spectroscopic binary

Binary star in which two stars are so close together that they cannot be seen separately, but their separate light spectra can be distinguished by a spectroscope.

The first spectroscopic binary to be discovered, in 1889 by US astronomer Edward Pickering, was the brighter component of Mizar. Many hundreds are now known, with orbital periods ranging from 82 minutes to 15 years. The most common period is a few days.

As the two stars revolve around their mutual centre of mass, they alternately approach and recede from the observer, resulting in a periodic Doppler shift (see Doppler effect) in the lines of their spectra. If the orbital motion happens to lie at right angles to the line of sight, there is neither approach nor recession so that such a system cannot be detected as a spectroscopic binary. In about one case in six, the component stars are sufficiently similar in brightness for the spectra of both to appear; then as one star approaches, the other recedes, so that the spectral lines appear sometimes double, sometimes single, giving a double-line spectroscopic binary. The line-of-sight velocity of the brighter star, or of each star in a double-line spectroscopic binary, is measured from the Doppler shift at various stages of the orbital period. Analysis of these velocity curves gives a lower limit of the combined mass of a single-line binary, or of the mass of each component of a double-line binary.