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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.)