 Greetings and welcome to the Introduction to Astronomy. In this week's special topic in astronomy we are going to talk about the HR diagram and how that is used to understand stars. So what is an HR diagram? Well, an HR diagram is also known as the Hertzsprung-Russell diagram, named after two astronomers who independently developed this tool back in the early 1900s. When the scientists want to study properties of something they often will graph them and look for patterns. So patterns show that there is often a relationship between these. Well, in this case we are graphing the luminosity or the brightness of a star against the temperature. So here we see luminosity on the y-axis increasing upward and we see temperature on the x-axis increasing to the left. And what was found was that most stars, 90% of stars, lie on the main sequence. So this is a diagonal going from the upper left down to the lower right and that holds the vast majority of stars. And what it's telling us is that is where stars spend the majority of their lives. Stars don't live a very, stars live a very long time and therefore we only see a snapshot. We get one instant of them in their lives. So by looking at a large sense of stars, looking at thousands and thousands of stars, and plotting their values together, we can get an idea of how common each type of star is. The more stars we happen to be catching on the main sequence, the more common that is. And that means that stars spend about 90% of their lives on the main sequence. So what do we find? Well we find this is a very useful tool for studying the evolution of stars. How do stars change over their lives? Well as they do, we'll see that their temperatures and luminosities change, which will cause their location on the HR diagram to change. So we can track that out by making models of stars and studying how they would appear to change. Now when we talk about temperatures, we mean the surface temperature. What temperature do we see for the star? The interiors are going to be much, much hotter than this, but this will give us a guide as to what we're seeing for either a very low temperature or a very high temperature of the star. Now here we see evolutionary tracks for several different stars. We see what we call the zero age main sequence, which is where the stars first form. That is where the stars first reach the main sequence. So even a star like our sun is slowly changing. Its temperature and luminosity are changing. We'll never notice it over our lifetime. It is much too small of a change for that. But over millions and tens of millions and hundreds of millions of years, the sun is slowly getting a little bit brighter. And we would see that as a slow change on the HR diagram. Now here we're seeing it for 4 different stars from a lot more massive than our sun 60 times to a little less than half the mass of our sun. Let's go ahead and concentrate for the purpose of this lecture just on one of those. Let's look just at one and let's look at something much closer to the mass of our sun. So we see it on the main sequence here and what the sun is doing right now is slowly moving off the main sequence. This will take hundreds of millions of years. So again, we will not notice that the sun is slowly brightening. It's not something that is noticeable except over extremely long time spans. What's happening here is it's using up its fuel and as it does that it continues to expand. And this pace will quicken and it will continue to go up here getting cooler. Notice how the temperature is decreasing as you move to the right and many times brighter. The star is becoming a red giant and then a red super giant star. Now at this point there's what we call a helium flash which I'm not going to go into but essentially it's starting a new source of fuel where it becomes hot enough to fuse helium into carbon. At that point the star starts to heat up again and get fainter and settles down. And then eventually it will use up that fuel and the process will continue and then eventually it just moves off becoming a planetary nebula and will eventually end up down below the main sequence as a white dwarf star. So this allows us to use models to be able to study how stars evolve using things like the HR diagram. Now how do we compare them to what actually happens? Well we can look at clusters of stars. Star clusters will show stars that all formed at the same time from the same material and therefore we can look and see what they, how they're evolving. All stars that are currently evolving are right around the same mass. So we can kind of follow an evolutionary track of those directly even though we only get a snapshot of each individual star and that helps us to better confirm the models that we're using for the evolution of stars using things like our HR diagram. So let's go ahead and summarize what we've looked at here. We looked at the HR diagram as a graph that is used to compare the properties of stars. Most stars will fall on the main sequence and we can use this to study the evolution of stars by looking at their tracks, how their temperature and luminosity change on an HR diagram. So that concludes this lecture on the HR diagram. We'll be back again next week for another special topic in astronomy. So until then, have a great day everyone and I will see you in class.