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The Question

(Submitted March 30, 1998)

How can binary stars be detected even when a telescope like HST can't even see them together?

The Answer

There are stars called spectroscopic binaries that are detected when a spectrum is seen to contain two sets of lines which move slightly relative to each other, in step with the two stars movement about their common center of gravity. If we are roughly in the plane of the binary orbit, we will see first one set of lines slightly blue shifted and the other slightly red shifted (as one star moves towards us and the other away) and then the opposite effect (after another 180 degrees of mutual revolution). If the two stars are similar the two sets of lines will be similar, but if the stars are quite different the lines will be too.

Paul Butterworth
for the Ask an Astrophysicist team

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