 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to look at methods of testing models of stellar evolution. So how can we determine whether our models as to how a star evolves consider that we cannot really see inside the star to really test things directly but there are methods we can use to test how our ideas of stellar evolution work. So let's take a look at this and get some ideas of it. First of all, the first thing we want to look at is how can we possibly understand the life of a star. What we have to think about is that the timescales are at minimally millions of years and more likely for stars like the sun billions of years meaning that we can never watch a star go through all its stages of life. We cannot watch one star even over multiple human lifetimes go through all of its stages. In fact even over multiple human lifetimes stars are unlikely to change at all. So one way that we can look at this and try to understand how we can understand stars even though we can't watch an individual star go through its life is that how can we understand how humans might change over their lifetimes and you can imagine a semester project to study human aging. Well in a semester you cannot follow one person through all the stages of their life but you can study all different stages from infants children adolescents young adults adults middle-aged elderly and so on you can study all those various different groups and put this together and you could still be able to understand how humans do change over their lifetime even without studying just one human and watching one human change their lives well we can do the same thing with stars we can look at stars at various stages of their lives and then use that to be able to better understand the life cycle of a star so let's take a look at some one of the ways that we're able to do this and one way that we can do this is using star clusters. Star clusters are essentially laboratories that we can use to study the evolution of stars the reason they are such good laboratories is that the stars in a cluster formed at essentially the same time from the same material they differ only in one thing they only differ in their mass how much material formed each individual star think of this as a controlled experiment if we just look at random stars in the sky they could have formed at different times the materials could have been slightly different and their masses could have been different there is not we're not able to look at just one variable this can be a controlled experiment in which the times and the materials are the same so now we're only difference is the masses so it allows us to study the effects of mass on stellar evolution and what we find is that different mass stars will be at different stages of evolution remember that high mass stars evolve very quickly taking only millions of years low mass stars can take billions or even trillions of years to evolve so we can see all of those different stages at different cycles in a cluster so let's look at an examples and see what these clusters are like first of all there are a couple different types of clusters that we want to look at and one type is what we call a globular cluster which is pictured here these are located in the outer region of our galaxy which we call the halo the spherical region of our galaxy they can contain a hundred thousands hundreds of thousands of stars and are typically many billions of years old so these are very old clusters that have been around for a while and there are other types of clusters that we can look at which are much younger clusters and these are what we call the open clusters so an open cluster by comparison is located in the disk of our galaxy so in the flattened portion of our galaxy they may only contain several thousands of stars and they're typically less than a few hundred million years old so an open cluster here as we can see by comparison is much more spread out then the globular cluster that we looked at previously for fewer stars has a lot of younger stars and they are more spread out an open cluster will eventually dissipate out into space whereas a globular cluster has enough material there to remain bound together for billions of years so how can we what are other one other type that we can look at is what we call the OB associations these are even younger than the young open clusters this is a group of very hot young stars that have formed together and we know that they must have formed recently because they contain stars that only live a million years or sometimes even less so they must have formed very recently otherwise those stars would not still be here however they also form many smaller stars so low mass stars which are still in the process of forming just as a star goes through its life faster at a higher mass it also forms faster at a higher mass so when stars begin to form from a cloud of gas and dust in space the first ones will be the most massive stars will form first and the low mass stars will take a much longer time so these ones these OB associations are associated with star forming regions now let's look at how we can study stellar evolution in clusters like some of these and how we can go about doing that is first of all to look at the HR diagrams what we can do is compare models to the actual star clusters so we can make a prediction as to how old a cluster should be and then how how it should age over time so as we look here we see the young cluster HR diagram so what should a very young cluster look like well it should have stars on the main sequence up here and it should have these lower mass stars still in the process of forming these are ones that are slowly moving towards the main sequence when we look at an actual HR diagram of a young cluster in this case NGC 2264 we see something very similar the higher mass stars are on the main sequence up here and there are low mass stars still forming down here now there is some a difference in the amount of spread here meaning that maybe these stars didn't all start forming at exactly the same time as they did in the model and that some of them formed a little earlier and some of them formed a little later so some of them are still in a little wider range reaching down to the edge here now we can look at older clusters as well so let's look at a slightly older cluster this is a cluster at about 100 million years old and at that age what we begin to see is that stars have now reached the main sequence but stars up in the upper part are actually evolving off and are becoming red giant stars so they have evolved off the main sequence and become red giants so these are not stars that are forming as they were in the first one first slide these are actually ones that are moving off and if you notice what we have here is what we'd call the turnoff point the point at which stars are beginning to leave the main sequence that is a way to be able to determine the ages of these clusters by looking at what that turnoff point is a slightly older cluster would have again even more stars on the main sequence all the stars have reached the main sequence down on the lower portion now but all other stars are starting to evolve off there are no stars on the upper main sequence they are gone they have all gone through their lives and there are none left to be to be present at this point in this so this is a cluster that is about four billion years old about the age of the sun a little bit younger than the sun but about that age so stars in this in this region star like the sun would be just getting ready to make its evolution off of the main sequence now we can also look at a very old cluster and let's take a look at one of those and this is a cluster week this is an example of a globular cluster and what we see in these this is about a 10 billion year old cluster and now we start to see a lot of features appearing here is the main sequence only colder stars stars like the sun even at six stars like the sun here are just at the point of evolving off the main sequence so at 10 billion years the sun would be evolving off and a star like the sun would now be beginning to make its journey and we can actually see the path that it is going to take so we start to actually see some of these tracks and that we know that the sun will follow a path something like this up to this point and then we'll come back down over here so we can see that as the stars slightly more massive than the sun have already gone through already in the process of going through their lives but this turnoff point again is what is important because that allows us to determine the age of the cluster where this point happens to occur what type of star is just turning off the main sequence gives us the age and we can then learn how old a cluster is so let's get an idea of how old these clusters are what do we know about some of these the various youngest clusters must be a million years old or less that's because stars that we see there will not live any longer if these clusters were two million years old those stars could not be there anymore the oldest globular clusters we find more than 11 billion years old these were some of the earliest groupings that formed in the history of the universe and can give us an estimate of the age of the universe because certainly the universe cannot be younger than the oldest stars in it the universe has to be at least as old as the oldest stars because stars could not have formed before the universe so let's finish up here as we do with our summary and what we've looked at this time is that first of all we have a way of understanding stellar evolution by looking at stars at their various stages so we need not see stars at all stages or one star at each stage we can look at those various stages of evolution and use those different stages to go back and understand stellar evolution itself the star clusters provide laboratories for studying stellar evolution they give us a way to form a controlled experiment because all of the stars formed at the same time and from the same material so the only difference is the mass and the mass is the one thing that we are looking at and then we look at these older clusters as a way to be able to help us get estimates on the age of the universe something that we can study when we look at cosmology and the history of the universe so that concludes our lecture on testing models 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