 Greetings and welcome to the Introduction to Astronomy. In this video we are going to begin talking about the deaths of stars and looking specifically this time at low mass stars. So what happens to stars like our sun at the end of their lives? Well we looked at them previously and they had formed things like a planetary nebula. So we knew that the outer layers were ejected out into space and here we see an example of one of those, but the question is what happens to the core? Well first of all let's just consider cores less than a certain mass and that is a core less than 1.4 times the mass of our sun. Now why such a small amount? Well this actually includes the vast majority of stars in the universe. If their original mass was 10 solar masses or less they end up with a core of about 1.4 solar masses or less. So if we're not limiting it we're actually looking at the vast majority of stars and we'll look at more massive stars later on. What this does is what happens to the material is that the core contracts until it becomes degenerate. What degenerate matter is is that the electrons are pushed as close together as possible. This is what we call the poly exclusion principle and it essentially says that two electrons can't occupy the exact same state at the same time. So it is a quantum mechanical limit and that then produces a pressure to hold the star up. Remember that a star is being pulled down by gravity. Everything is. So everything wants to be pulled down to a point by gravity. There's always something holding us up against that. For earth it's just the structure of earth. The material pushing against each other is strong enough to hold up against gravity. For the sun it was the pressure from the nuclear reactions inside, but a degenerate star like this white dwarf will no longer have that possibility and this electron pressure can then support the core against gravity and it becomes a stable white dwarf. It's an object about the size of earth but still has the mass of a star. The density is incredible which is a million times the density of water, more dense than anything we can imagine here on earth. And this white dwarf is completely stable and will remain stable forever if we have no external influences. So if this is a white dwarf star all by itself it will just remain there and slowly cool off. Now let's go ahead and look at a little bit about why we picked this 1.4 solar mass limit. And that's because there is a limit to how massive electron degeneracy can support an object. What does that mean? Well imagine a rickety old table and you start putting piling heavy books on it. You can put a few books on it and a few more, eventually you're going to come to the point where you put one too many books on it and the table will collapse. That's the same kind of thing that would happen here. There is a limit to how much pressure those degenerate electrons can do. So how much pressure they can put out. And if you exceed that limit it happens to be 1.4 solar masses, also known as the Chandra Shekhar limit after the astronomer who calculated this. And if you go over that limit then it collapses. So these are the ones that become unstable, they will continue to collapse. And here we see that limit. When we look at these, the follow the green line here, it starts to as it goes down here it will just become asymptotic towards this ultra-relativistic limit. It cannot go past that limit. So anything with more mass than that will have to continue to collapse. Note one other thing, because of this pressure, because of more and more pressure, the more mass you put in a white dwarf the smaller it becomes. So the largest diameter white dwarfs are going to be those with the least mass, the least pressure pushing those electrons together. The more mass you get the smaller those white dwarfs will become, so a very massive white dwarf will be far smaller than a much less mass white dwarf. Now how can we detect these? Do we know that white dwarf stars, weird stars like this exist? And yes we do. We can have visual detection. There are some that are visible, including here very nearby in Sirius B. And we see down below there, if we look at the star here, that is actually the companion star. So this small dot is actually the white dwarf star itself, which is a star like Sirius but a little bit more massive and had already gone through its life. And it sits there now, it's very easily visible to be seen. So we can actually see this and many other white dwarfs. We can also detect these in binary systems. How can we detect them in a binary system? Well through their gravity. So material from one star can be pulled into an accretion disk around another star and as it swirls around it can build up on that and we can see the energy as it builds up here and eventually if enough material builds up it will undergo what we call a nova explosion, nuclear fusion on the surface of that white dwarf. So if enough material is transferred you can actually have another way to be able to detect these white dwarfs even if you don't see it directly or if it's too close to the other star for you to be able to visually separate it. Now what happens to a white dwarf over time? Well a white dwarf can't do very much. It has no energy source. So it starts out extremely hot and all it's going to do is continue to cool. It's going to continue to cool off and it will start up here and it will just work its way down this white dwarf sequence down parallel to the main sequence. It will continue to get colder and colder. Now what that means is that its color is going to change. So it's going to go from white as it's very hot now to red to black. Meaning that it's giving off mostly infrared light. So eventually it will become what we call a black dwarf star. Now this will take many billions or even a trillion years. It will just be completely dark giving off primarily infrared light and very little of other kinds of light. However these are very small and that means that they cool slowly. It takes a long time for this to occur. In fact such a long time that no such stars have had time to form in the history of our universe. So even the very first white dwarf to form has not had enough time to cool off to be a black dwarf star. However this will eventually happen to all the white dwarf stars. So come back in trillions of years and the universe could be dominated by these black dwarf stars. And that is the eventual end state for all of the white dwarfs. Unless they happen to be in a close binary system with another star. And we'll look a little bit more about what happens there in a future lecture. Now the other question is can a star really lose this much mass? How do we lose so much mass? How about a 6 solar mass star? I said that we could talk about 10 solar masses. But if a 6 solar mass star is going to become a white dwarf it has to lose 4.6 solar masses, 75% of its mass. How can you do that? How can it possibly lose this much mass? Well to be fair we don't know. But is this possible? And the answer is yes. How do we know this? Because white dwarfs have been detected in very young star clusters. If we can judge the age of a star cluster and we can say that only stars of at least 6 solar masses have had time to evolve to become a white dwarf. Since we detect white dwarfs in these star clusters they must be able to lose sufficient mass. We don't know the complete mechanism. We know that these stars do give off some material that some is expelled out into space. But we can't necessarily explain how they can lose such a tremendous amount. But we do know that it can happen. So let's go ahead and finish up this section with our summary. And what we looked at is that the low mass star aging the outer layers become a planetary nebula while the core contracts to a white dwarf. A white dwarf star a million times denser than water and about the size of our planet Earth. Now and we also know that the white dwarf will cool slowly over time to become a black dwarf. But there has not been enough time in the history of the universe for this to have yet happened to any white dwarf star. So that concludes this lecture on the deaths of low mass stars. 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.