimatter meet, we get the complete conversion of matter into energy. So an antiproton meeting a proton, they would annihilate each other. The mass would be completely gone. And based on Einstein's equation, that would be completely converted into energy. We could take the mass of the proton and the mass of the antiproton, add them together, multiply that by the speed of light squared, and that would give us the amount of energy that is created when two of those collide together. Now, one other elementary particle we want to be able to understand is what we call the neutrino. These were proposed to exist without ever having been detected because it looked like some nuclear reactions appeared to lose energy. And if we recall, a conservation of energy says we cannot. We cannot gain or lose energy, although we can change forms. So having another particle there, it would be an interesting particle. It was very massless or very nearly massless. And it was a neutral particle. So it didn't interact through electromagnetic interactions. And it didn't even interact at all with regular matter. It was very rarely interacting with any other material, any other material in space. So neutrinos were proposed for this reason, and we'll see that they become very important in our understanding of the sun. So let's look a little bit about what's happening here in different types of nuclear reactions before we look specifically at what our sun is doing. We have two types. There is nuclear fission and nuclear fusion. Nuclear fission is what we use in nuclear reactors here. So nuclear power plants on earth use nuclear fission. They take large atoms such as uranium or plutonium and split them apart into smaller atoms. That releases energy and gives off energy and then can be used to generate heat energy which can be converted into electrical energy that we use. However, this is not useful for stars because stars are not made up of uranium and plutonium. They are made of lighter elements, hydrogen and helium. So remember that most all of the composition of the sun and the stars is all hydrogen and helium. So this is what we use on earth and the stars use nuclear fusion which fuse the light atoms and particular hydrogen into heavier atoms. Problem is how do we overcome the repulsive force of two positive nuclei? So if we have a hydrogen nucleus here with a positive charge and we have another hydrogen nucleus with a positive charge, if they're moving towards each other, then they will not keep moving towards each other. They're gonna be pushed away by their electromagnetic force. So how are they going to combine together? Because their electromagnetic forces will get stronger and stronger as they get closer together and keep them from getting too close. However, remember what temperature means and temperature means very fast speeds. If we have temperatures of about 10 to 15 million degrees, then they are moving fast enough that they can get close enough together that they can actually bind together before the electromagnetic force has a chance to push them apart. So in other words, we need two things to do this. Not only do we need the very high temperatures that exist at the center of the sun, but we also need very high densities. We need lots and lots of particles close together in order to be able to get these nuclear reactions to occur. So let's look at how they work inside the sun. And what we see is that what we use is what is called the proton-proton chain is how we believe the sun produces its energy. In that result, it is to fuse four hydrogen nuclei into one helium nucleus. However, this can't occur in one step. That would require four positive charges all meeting together at the same time. So you would have to get all of these in the same place at the same time to try to fuse them together. And that's just not going to happen even at the temperatures and densities that we have in the sun. So what we need to do is to do this in various steps. And the first step we want to look at is to fuse two hydrogen nuclei together. So instead of getting four together at once, we just get two of them. And when those two hydrogen nuclei combine, they combine together and they form another version of hydrogen that we call deuterium. It is deuterium is hydrogen because it has one proton, but it also has a neutron. So it is a heavier version of hydrogen. And it also, because the charge has changed, we had a charge of plus 2 here and plus 1 here. We can't lose electrical charge, like mass and energy. Charge has to be conserved. So we also get this thing here, which is called a positron or a piece of antimatter that forms inside the sun. So we have two positive charges coming out, two positive charges going in. And in order to get the energy balance, this is the neutrino that forms as well. So this neutrino has to come out as well just to keep everything balanced. So the first step takes two hydrogen nuclei. We get deuterium out of it. We also get a positron, which will, of course, immediately annihilate with an electron producing energy and a neutrino. Now in the next step of this, what we get is the deuterium nucleus here now forms, merges with another hydrogen nucleus, to form helium. But this is not the ordinary helium that we see. This is helium 3. So it's a lighter version of helium with two protons and one neutron. Normal helium would have two protons and two neutrons. So we're getting to helium, but it's not the final product yet. This also gives off energy in the form of a gamma ray. So it gives us a little bit more energy as well. And then finally, in the last step, we take two of these helium nuclei, combine them together. That forms the stable helium 4 nucleus, which is ordinary helium, and two protons. And these two protons go back out at high speeds and begin the process all over again. So it continues. So what we've taken in net is we've taken four hydrogen nuclei, produced one helium nucleus down here, and produced some energy. So we've taken that four protons, converted them into helium, and gotten some energy out of it. Energy in terms of the positrons that annihilate and create energy, and in terms of the gamma rays that form in some of these reactions. So how does this give us energy? Why does this give us energy when this occurs? And what we see is the difference in the mass of the helium nucleus and the mass of the hydrogen nuclei that formed it is about little less than 1%. So we take that 1% of mass that is then converted to energy by E equals MC squared. So if we took 1 kilogram of hydrogen, would produce this large amount of energy, a large number of joules of energy, and enough power, 1 kilogram of hydrogen would be able to provide the electric supply, electric energy of the US, for two weeks. Just simply 1 kilogram of hydrogen converted to helium would be able to provide that much energy. Now there is another cycle that's used for more massive stars. We call the CNO, or the Carbon Nitrogen Oxygen Cycle. It is not significant in things like the Sun, but it does provide energy for more massive stars and kind of uses carbon as an intermediary to build the helium nucleus. Essentially, you start off with a carbon nucleus and add protons to it and build that up and into nitrogen and then into oxygen. And then when you get to a certain point, it splits off. The helium nucleus splits off. And you have the same net process you have with the proton-proton chain. You're converting for hydrogen nuclei into one helium nucleus. And the carbon comes back to start the process again. So the carbon is just used as the catalyst in it. It's rebuilt upon the carbon. But the carbon in the long run just remains right where it was. So let's finish up with our summary. And what we've gone over this time is certainly we talked about the puzzle of energy generation in the Sun. And it was solved by Einstein telling us that there is a relationship between matter and energy and that while we're not creating or destroying energy, mass is just another form of energy. And we can convert mass to energy, giving us large amounts of energy. The Sun produces energy by using the proton-proton chain, which is four hydrogen atoms in net becoming one helium atom. The mass difference between the hydrogen atoms and the helium atom is converted fully to energy by Einstein's equation. And that going on consistently is sufficient to power the Sun for 10 billion years. So that concludes our lecture on solar energy generation. We'll be back again next time for another topic in astronomy. So until then, have a great day, everyone, and I will see you in class.