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Astronomy with an online telescope
Astronomy with an online telescope

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2.2 Eclipsing variables

The second type of variable star mentioned by Jo in the video is the eclipsing binary. This type of system consists of two stars orbiting one another closely in such a way that one star passes in front of the other as seen from the Earth. As this happens, the light from the more distant star is blocked, causing a dip in the overall brightness of the system as measured here on Earth. you have already seen in Week 6 how the periods of these eclipsing binary stars can be used to work out their masses. Here you will look at the light curve in more detail:

Download this video clip.Video player: boc_aot_1_video_week7_eso1311b.mp4
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The light curve of such a system is very characteristic, having two dips in brightness per orbit. If the two stars are of similar size and brightness then the dips will be similar in size. In many systems, however, the stars are of very different brightness – in this case, there will be a small dip when the light from the fainter star is blocked and a larger dip when the fainter star passes in front, blocking light from the brighter one.

By analysing the light curves of these eclipsing binary stars, it is possible to learn a lot about the two component stars, even if they are too close to be seen separately. In your observing project next week, you will use COAST to make measurements of an eclipsing binary star and combine your results with measurements made by others to produce an overall light curve for the system.

A related type of system is the transiting exoplanet in which the light from a distant star is blocked by a planet orbiting around it, again producing a dip in the light curve. As you can imagine, the reduction in light is very small (perhaps less than 1%) but with modern detectors, these small changes in light can be detected even using small telescopes, as you shall see in a later section.