 Greetings and welcome to the Introduction to Astronomy. In this video, we are going to discuss one of the most important tools in astronomy in what we call the HR diagram. And it is a way of organizing the stars. When we get a bunch of data, what astronomers and scientists in general like to do is to plot it out by various properties and look for patterns that occur. And that is what happened in the early 1900s when a couple of astronomers, Henry Norris Russell and N. Y. R. Hertzsprung actually took the properties of stars that had been catalogued. And what they looked at was the spectral class of the stars. So the O, B, A, F, G, K, M spectral class. And that they looked versus the magnitudes or the luminosity of the stars. So when you plot those two, you look for patterns that occur. And what they found was that there was a relationship between the temperature of a star and the luminosity of the star, not for all stars but for most stars. And we see that there is a sequence here starting in the upper left going down to the lower right. And that main sequence is a relationship that there is a very distinct relationship between the temperature and the luminosity. We don't find a lot of stars down in the lower left-hand side. We don't find as many stars up in the upper right-hand side. There are some, and there are other reasons for that that we'll look at. But in general, we did find this pattern. So when we look at an HR diagram, what we find is that there are different types of stars and they are classified as primarily main sequence stars. So 90% of the stars that we see when we plot an HR diagram are main sequence stars. There are white dwarf stars, which are about 10%, and those are the ones that fall down in the lower left-hand side. And then there are giants and supergiants. Well, if we take our 90% and add our 10%, we've got 100% right here. But of course, due to rounding errors, there are still some, and the giants and supergiants are less than 1% of the stars. So there are a very small percentage of stars in the HR diagram. However, these are the ones we call, these are the ones that we can see over vast distances. So they are so large and so bright that we can see them even over very great distances. So let's go ahead. We've looked at a couple of HR diagrams, but let's plot one out for ourselves. So when we look at this here, we have to first of all draw the axes of the HR diagram. So the first thing we want to do is draw the axes. So we put an x-axis here. We put a y-axis here. And then we have to label what we're going to plot on each of those axes. So on this axis, it is temperature. Now we can measure temperature in various ways. It can be a spectral class you can plot. You can look at the color index. You can look at the temperature itself. But the thing to note is that when we plot the temperature, we plot it backwards. So the temperature increases this way so that we have hot stars over here and cool stars on this side. So numerically the temperature is going to be plotted backwards. Now the other thing that we want to look at and plot on the y-axis is going to be the luminosity. So we put the luminosity here. The luminosity would increase going here with bright stars at the top. So you would have that. That would be the second thing that you would plot. Then what we would find are the various parts of the main sequence of the HR diagram. When we start to plot stars here, what we do not find is that they do not just randomly fall all over the main sequence. That would show us that there is no relationship between them. Instead what we find is that there is a very distinct pattern between these when we plot the temperatures versus the luminosities. And what we find when we plot those are that we find, for example, the main sequence runs from the upper left, curves, and goes down to the lower right. So the blue line there is the main sequence. We would then have the giants and super giants would be out over here. So giant stars and super giants would be out over here. The white dwarf stars would be very hot stars, but very cool. And they would be down here in the purple. So they would be down below the main sequence. And where would the sun be? Our sun is right about in the middle of the main sequence. So our sun would be right about here. So there's our sun. And then we'd have all of the other types of stars that we would see. The other thing to note is where are the largest stars? Well, the largest stars are up in the upper right-hand corner. They're very cool, but they are also very large. These are the super giants. These are the giant stars. We looked at the others. We looked at the white dwarf stars here. And then we looked, again, originally at the main sequence. So the main sequence right here. So we have all of these different types of stars that we find located in the HR diagram. Now what we will learn is, you know, what can we learn from these? So let's go back to another HR diagram that we looked at, clear ours here. And let's look at another HR diagram. And what we see is that on the main sequence, the larger and more massive stars are in the upper right. So large stars are here. Those are the large and massive stars. Down here we have cool and low mass stars. So there is a very distinct pattern on the main sequence. The ones up here might be 50, 60 times the mass of the Sun. The ones down here might be one-tenth the mass of our Sun. And this is related to star-forming models. The temperature will be related to the amount of mass within the star, which actually also tells us their lifetime. And lifetimes for stars like the Sun might be 10 billion years, but for more massive stars up here might only be 10 million years, and for very low mass stars, maybe trillions of years. So star-formations models will tell us this. However, how then do we get all of the other types of stars? Where do the super giants and the white dwarfs come from? Well, stars will change their temperatures and their luminosities over their lives. So the Sun has a very specific temperature and luminosity, which puts it at a specific spot on the HR diagram right now. That is not where it will always be. So over time, eventually, the Sun will change. It will cool off and it will get brighter, so it will actually move up into the giant region. Nothing is moving about the star. It simply means that its temperature is changing. It's cooling off, so it moves to the right when we plot it on here, and it's getting bigger and brighter, so it's moving upward. So the Sun and other stars will change their positions over time. And that was what we'll learn is how stars evolve. So there's a formation of stars that we will look at, and then there is then the end of the lives of the stars, where they will change. So they will move around the main sequence that is very different from them moving around through space. So let's look at a few of the extremes of the stars as well, just to finish up here. When we look at the extremes of the stars in terms of diameters, the smallest stars are in the lower left. So very, very tiny stars, smallest stars will be right here. The largest stars will be up in the upper right. So star size kind of increases as you move from lower left to upper right. Star masses, this applies to the main sequence only. We can't tell anything about the masses within the white dwarfs or within the giant stars, those are completely different. But along the main sequence, the mass decreases as you move down the main sequence. So more massive and less massive down here. The densities of the stars are least dense in the upper right with the largest stars and more dense in the lower left with the white dwarf stars. In fact, these stars, which we'll talk about in future videos, are extremely dense, so dense that just a teaspoon full of their material would weigh 50 tons. That is how dense and how compacted they are. They essentially have had all of the space squashed out of them and are as tiny as a star can possibly get and still be made of ordinary atoms. They are just compressed so close together that their electrons are essentially touching. And we will come back to those in future lectures. But really what we want to see is that we can then learn about stars by how they move around on the HR diagram. Now within our lifetime, a star will never change its position. Our sun, somewhere in the middle right here, is going to stay there. It's been at this location for about 5 billion years and it will stay very close to this location for about 5 billion more years. It's only when it has exhausted its supply of energy in about 5 billion years that we'll notice that it will start to move up towards the giant branch. So our sun will eventually become a red giant. It will grow larger and it will cool off. So that is some of the things that will change. Again, it's not something that we would see within our lifetime. Every star that we see for the most part will not change in a human lifetime or even several human lifetimes. It takes in many cases millions or even billions of years to be able to see significant changes. So let's finish up with our summary. And what we see is that we create an HR diagram that when we plot the luminosity or some measure of the luminosity of a star versus its temperature. Remember that luminosity could be magnitudes. There are some ways to do that. Temperatures could be spectral class. Could be color index. It really depends on what is being done and what measurements are being made. But both of these are related to temperature. Magnitudes are related to luminosities. So there are various things that can be plotted. And the HR diagram shows how these are related, giving us the main sequence. Now, stars will spend most of their lives on the main sequence and that's why 90% of the stars that we see are on the main sequence because that is where they spend most of their lives. But they will change their positions as they age when their temperatures and luminosities change. And that is something that we will look at over coming lectures. So that concludes our lecture on the 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.