 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to begin talking about the evolution of stars and specifically what stars do as they evolve off the main sequence. So as they begin to move and use up the hydrogen in their core and then begin to change. So for the longest time a star will remain exactly the same but we will find that as it goes through its life significant changes will occur in at least astronomically speaking relatively short periods of time. So let's go ahead and start looking here at what we have and first of all let's look at the life of a star on the main sequence. Now there isn't a whole lot to say here because this is as we say the very boring part of a star's life. It's about 90% of the life and it does not change. Now depending on the mass of the star you might not have changed for millions, billions, or even a trillion years because the star is simply converting hydrogen into helium in its core. So hydrogen to helium and as long as it has that supply of hydrogen it is able to continue this process. This is only going on in the core so the outer layers are unchanged. We do not have a change in the outer portions of the star. So what is happening in the core is that the core is becoming depleted in hydrogen so less hydrogen and more helium but this does not change what we see in the star itself. It will change the core and that the core temperature will slowly increase over this time. Now for something like the Sun this would be 10 billion years. So over those 10 billion years we would see the core temperature of the Sun slowly increasing which will increase the rate of fusion and slowly the luminosity but it is a very slow change not something that occurs rapidly at all. So what would happen then as we continue to look through this would be let's look at some of the lifetimes of the stars. How long does a star live? Well it depends on one thing and it really depends on the initial mass of the star how much material it was formed with. So a larger star has more mass more fuel but it goes through it at a much faster rate. That means that it will use up the fuel in its core in maybe only a few million years. Smaller stars have less fuel so they have less mass and therefore less fuel but they go through it far more slowly and their fuel may last a trillion years. So the table here shows us various types of stars going from a very hot class O star down to a very cool class M star. We can review their temperatures we can see roughly what their masses are and you note that it's not a tremendous difference in mass that it goes from 0.4 down here for an M zero star to 40 for an O five star so it is a factor of a hundred times in the mass. However the change in the lifetime is much much more that O star may only live for one million years and the M star may live for two hundred billion years. Now if we think about that two hundred billion years our universe is only about fourteen billion years old. Meaning that any star of this type that ever formed in the universe is still present. There has not been enough time for even one of these stars to have gone through their life in the entire history of the universe. So every M zero star that ever formed is still around is still around. No O five stars are around that formed more than a million years ago so those that formed even two million years ago are gone so it really depends on the mass of the star. Now let's take a look here at what it happens as we begin to leave the main sequence because eventually whether you have a big gas tank or a little big gas tank the hydrogen is going to be consumed whether that be in a million or in a billion or in a trillion years once it runs out of fuel the star must change it does not have any choice because it is going to begin to change its core. So as a star on the main sequence has a hydrogen burning in its core here eventually it starts to build up helium down in the core and has hydrogen burning in a shell around it so the core, the central part of the core becomes solid helium and that will then begin to contract and heat up. So hydrogen will then fuse in a shell around the core and will actually cause this to expand so we're getting out of that equilibrium that we had four stars that as the star went we were able to keep a balance between pressure and gravity now the pressure in the core here the core is collapsing down slowly contracting and the outer layers will now begin to expand so a star like the sun will eventually become something like this a red giant star and the outer layers of that star will expand creating a red giant. So this is the again the first step and it's going to happen to every star because eventually they will use up that hydrogen in their core and there is no choice except for that core to collapse and that will cause the outer layers then to expand. So what are we going to see in the star if we look at the star we would watch the star have two things it would get larger and it would get cooler so as it gets larger even though it's getting cooler it's going to actually get brighter the luminosity is going to increase and the temperature is going to decrease that means that it's going to change its position on the HR diagram because the HR diagram is as we know a plot of surface temperature of the star versus the luminosity. So if the luminosity increases the star is going to move up if the temperature decreases the star will move to the right and that means that overall the star is going to move from the main sequence towards the upper right portion now the exact tracks are shown on the diagram here it's kind of a little more complicated process than that but it is moving off and is going to become a red giant star so this is something that will happen to the Sun in about five billion years when it begins to exhaust its fuel it will follow the track here shown for about one solar mass and it will slowly over the next five billion years increase its luminosity a little bit and then the rest of the phases will happen relatively quickly as it goes on so this will take a lot of the time then it will start to move relatively quickly up into the red giant region eventually engulfing much of the inner solar system including the Earth. So let's continue as we like to conclude with our summary and what we find is that stars do spend the vast majority of their lives fusing hydrogen into helium on the main sequence so this is what stars do like the Sun for the majority of their lives eventually the hydrogen will be used up the core will begin to contract it will get smaller and smaller compressing together and the outer layers will expand making the star a red giant many times larger than it was before and as I said eventually engulfing the entire inner solar system including the Earth so that concludes our lecture on leaving the main sequence and we will 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