Astronomy with an online telescope
Astronomy with an online telescope

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Astronomy with an online telescope

2.2 Building the HR diagram

From looking at stars in the cluster M29 and in the constellation Orion, we have learned that stars have different colours related to their temperatures, and that the brightness of each star is also related to the colour and temperature.

We are now in a position to put all this together. In science, one way of seeing how properties are related is to look for patterns by plotting one quantity against another on a chart or diagram. In this video, Jo and Alan explore the diagram that results from comparing the temperatures and luminosities of a large number of stars.

Download this video clip.Video player: boc_aot_1_video_week6_2_hr-diagram.mp4
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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.
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.
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.
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?
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.
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.
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.
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.
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.
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This plot of the surface temperature of a star against its luminosity is called a Hertzsprung-Russell diagram after the astronomers Ejnar Hertzsprung and Henry Norris Russell who first plotted it in the early 1900s. This diagram makes it easy to see the patterns and correlations between the two parameters. In particular, most stars lie on the Main Sequence – a band running from the top left to the bottom right of the diagram. Within this main sequence, which represents the stable part of a star’s lifetime, the hotter a star is the more luminous it is (and hence the cooler a star is the less luminous it is). Temperatures on this diagram are measured in Kelvins (K). On the Kelvin scale, the freezing point of water is 273 K and the boiling point 373 K. To convert from Kelvins to degrees Celsius, subtract 273.

This diagram can also be used to answer the question – posed towards the end of last week – of how the Sun compares to other stars. As explained in the video the Sun is entirely average – with a modest luminosity and a yellowish-white colour, the Sun is not among the hottest, most luminous stars and it is not among the faintest and coolest stars either. Jo placed the Sun directly on the main sequence, slightly to the right of centre, indicating that it is indeed a fairly small and average star.

As you shall see later this week when thinking about stellar lifetimes, the fact that the Sun is such a relatively modest star is actually a very good thing for life on Earth.

While the main sequence represents the stable main part of a star’s lifetime, there are other groupings on the diagram representing different phases of a star’s evolution. In particular, many stars expand and cool to form red giants as their nuclear fuel runs out towards the end of their lifetimes, and in the process can become unstable and variable. Stars similar to our own Sun will eventually collapse to become white dwarfs – very hot but very compact bodies, seen at the bottom left of the diagram.


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