 Greetings and welcome to the Introduction to Astronomy. In this lecture, we are going to look about creating an HR diagram, how we can go ahead about plotting those and what we can learn from this diagram. So let's see what we're looking at first. When we think of that, what do scientists do when they want to find a relationship between two things? Well, a quick thing to try is what we call a scatter plot, which is just plotting two variables against each other, and if there is no relation to them, the points will just be scattered all over the graph and they can move on to something else. If there is a relationship between variables, we see a clear trend in the graph, and that's what you see here with weight and height. So there's definitely a trend that those who are taller tend to weigh more. Now is it perfect? No, there are certainly some outlying points here and some outlying points here, but overall there is a general trend that we see between these. And in the early 1900s, two astronomers, Hertzsprung and Russell, looked at plotting the luminosity versus the spectral class. And what you see there when you plot that is that there is a relationship between the temperature and the luminosity of the stars, so these two are related. And that most stars, again, fall on what we call the main sequence, going from upper left down to lower right. And while there are scattered stars down below and up above, they are a much smaller percentage of the overall stars. Now when we look at plotting this, one of the things we have to understand a little bit is what we call the color index. Now we looked at this previously as a way of being able to measure the temperatures. One color index that we use is called B minus V, and that uses the blue filter looking at blue light and the visual filter looking at yellow light. If you measure these two magnitudes and then subtract them, if you get a positive value that means a redder star. So if the blue magnitude is 3.5 and the V magnitude is 2.5, subtracting them gives you 1, and that means it's going to be a redder, cooler star. Remember that magnitudes are backwards? So the B magnitude means it's fainter in the blue than in the visual here. A negative index tells you a bluer or hotter star. So if B is 1.8, V is 1.9, B minus V is negative 0.1 means that this is a very hot star. It is emitting more light in the blue portion of the spectrum than in the visual, and again remember that magnitudes are measured backwards. Now let's look at the components of the HR diagram and kind of how this ties in. First of all on the horizontal axis we have to look at a couple things. We look at a measure of the temperature, which could be temperature, spectral class, or color index. Those are three different things that we can plot on the horizontal axis. Note that temperature increases to the left while color index numbers increase to the right. Now on the vertical axis we measure the intrinsic brightness. That means the luminosity, the absolute magnitude, or in special cases the apparent magnitude, and that would be when we're looking at star clusters where they're all essentially the same distance away from us. When we graph these what do we find? First of all we find the main sequence, the larger and more massive stars to the upper left, and the smaller and less massive stars down to the lower right. Then we find the, above that we will find the giant branch. The giant branch is the evolved stars which are burning helium into carbon. So they are stars that have already gone through their main sequence lives and evolved off of it. As we continue upward the supergiant stars shown here, these are the largest stars burning heavier elements up to iron. Down below the main sequence we see the white dwarf stars. Those are stars that will be much fainter than the typical main sequence stars, but they can still be very, very hot. So again note the difference in temperatures. White dwarfs are very cool stars, very hot stars, while the giants and super giants tend to be relatively cool. So all the white dwarfs are stars that have been compacted down to the size of earth, but still have a mass comparable to the sun. A teaspoon, just a teaspoon of this material would be fifty tons. That is how compacted down this material is. Now we see the sun located there on the main sequence, as pretty much in the middle range of a relatively typical star, and the largest stars up to the upper right edge of the diagram. Those are super giants and hyper giants. These are things that would fill the inner portion of the solar system. These are incredibly large stars, and the further you get up to this corner the larger the stars are. Now we can also learn something about the masses of stars, but only those on the main sequence. So it does not apply to anything else other than those that are on the main sequence. The most massive stars are in the upper left. The lowest mass stars would be down in the lower right. Now what can we learn about stars? We can learn a little bit about how they go through their lives by looking at their paths they take on the HR diagram. So we can watch the path of a star on an HR diagram. We can't watch any individual star because they take this time frame is too long, and most of a star's life is spent on the main sequence. However, once it exhausts the hydrogen in its core it moves upward and follows the track here, and will eventually end up down on the giant branch of the HR diagram. And that will be in what we call the horizontal branch that we see here. And that is where on the horizontal branch it is burning helium in its core. We'll find that the star has reached a new stability with a new energy source. It has already exhausted all the hydrogen in the core and now found a way to burn helium. However, that does not continue. The helium is exhausted relatively quickly compared to the hydrogen and the star begins to cool off again and get larger and larger, becoming a super giant star and then following along and eventually becoming a planetary nebula. And then cooling off, expelling that outer layers and cooling down to become a white dwarf star, which will eventually cool off to become a black dwarf. Now, a black dwarf is the eventual end state of the vast majority of stars, however because of the amount of time it takes a white dwarf to cool, there are no black dwarfs that have yet had time to form. There has not been sufficient time in the history of the universe to yet make one of these stars, however trillions of years from now they will be the vast majority of objects within the universe. So let's go ahead and finish up with our summary and what we look at is plotting, luminosity and temperature shows patterns in the different types of the stars, our HR diagram shows how these quantities are related, and we saw that how stars, while they spend the majority of their lives on the main sequence, they change positions as they age because their temperatures and luminosities are changing, causing them to move on the HR diagram and change their position. So that was our lecture for today on creating an HR diagram. 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!