 Greetings and welcome to the Introduction to Astronomy. In this lecture, we are going to talk about the misconception that the Sun is burning. So we sometimes look at the Sun as a great ball of fire in the sky, and that's not really quite what it is. And it is certainly not burning in the sense that we normally think of fire here on Earth. So our quick answer to this, what is the Sun burning? No. In order to burn, in order to undergo combustion, you need a concentration of oxygen. And yes, the Sun has a lot of oxygen, but it's so diffuse and spread out, and in fact we find out that most of the Sun, in fact more than 99% of the atoms in the Sun are hydrogen and helium. And these are not going to burn as the wood burns here in the fire without oxygen. So nothing would burn in space. You could not start a fire on something out in space. You could not start a fire on a planet that has no oxygen. You would not be able to start a fire, for example, on Mars or on Venus. There is no oxygen there to combust. So there would be no fire elsewhere than someplace that has a concentration of oxygen. Now, so what is the Sun? Well, the Sun is really a large mass of plasma. It's a bunch of ionized gases at a very high temperature. Energy in the Sun is only produced in the core of the Sun. We can't see that here. So the core would be deep down inside the Sun here, and we cannot see that. But what's going on in the core is that there would be nuclear reactions going on. Now we sometimes call this hydrogen burning, but it is not burning in the same sense we think of here on Earth. We can take a quick look at what those reactions are. And in fact what it does is it takes hydrogen atoms and fuses them through a series of steps here. We don't know to go into all the details, but essentially it takes several hydrogen atoms and ends up with one helium atom. Now that produces energy because there is a mass difference. Four hydrogen atoms weigh a little tiny bit more, a fraction of a percent more than one helium atom. And in the reactions that is converted to energy. By Einstein's equation, E equals MC squared, so that little bit of mass becomes a reasonable amount of energy. So why is the Sun glowing? Well, again, it glows not because it is burning, but because it is at very high temperatures and it is that big ball of gases. So everything you see here is not fire. There are not flames sticking up over on the side here. Those, these are not flames. These are actually jets of plasma, or material that has been lifted off the surface, and that's what you see around the edge of the Sun. They may bear a resemblance to flames, but there is no oxygen there and they are not burning. The Sun glows because of all that energy being produced at the core. As it diffuses outward, it then heats up the rest of the Sun, which is at very high temperatures. So the outer layers of the Sun that we see are this plasma at about 6,000 degrees. However, in no sense is it actually burning as we think of objects here on Earth that burn. So let's go ahead and finish up with our summary, and first of all, what did we find out? Well, the Sun is not burning. Burning requires combustion of oxygen, and that is a tiny percentage of solar material. Yes, there is oxygen in the Sun, but not in any concentration like the 20-some percent it is within our atmosphere. It's a tiny fraction of a percent of the solar mass. The Sun is really a large mass of plasma, ionized gases, that glow because of its high temperature, thousands of degrees, and when you heat something up to very high temperatures, it will give off light. This is how an electric stove works. When you see the burners starting to glow, they are getting very hot and getting to the point where some of the light they give off is visible light. So the Sun is heated up as well by a different mechanism, but it is hot. So it is hot, and therefore it gives off a temperature, gives off energy based on that temperature, and that is what we see as the light being emitted. It's only because it is so hot that we see it glowing. So that concludes this discussion on the misconception that the Sun is burning. We'll be back again next time for another misconception in astronomy. So until then, have a great day, everyone, and I will see you in class.