(binary star detected by movement indicated by Doppler shifts in the spectrum)
A spectroscopic binary (SB) is
a binary star system which appears to be a single point
of light, but for which movement of one of the pair is
detectable by noting Doppler shifts in the spectrum indicating
movement to and from the observer, showing the cycle of motion
characteristic of an orbit.
It is one of a number binary-star classes based upon how the star is determined to be binary. Others:
A double-line spectroscopic binary (SB2) is one where a shifting
spectral line can be seen for each star.
A single-line spectroscopic binary
(or single-lined spectroscopic binary or SB1)
is one where shifting lines are seen only for one of the two
stars, i.e., one star is so bright that the other cannot be distinguished.
Detection of the latter is identical to a single star with
an extra-solar planet detected through the radial velocity method,
and some effort is necessary to distinguish between a
binary star versus and a single star that is hosting an orbiting planet.
(The term spectrum binary is used for a star that appears as a
single point, but has a spectrum suggesting a blended spectra
of two stars.)