 Greetings and welcome to the Introduction to Astronomy. In this lecture we will talk about the HR diagram, which is a way of looking at the properties of stars in a graphical manner. So let's go ahead and take a look at this, and this was developed in the early 1900s by Henry Norris Russell and Enyar Hertzsprung, who made graphs of these properties of stars. And what they looked at was the luminosity as compared to the spectral class or temperature. So what they found was that there was a relationship between temperature and luminosity for most stars. So for most stars, we saw that there was a line kind of going from upper left down to the lower right, and that most of the stars fit there. Now there were certainly exceptions to each side, and we can look at those later, but it was found that the vast majority of stars did fit what we call the main sequence of stars. So when we look at these, we see that the vast majority, 90% of the stars were found to lie on the main sequence, and that's the one that starts in the upper left and runs down to the lower right here. White dwarf stars accounted for about 10% of stars, and the giants and super giants, some of the more prominent stars that we see in the night sky, were less than 1% of the stars that we see. Now let's look at an HR diagram and how we can plot one, and we're going to do a rough plot here, and we'll look at this a little bit more detail in a future lecture. So first thing we want to do is draw the axes of this. So we draw an axis here for the y-axis, and we'd go across for the x-axis. On the x-axis, we graph something that relates to the temperature. Now it could be a number of different things, and we see that we can use things like temperature itself, we can use spectral class, and we can use the color index, which we've talked about in a previous lecture. Temperature increases to the left. Now that's the opposite of how we often graph things, but for this HR diagram, the temperature will be to the left. On the y-axis, we plot the luminosity, and the luminosity increases going up as you might expect. Now when we plot this, we can locate the main sequence of stars going from the upper left down to the lower right. So that's our main sequence, the giant branch up above the main sequence off to the right, the supergiants up above that, the white dwarf stars down below the main sequence, the sun maybe somewhere towards the middle here, and then the largest stars up in the very upper right-hand corner. So we could just do a quick sketch here just looking at these. So again, we have the white dwarf stars down below, the main sequence stars diagonally, the giants, and the supergiants stars. So scattered across the diagram, but there was again that definite pattern with the main sequence stars. Now again, we're going to look at this in more detail, but what can we learn from the HR diagram? Well, the main sequence, we find that the larger and more massive stars are on the upper left, the smaller, less massive stars on the lower right. And this has been confirmed with models of star formation, which predict this to be the case. Stars do not stay in one spot on the HR diagram. Over their lives, the temperatures and luminosities will both change, which means that their position on the HR diagram will change. And we will see that as evolutionary tracks, paths that stars take over the course of their lives. Now when we look at some extremes here, we have the star diameters. The smallest stars are down in the lower left-hand side, the largest diameter stars in the upper right. Now star masses we can tell, but only along the main sequence. This doesn't apply to giants or supergiants. We find the most massive stars on the main sequence to the upper left and the least massive down to the lower right. Densities, the least denser in the upper right with the largest stars, so the largest stars would also be the least dense. The most dense in the lower left would be the white dwarf stars, which are where the smallest stars are. And white dwarf material is extremely dense with a teaspoon weighing 50 tons. So this is a basic look at the HR diagram, and then we will go over in the next lecture and look at this a little bit more detail as to how to create the HR diagram. So let's look at our summary first, and we plot the luminosity and temperature, and that gives us patterns that we see in the different types of stars. The HR diagram shows how these quantities are related, and we find that stars spend most of their lives on the main sequence, but will change position as they age, which means that their temperatures and luminosities will change. So that concludes this 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.