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Eclipsing Binary Stars

© Copyright 1999, Jim Loy

Binary (or double) stars are a pair of stars which orbit each other. In rare cases, the plane of the orbit of the two stars is lined up with our line of sight, so the two stars may eclipse each other (animation, above right). In some cases, this is the only evidence that there are two stars there. Quite a bit can be deduced from their light curves (graphs of the light intensity, over time). I show two simplified examples, to the left. In the first, the larger star is the brighter star. In the second, the smaller star is the brighter. You can see how those curves differ. Mainly, there is a much greater decrease in brightness, when the brighter star is completely blocked.

Other, more complicated, situations are possible. If the stars are not as well aligned with our line of sight, then the eclipse may not be total, and we do not get a flat bottom for the minimums, on the light curve. The orbit may not be very circular. The stars may not be very spherical. There may be gas clouds around one star. One star may heat up one side of the other star. The two stars may be almost touching each other. The eclipsing of sun spots (star spots, actually) may even be recorded. The light curve can give clues to all of this.

Besides the light curve, the changing spectrum gives other clues to the details of such a star system.

There are star pairs called "visual binaries" which are stars that are not related in any way, except that they are almost exactly in the same direction from earth. So they are not really binary (orbiting) stars.

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