)
spectroscopic binary
(spek-trŏ-skop -ik) A binary star in which the orbital motions of the components give rise to detectable variations in radial velocity, revealed by changes in Doppler shift in their spectral lines (see illustration). Because a binary star is more likely to be detected spectroscopically if its orbital period is relatively short and the orbital velocities are relatively high, most spectroscopic binaries are close binaries (see binary star). Spectroscopic detection is not possible if the orbital plane lies at right angles to the line of sight.When the components are nearly equal in brightness both spectra are visible and the spectral lines appear double for most of each orbital period, coinciding only when the stars are moving at right angles to the line of sight. Such a system is called a double-lined spectroscopic binary. Comparison of the Doppler shifts in the component spectra gives the relative velocities and hence the relative masses of the two stars:
The individual masses cannot be determined unless the inclination of the orbital plane to the line of sight is known (see eclipsing binary).
The presence of a comparatively faint component is revealed only by its gravitational effect on the motion of the brighter star. Invisible components of low mass are rarely detected because doppler shifts corresponding to orbital speeds of less than about 2 km s–1 are obscured by observational errors. See also mass function.
Spectroscopic Binary
a binary star whose component stars are so close together that they cannot be seen separately with even the most powerful telescopes. A spectroscopic binary can be distinguished from a single star only by observing periodic shifts or doubling of the binary’s spectral lines. This shift or doubling is caused by the Doppler effect, which results from the orbital motion of the component stars.