A spectroscopic binary is a binary star in which the two components are so close to each other, or so far from the Earth, that they can not be resolved by sight, not even by using powerful telescopes. However, its binary nature can be established by the Doppler shift of its spectral lines.
Turning over the common center of mass, the star successively approaches and moves away from the line of sight. This movement generates in the combined spectrum of the two stars a regular oscillation or splitting of its spectral lines. Spectroscopic binaries with double lines have two sets of lines that oscillate with opposite phases, each corresponding to a star. Spectroscopic binaries with a series of lines show the oscillation of a single group of lines, because the secondary star is very dim.
Mizar A was the first spectroscopic binary discovered in 1889 by Edward Charles Pickering. Its orbital period is 20.5 days and both components are approximately equally bright. The following table shows some of the best-known spectroscopic binaries:
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