As the stars revolve around their common center of gravity, they alternately approach and recede in the line of sight. This motion shows up in their combined spectra as a regular oscillation or doubling of the spectral lines. Double-lined spectroscopic binaries have two sets of spectral features, oscillating with opposite phases. Single-lined spectroscopic binaries have only one set of oscillating spectral lines, owing to the dimness of the secondary component.
In most cases, the components of a spectroscopic binary are so close together that each is distorted into a nonspherical shape.
The first spectroscopic system to be discovered was Mizar (Zeta Ursae Majoris) in 1889.
Related category• TYPES OF STARS
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