Image credit: ESA.
A spectroscopic binary is a binary star system in which the two components are so close together (see close binary), or so far from the Sun, that they cannot be resolved simply by looking at them, even through a powerful telescope. Their binary nature can, however, be established because of the Doppler shift of their spectral lines.
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.