 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to look at ways we test the stellar evolution models and how we can compare them to what actually goes on in the universe. So we will see what our laboratories are for testing stellar evolution. So first of all, how can we understand the life of a star? How can we discuss it? The time scales are much too long, even for the shortest-lived stars we're talking millions of years, for stars like the Sun many billions of years. So we can never watch a star go through all of the stages of its life. We can look at this similarly. How can we understand how humans change over their lifetimes? Well we could watch humans as they change over their lifetimes and do this. But how would you do it if you were doing a semester project to study human aging? You could not follow one person to see them through all of the stages of their lives. However, you could study various groups of people, from infants to children, adolescents, young adults, adults, middle-age, elderly, etc. You could look at all those different stages, put this together, and you could still understand how humans change over the course of their lives even though you're not studying any one individual person. We can do the same thing by studying stars at various stages of their lives. How do we do this? Well we use star clusters. Star clusters are our laboratories to study stellar evolution. How does this work? Well, why are they good laboratories? Because stars in a cluster formed at the same time from the same material. We're eliminating a lot of the variables, making it, in a way, kind of a controlled experiment. That's what you want to do. You want to leave one variable that is present and fix all the others so everything else is the same. It's not perfect because the material isn't identical every place from which they formed, and they don't all form at exactly the same time. It takes a little bit of time. But overall, really the major difference between these stars is their mass. So we can study the effects of mass on the evolution of stars. Because different mass stars will be at different stages of evolution inside that cluster. So first of all, let's look at a couple different types of clusters that we see. We will see the globular clusters which are located in the outer spherical region of our galaxy we call the halo. These consist of 100,000 stars or so and can be billions of years old. So they've got a lot of stars, they're very, very old. On the other hand, an open cluster, these are located in the disc of the galaxy, and they consist of maybe a thousand stars or so, and are typically less than a few hundred million years old. Globular clusters are bound together gravitationally and remain together their entire lives. Evolution clusters are not gravitationally bound together and will eventually spread out and disperse throughout the galaxy. One other type of cluster that we can see is what we call an oboe association. These are groups of very hot young stars that form together in a very small region of space. They had to form recently because they don't live very long. They will also contain many small lower mass stars which are still in the process of formation. Here we see the central region of the trapezium star cluster in Orion, which would be an example of this type of formation. They're generally associated with gas and dust left over from that star formation, which is in this case still ongoing. How can we use these star clusters to then study the evolution of stars? Well, we compare models to actual star clusters. Here we have a model of the star cluster, and that would show a three million year old star cluster, and here we have the actual observations from a star cluster. This is from NGC 2264. Now we see a lot of those stars are still approaching the main sequence in this case. So some stars have not yet reached the main sequence. Those lower mass stars take a longer time to form, but most of the upper mass stars are still on the main sequence, and we see that in both cases here. That there are no red giant stars visible because they have not yet formed. All stars are still on the main sequence at this point. Now if we zip forward a hundred million years, things have changed a little bit. Now we look at another cluster and we see that those stars have now reached the main sequence in the lower masses, but we see that some of these stars are just turning off, and we have a small population of red giant stars. Now some stars of course have completely gone through their lives, but at a hundred million years we're starting to see some structures. So really what we're seeing here is the pattern that they will follow. So these stars are of slightly different mass and took a little bit more or less time to evolve, and we can start to see the pattern that they take working their way up off the main sequences they start to evolve until they reach that red giant phase. If we jump ahead another large amount of time going to four billion years in this case, we have that we can see again all these stars on the main sequence we have a distinct turn-off point, and we can start to see a little bit about how these stars evolve and go through. We can look at an evolutionary track for those stars that have lives of about four billion years. Stars are a little more massive than our own sun. And then finally we can look for stars like our sun, and that would require a ten billion year old star cluster, and we see that here that would be a globular cluster. Again, we see the main sequence here, all the stars there, a very distinct turn-off point and pattern that they're following. These stars take much more time to do this, so we can see how they go up to the helium flash, and then drop back down to the horizontal branch here. So we can start to see all of these different stages of stellar evolution that occur. So we can use models then to try to tie into this and make sure that our models of how a solar of star that lives for ten billion years, that would be a star like our own sun, about one solar mass, how it goes through its lives, what do we predict for it to do, what do we predict for it to move, and does it follow the patterns that we see within these clusters. Now we use that turn-off point helps us to determine the age of the cluster. So turn-off point T tells us what the age of the cluster is because those are the stars that are just finishing their lives, that are just exhausting the hydrogen fuel in their core and beginning to evolve. By models we'll know what mass stars those are and we can then determine how old the cluster is based on that. So let's look at that a little more here. Again, the youngest clusters must be a million years old or less. The stars that we see them do not live any longer. So we see very young clusters and again we know because stars that around that only live a million years, then these clusters can't be more than a million years old or those stars would be gone. Oldest globular clusters more than 11 billion years old. Early some of the earliest groupings of stars to form within the universe and putting a limit to the age of the universe and helping us to estimate that age that we will look at later on. So let's go ahead and finish up here with our summary. And what we've looked at this time was that we can study stellar evolution by looking at stars at various stages. Star clusters are our laboratories for studying stellar evolution because they formed at the same time and from the same material. And the oldest clusters, those old globular clusters can help us get estimates of the age of the universe. So that concludes this lecture on testing stellar evolution models. 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.