Transcript

INSTRUCTOR:

So we've spent some time looking at the constellation of Orion, and we're seeing that the stars in that constellation are different colours. Those colours directly relate to temperature. And temperature, along with luminosity, are the two variables that we can measure for a star.

If we plot those variables out on a diagram, we get a very distinct relationship between the two. This diagram is called the Hertzsprung-Russell diagram. And the horizontal axis at the bottom here, we have temperatures ranging from about 2,000 degrees at this end all the way up to about 30,000 degrees at this end.

INSTRUCTOR 2:

Now our second variable is luminosity, and that's plotted on this axis here, the vertical axis. And we start with the faintest stars at this end, thousands of times less bright than our own sun, and coming all the way up to the brightest stars at this end, maybe a million times brighter than our own sun.

INSTRUCTOR 1:

So what we then have running diagonally across the middle of our diagram is what we call the main sequence. Down in the bottom corner here, we have our very, very faint and very, very cool stars, and we call them red dwarfs.

INSTRUCTOR 2:

And as we come up the main sequence, we've got progressively hotter and more luminous stars coming all the way to the top, until we've got the very brightest blue, very hot stars like Rigel in the constellation of Orion. So where would our own sun fit on this main sequence?

INSTRUCTOR 1:

Well, our sun is a lot cooler and a lot fainter than Rigel is. In fact, it's considered a bit of an average star, really. So I'm going to pop it much lower down on the main sequence there.

INSTRUCTOR 2:

OK. So that completes our main sequence. And this is where stars will spend the majority of their lifetime in this stable main sequence. So now let's think about what's going to happen at the end of that lifetime.

INSTRUCTOR 1:

So when stars move off the main sequence and evolve into their old age, they will all come up to this area of the diagram. So they're getting cooler, but they're also getting brighter. Exactly where they go depends upon where they started on the main sequence.

But generally speaking, we're going to get some that will go up to giant stars, which will sit around there on the diagram, and we'll get some that will go even further. So we have these supermassive stars that come and sit right at the top here.

And complete top of the diagram there. These can get quite unstable.

INSTRUCTOR 2:

So our stars have expanded to form these red giants and red super giants, but this is a much shorter phase of the stars' lifetime, and they're not going to stay in that place for very long. Eventually, they're going to collapse down and travel all the way down to this lower corner of the diagram and become white dwarfs.

So our own sun will end up somewhere here on the diagram, being a white dwarf star. The more massive stars could have an even more dramatic fate, becoming a supernova and collapsing down, finally to form a neutron star or possibly even a black hole for the very most massive stars.

INSTRUCTOR 1:

Now quite a few stars in the process of evolving their way around this diagram will go through a phase where they become quite unstable. And we have what's called an instability strip sitting about here on the diagram. And it's these unstable stars that make up a lot of the variable stars that we're going to start to look at next week.