Astronomy with an online telescope

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# 3.2 Interpreting the light curve

In working through this section you may want to refer back to Section 2.2 in Week 7 and the animation in Figure 8 in that section.

As more points are added to the light curve it should be possible to see a pattern emerging. Use the Fold data option to make the pattern easier to see.

Having produced the light curve, what can be learned from it? The light curve can be used to determine a number of important properties of the binary system:

The period of the binary star

• The most obvious property of the system is the period of the orbit. The light curve of a binary star repeats itself on a regular basis as the stars orbit one another, typically with two dips per orbit as first one star, then the second star passes in front of the other. The time between these pairs of dips tells us the period. This information is used in folding the light curve as described above, by overlaying points from different orbits to give a single curve covering one orbital period.

The masses of the stars

• As described in Week 6 and Week 7 the period of the binary system, combined with Newton’s laws of gravity, can be used to work out the masses of the stars. This is one of the primary methods of determining the masses of stars on the Hertzsprung-Russell diagram.

The relative sizes and brightness of the stars

• The size of each dip in the light curve provides information about the size and brightness of the stars in the binary system. Two identical stars would produce dips of equal sizes and shapes. If the stars are different in luminosity then there will be a small dip when the fainter star is eclipsed, and a larger dip when the fainter star passes in front of the brighter star. The shapes of the dips can be used to work out the sizes of the stars.

As you can see, there is a wealth of information in the light curve. In this investigation we have looked at binary stars, but the same techniques taken to extremes can also be used to detect the very tiny dips in brightness caused by a planet orbiting a distant star. To date, several thousand exoplanets have been discovered in this way.

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