 Greetings and welcome to the Introduction to Astronomy. In this video we are going to talk about the later stages of stellar evolution in a sun like the star. So in a sense we're getting a preview of what our sun will happen to our sun over the next 5-6 billion years. We know that not a lot will happen in the near future, that it will just remain burning hydrogen into helium as it has done for a long time now. However, eventually it will run out of that fuel. So what we want to look at, and we know that it will become a red giant stage, and what we want to look at is what is going to happen beyond that stage, beyond when the core helium starts to build up to very immense temperatures. So let's take a look at some of the things that will happen here. First of all, let's review a little bit. A star that exhausted its core hydrogen, its core was contracting, getting smaller and smaller. And this is a very slow contraction, but it does increase the temperature. So the temperatures are going up, and eventually the helium will begin to fuse in the core. It takes temperatures of 100 million degrees to fuse helium. And helium is fused through what is called the triple-alpha process. In this case, three helium nuclei will fuse together to form one carbon nucleus. And the reason it's called the triple-alpha process is that scientists call the helium nucleus the alpha particle. So that's where that gets its name. Now the question could be, why can't we just fuse two helium nuclei together? That would require lower temperatures. And what happens when you fuse two helium nuclei together is the product is completely unstable and breaks apart almost instantly. And therefore, there is no time for it to form it or to become anything else. So it will form, but it will not last long enough. And we're talking tiny, tiny fractions of a second. And it will immediately break apart. And therefore, we have to wait until we can actually get high enough temperatures that three helium nuclei are able to do this. Now, how this begins depends on the mass of the star. So a very low mass star, like our sun, undergoes what we call a helium flash. And that means that it has built up a very high density of helium. And the helium is extremely compressed together when it reaches the temperature of 100 million degrees. So once it reaches this temperature, everything is so dense that it just begins to ignite immediately all together. And it's a very rapid process. In a high mass star, on the other hand, the helium fusion will start gradually. It will reach a high temperature before very high densities have been reached. And this gives us a way to reach a new stability in terms of how the star can now be stable again. So what we mean, we had a stability when it was fusing hydrogen into helium. But after this flash occurs, the star will actually contract and get smaller again, and it will heat up. So what happens is that it will change its position on the HR diagram and will go from where it was as it tracked up here, up the red giant branch. And it tracks up here, this is the point where the helium flash will occur. And then very quickly, the star then jumps down to this area. So it will come down very quickly to this region. So that's what we mean when a helium flash will occur. It will become now, we have seen that the temperature has increased. So it's moved to the left, and the luminosity has decreased so that the star has moved down on the HR diagram. And it will eventually settle in to what we call the horizontal branch of the HR diagram. And that is where it is burning helium into carbon in its core. And hydrogen into helium in a shell around that. So we're going to start building up layers within the star. But this is a new type of stability in that the now that we are now stable. That we have a balance between the pressure pushing outward and gravity pushing inward. So we have completely balanced that. At this point, we would have the helium core that is there and helium fusion going on. And that would be forming a new set of layers within the star. So we would have the core helium fusing into carbon. So carbon would be forming down at the core here. But then we would have helium fusion going on and hydrogen fusion going on. And then we would have the outer layers of the star which have had their composition essentially unchanged. But now gravity trying to push the star downward is balanced by pressure from this new source of energy. And that's what we mean by a new stability. We have actually reached a stable portion of the star's life, much as it was on the main sequence. However, just as with the main sequence this couldn't last forever. And in fact, it will last a much shorter period of time. Because the star will continue to go through its energy faster and faster. So the star is now building up a core of carbon. And that core will continue to contract and heat up. But stars less than two times the mass of our sun will never get hot enough to fuse the carbon. So this is the end of the stage for our sun. It is going to build up a core of carbon. And that core will continue to contract and get hotter and hotter, but never get hot enough for the outer layers to, I'm sorry, for the core to begin burning carbon. It will again become a giant or a super giant. So when we look at it on the HR diagram here, we started off here in the horizontal branch region. And then we start to move up again as it exhausts its helium and builds up carbon in its core. It gets up larger and larger and larger and will become up in this upper right-hand corner where we find the largest stars on the HR diagram. It will become a red giant again and then a red super giant star. And it follows what we call the asymptotic giant branch tracing its way up. The main sequence up to the upper right corner becoming very large and becoming very bright, but also very cool in terms of its temperature. So looking overall at the evolution of a star much like our own sun, what we see is that here's kind of the range of what would happen to it. And we remember that 90% of a star's life, 10 billion years, is going to be sent bent on the main sequence. The remaining 10% of the life will be spent as a red giant star undergoing helium fusion. All of the other ranges will take a much shorter period of time. So here we see how long does it spend in this stage. Well, there is almost all of the range. Once it becomes that giant star again, it's a very short period of time. It's only 20 million years. And while that may seem like a long time to us, in terms of having already spent 11.5 billion years going through its life, this is a very short 20 million years is just the blink of an eye for that. But this is when it becomes the most luminous, becoming 5,000 times the sun's luminosity and nearly 200 times its diameter. So the sun will become completely engulfing the entire inner solar system at that point. Now as it becomes larger and larger, it becomes unstable. So it will eventually have become so large that the outer layers will be pushed out. And what we see is what we call a planetary nebula. So a planetary nebula is when the stars become very large, we get a much lower escape velocity and the outer layers are not held very strong. So the outer layers then become expelled out into space because of instabilities and push those outer layers here. So at the center, we have what we call a white dwarf star. That is the dense core of the star left behind. And within that, we also see the outside of that, we see the outer layers. So this nebula around the outside was originally the outer layers of the star. So it has been expelled out into space and will continue to expand outward, eventually becoming seed material for other stars. Now, not all planetary nebulae look like this. There actually are some significant variations in them. And we see that planetary nebulae can vary and we get a number of different. Here are just four different planetary nebulae that we see here. And why the question can be why is there such a variation in these? Well, we have to remember that many stars are present in binary systems. Could the presence of another star and two stars orbiting around each other, as they occur, as they orbit around each other, could that eventually cause differences in how the planetary nebula looks? We also know that the nebulae change over time. So there could be a number of other impacts that could affect this. Could there be other material around it and the different numbers and again, the different numbers of stars that might be present in that system with the star that formed the planetary nebulae? We also know that this is an extremely short stage of stellar evolution, only 50,000 years. Again, remember we were talking with the sun between its various stages, maybe 11 and a half billion years. So very short stage, just tens of thousands of years. So the ones that we see, like the images here, are ones that we've just happened to catch at that stage. Had we looked at them 100,000 years ago or 100,000 years from now, we would have seen something quite different 100,000 years ago. They wouldn't even have been a planetary nebula. They still would have been a red super-giant star. And at 100,000 years from now, they will have spread out so much that we would no longer be able to see them. And the central white dwarf would have cooled off so much that it would no longer be able to cause those stars to glow. And as we've looked at previously, this is one way enriched material can be expelled back out into space to form seed material for other stars. So let's finish up as we do with our summary here. And what we've looked at this time, in the later stages of evolution for a star like the sun, we see that helium will be fused into carbon. And that will be the last stage for a sun-like star. Carbon will be the end state that core will be essentially made of carbon and will eventually form what we call a white dwarf. And that will be it. So our sun will eventually become a big ball of carbon after it reaches the planetary nebula phase. It will build up layers in its interior. Those layers can be inert, like the carbon core that never will fuse. It can be layers around that, like the helium burning layer and the hydrogen burning layer, as well. And we'll see in even more massive stars that we can build up more and more layers. And then eventually the outer layers will be expelled out into space as a planetary nebula, a very short-lived stage of stellar evolution, and then leaving behind after that just the white dwarf star sitting there to cool off for the rest of its life. So that concludes our lecture on the later stages of stellar evolution. 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.