 Greetings and welcome to the Introduction to Astronomy. In this lecture, we are going to talk about stars as they evolve off the main sequence. What happens to them after they have used up their fuel? So, let's first of all remind ourselves what happens on the main sequence. This is, in essence, the boring part of a star's life. Not a lot happens. That's about 90% of the time a 90% of the life is spent on the main sequence. And not much changes for millions, billions, or even a trillion years depending on the mass of the star. What the star does do is converts hydrogen to helium through nuclear fusion. It's relatively stable. We don't see any significant changes, especially not over short periods of time. Remember, this only occurs in the core, so the exterior of the star looks identical. It has not changed. The outer layers look essentially the same during this time. So, their composition does not change. They're still a mixture of hydrogen and helium. But the core is becoming depleted in hydrogen and enriched in helium. So, what will happen is that the core temperature because of this is increasing. The core has to compact down a little bit more to produce enough energy to support the star against gravity. That increases the fusion rate and the luminosity of the star. So, the star will become more luminous as it goes through its life. So, not a lot changes, but it does slowly increase in brightness over that lifetime. Now, how long is that lifetime? Well, it really depends on the star. And in fact, it depends on the initial mass of the star. Larger stars have more mass, but go through it at a faster rate. So, a star 40 times the mass of the sun may only live a million years on the main sequence. Even though it has 40 times the amount of fuel, it burns through it much, much faster. Our sun is around the 10 billion range. This is a little bit more massive than our sun. But it would live around 10 billion years. And a very low mass star here at less than half the mass of the sun could live 200 billion years. Now, since our universe is about 14 billion years old, that means that any of these stars that ever formed are still around. So, the large stars go through it at a faster rate using up the fuel quickly. Small stars have less fuel, but go through it more slowly. So, this fuel may last a trillion years for those very slow mass stars. But eventually, regardless of which star we look at, it will use up that hydrogen in its core. At this point, the star must change. It cannot remain the same. It no longer has an energy source in its core. So, the core contracts and heats up. And as it heats up, the area around it becomes able to burn hydrogen. We have a hydrogen burning shell around a helium core which is inert at this point. So, what happens as it does that, the outer layers begin to expand. And something like our sun will expand to a red giant star, becoming much larger. So, the core compacts down, much smaller at the center. The outer layers expand outward, becoming larger and larger. How big do they become? Well, let's look at some examples here between the sun and the red supergiant beetle juice. Beetle juice is only about 16 times more massive. However, in terms of size, it's nearly a thousand times larger than the sun. Temperature is a little bit lower. Core temperature many times higher. So, 10 times higher in the core, 160 million Kelvin. It's luminosity 46,000 times larger than our sun's, but its density is next to nothing. All the material is compacted to the core. And its age is only about 10 million years, as compared to 4.5 billion years for our sun. So, there are some big differences between these two that we see. Even though their masses are only a factor of 16, that little bit of difference in mass makes a big difference in all of these other properties. So, how is the star going to now begin to visibly change? Now we want to look at the visible effects on the star. And what do we see happening? Well, the star is going to get larger and cooler. What does that mean? It means the luminosity increases. When the star increases its size, it will become brighter. It has more surface area giving off light. The temperature will decrease, of course, if it's getting cooler. Well, temperature decreasing moves it to the right of the HR diagram. Getting larger moves it to the upper right. So, it's going to move up and over. And we see the tracks here that various stars will undergo changing on the HR diagram. So, for a star like our sun, it will move up and reach up into the red giant phase here. Now, we see, again, how long this will take. We can see the amount of time that it takes for this to occur. So, it might take 10 billion years for a star to move. One section getting up there only takes a small fraction of it. It only goes from 10 billion to another billion years. So, it takes a very small amount of time relative to the amount of time that it's spent on the main sequence slowly evolving off. So, it's a very slow change that's occurring. Now, if we look for the 15 solar mass star, please note the numbers here. This is 10 million years as opposed to 10 billion years. So, a thousand times faster that it goes through its life here becoming a red super giant star. So, the star is going to change and move off the main sequence becoming a red giant or a red super giant depending on its mass. How long this takes, again, depends on the initial mass of the star on the main sequence. Some stars, very low mass stars, have never, none of these have ever evolved in the history of the universe. Whereas other stars don't last a very long time. So, let's go ahead and finish this section with our summary. And what we looked at is that stars are going to spend the vast majority of their lives fusing hydrogen and helium on the main sequence. When the hydrogen is used up, the core contracts and the outer layers expand, it becomes a red giant star many times larger than it was before. And we will continue to look at what the changes are and what goes on within the star coming up in further lectures. So, that concludes this lecture on evolving off the main sequence. 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.