Week 7: Variable stars

Introduction

In recent weeks you have learned how stars are powered by nuclear reactions in their cores and seen how classifying stars on the Hertzsprung-Russell (HR) diagram allows us to visualise their characteristic groupings, in particular the main sequence on which most stars typically spend a large part of their stable lifetime.

This week we will look at how stars evolve once their main sequence lifetime comes to an end, and in particular at the instability strip on the HR diagram and the reasons why some stars become variable at certain stages in their lifetime.

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Transcript

NSTRUCTOR:

Well, the clouds have rolled in here on the Tenerife mountainside, so we've moved indoors for the moment. As we've seen from the Hertzsprung-Russell diagram, stellar evolution is generally a fairly slow process. Most stars spend a significant portion of their time on the main sequence, where their output is relatively stable. Our own sun is on the main sequence at the moment. And that's probably just as well for life here on Earth.

In other phases of a star's lifetime, things can change on much shorter timescales. And many professional and amateur astronomers are drawn to studying variable stars whose brightness varies on a regular basis. We’ve already mentioned the instability strip on the HR diagram, and we'll find that this is where pulsating stars vary their output as they expand and contract. This type of variability can be used for measuring interstellar distances or for mapping the scale of the galaxy.

Another type of variability is where one object passes in front of another, as with orbiting binary stars. And in recent years, exciting developments have enabled us to detect ever smaller changes, which leads us directly to the search for planets beyond our own solar system, which we'll find out more about this week.

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At the end of this week you will use COAST to start taking your own images of a variable star in order to measure how its brightness changes. Next week, you will combine your measurements with those taken by others at different times to produce a light curve which shows the variation of your star over time.

By the end of this week you will be able to:

• understand more about the life cycles of stars, in particular what happens to stars of differing masses when they reach the end of their main sequence lifetime
• understand how the mass of a star will determine its eventual fate
• understand the three main types of variable stars: pulsating stars, eclipsing binaries and cataclysmic variables
• be able to identify a suitable target star from a list of known variables and schedule your own observations with COAST.