 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to continue talking about stellar evolution specifically looking at the later stages and what happens at the end of the life of a star like our own sun. So we had had stars looking at stars in the red giant phase, so at this point we look at stars that it's exhausted, it's hydrogen in the core, and its core is contracting. That means that the temperatures will increase and they will get hotter and hotter. For the sun it started at 15 million Kelvin and eventually once it gets to 100 million Kelvin the helium will begin to fuse and they fuse by what we call the triple alpha process. It's called triple alpha because the alpha particle is the helium nucleus. So three of them will fuse together and this makes one carbon nucleus and releases energy much as fusing hydrogen into helium does. But you might question why not just two helium nuclei? Well if two helium nuclei fuse together they form an unstable element that immediately breaks apart. So you need essentially three to hit simultaneously in order for it to fuse together to make a stable carbon nucleus. And that's why we need such incredibly high temperatures that these are moving fast enough and there's a high enough density that they all occur at one time. Now how this happens depends on the mass of the star. For a high mass star the helium fusion just begins gradually and starts much as the hydrogen burning did. The low mass star is a little bit different it takes it a while to build up enough helium and enough pressure and enough temperature for it to undergo helium fusion. And in the meantime the material has compressed down that it has become so dense that it becomes what is called degenerate matter. Now normally when you increase the temperature then the material will expand so the pressure and temperature are related in a gas. However when you get into degenerate matter that does not occur so even though it starts burning it does not initially expand the core and the helium goes very quickly in a rapid helium flash burst burning up a lot of the core at once. Now that will slowly expand but it's a lot faster than it would be in a high mass star. The high mass star has more mass, more gravitational pressure hits that higher temperature before the material becomes degenerate. Now what this will do is give us a new stability. So after the helium flash for a star like the Sun the star contracts and warms up. It becomes hotter and fainter. So as it left the main sequence and went up this is where it's going and building up helium in the core and all of a sudden at the peak here that's where the helium flash occurs and the star jumps down really quick to the lower level here and that is then going to be where it is a new stability. What is it doing? It's burning helium into carbon in the core so it's down into the left on the HR diagram and hydrogen into helium in a shell around that. So now it has a new stability and it actually will sit here for a good amount of time hundreds of millions or a billion years or more depending on the exact type of star. So we start building up these layers. A star ends up having layers inside such as shown here and what we'll have is there is a carbon oxygen core that is forming so that's where the helium burned into carbon and you have the carbon oxygen ash at the center. Helium fusion in the green here and then out beyond in the yellow you have a region of helium so this is just a portion of helium that is not hot enough yet to burn and it is being fueled by this thin hydrogen shell so you have another shell of material burning hydrogen and then the outer area would be the hydrogen envelope in the red. Now that is the largest portion of this so this is not to scale. That red would be the vast majority and what's going on here in the core is the very small central section that we see. Now what is continuing to happen here? Well, we're building up that carbon core so for stars less than two times the mass of the sun that we will not get hot enough to fuse carbon. They're done. They cannot have any other energy sources. So for stars like our sun or anything less than at least two times the sun mass, sun's mass no carbon fusion and they're done. They have used up all of their energy sources and what happens is that the core continues to contract and heat up but never gets hot enough to burn carbon. It becomes a red giant and then a super giant and it follows what is called the asymptotic giant branch so essentially it went up here one time and underwent the helium flash then it kind of jumped back down and then it goes up again. Asymptotic it's following the same curve as it goes up into the giant and then the super giant region and at that point there's not much else that can happen to it. There's no more energy sources. Energy sources are all gone. So let's look at this. Let's summarize what we see with the sun-like star. So the main sequence was about 10 billion years, 90% of the star's life. The helium fusion time as a red giant is about 10% of the star's life. For the sun that's about one and a half billion years. The second red giant phase only lasts millions of years. So here we see these. How long does it spend there? 11 billion years or so. 1.3 billion years as a red giant. Then 100 million years for helium fusion and then about 20 million years for the giant again. Temperature decreases, then increases, and then decreases again. Luminosity, again same, decreases, increases first, then it decreases, and then it increases to an even larger amount. And the same thing with the size. So it's kind of jumping back and forth in terms of size getting bigger and smaller in terms of its size. Now, as it gets bigger, those outer layers become unstable. The star is extremely large. So the escape velocity is lower and the outer layers are not strongly held by gravity. Eventually, instabilities and pulsations can push off those outer layers and the outer layers are expelled out into space, giving us a planetary nebula such as the ring nebula shown here. So the outer layers are here. This is the outer regions. This is the star. This is what was the outer layers of the star being expelled out into space. The core is still here as well. And if you look right at the center, there is the white dwarf star at the center that is forming. That is the left behind extremely hot core of what once was a star, perhaps much like our own sun. Now, we see one example of a planetary nebula. Here, there's actually a wide variety of them. So they're not all looking exactly the same. Here, we see some that look very different. And why is there such a variation in these nebulae? Well, possibly binary systems account for some of that. If the stars are moving, that might have some differences in how the material is pushed out. Nebulae change over time. Perhaps there are various different stages. For example, we see some here where maybe there's one stage of material being expelled and maybe there was a previous stage. So maybe the entire outer layers are not always expelled at once. We may see things from different angles. Remember, we only get one point of view on any object in space. That's all we can see. Now, we also know this is a very short stage lasting maybe 50,000 years. After that, the material will have expanded out enough and the white dwarf star would have cooled off enough that the outer layers become invisible. And that material, that enriched material, is now expelled back into space to become the seed material for future stars. So let's go ahead and finish up with our summary. And what we looked at here is the later stages of a sun-like star helium is fused to carbon. So on the main sequence, it's hydrogen into helium. Here it is helium into carbon and it builds up different layers of materials in its interior. Eventually the outer layers are expelled out into space giving us a planetary nebula. So that concludes this lecture on 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.