In this course we've talked a lot about the Hertzbrung Russell diagram, but it's probably a good idea for us to talk about what the Hertzbrung 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, a blue star's 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 their 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 there's a rather parochial view with 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 Hertzberg 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 turn off this point on the Hertzberg 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 Hertzberg Russell diagram we can work out the age of a cluster or even of a whole galaxy.