 In this course, we've talked a lot about the Hertzsprung-Russell diagram, but it's probably a good idea for us to talk about what the Hertzsprung-Russell-HR diagram actually is. Well, this comes from the early days of trying to understand stars. And in the early days of astronomy, we only really knew two things about each star. We knew how bright it was, and we knew what color it was. So let's put color down here. We've got red stars over here, and blue stars over here. Okay, and I'll put faint stars down here, and bright stars up here. So take the sun, for example. It's a yellow star, so in between blue and red, and it's faint, but not that faint. So it might appear here on the diagram. Not that exciting for one star, but once you start adding other stars, you discover that most of them lie in a straight line. And we call this line the main sequence. And the main sequence is the region on this diagram where stars are burning hydrogen, or fusing hydrogen, to form helium. And that's where their energy is sourced from. And they're on a straight line precisely because of that. They form energy by the same process, and that's reflected by the straight line in this diagram. But not all stars lie on that straight line. So the second most important region is here. This is called the giant branch, and these stars head off to the red over here. And this is where the really big stars live, normally towards the end of their lives. And they're sourcing energy from a slightly different place. It's still nuclear fusion, but in this case, they're no longer fusing hydrogen to form helium. They're fusing heavier things, helium specifically, into other heavier elements. And so this is the picture for the whole of our galaxy, but you can also think about where individual stars move over the course of their lives. So take the Sun, for example, it would have started off somewhere up here, maybe, as a protostar and moved down onto the main sequence four and a half to five billion years ago. And it'll stay on the main sequence in pretty much that spot for four or five billion years, at which point it runs out of hydrogen fuel and begins to burn helium and moves on to the giant branch here. That probably won't last too long for the Sun. It'll blow up, become an enormous red giant, at which point it will then lose that shell of material and start to collapse down in size and become what's known as a white dwarf. And the white dwarfs all live over here in this part of the diagram. From there, the white dwarf will fade slowly over a long, long period of many billions of years, become a faint fading stellar ember. But so much for the Sun, let's get rid of that. That's a rather parochial view of the universe. I think let's go back to talking about stars in general. And we drew the diagram earlier with the main sequence and the giant branch for the whole galaxy. But let's look not at the whole galaxy, but let's look at a particular star cluster. In this case, let's look at an older star cluster. And the Hertzpring diagram for that cluster will look something like this. What's happened here is that the brightest stars in that cluster, because all the stars in the cluster formed at the same time, the brightest ones die first. They're the rock stars of the universe. So though they've got more fuel, they burn through it faster. So they live fast and they die young. And so you can tell by looking at what's called the turnoff, this point on the Hertzpring Russell diagram, where the stars have become old enough to be giants, gives you the age of the cluster. And that's why this is so important. Just by looking at the magical, mystical Hertzpring Russell diagram, we can work out the age of a cluster, or even of a whole galaxy.