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A binary star whose nature is revealed from its spectrum even though the components cannot be resolved visually. In a double-lined spectroscopic binary, two superimposed spectra can be detected. The lines are shifted relative to each other in a periodic way by the Doppler effect, as the two stars orbit about their common centre of mass. In a single-lined spectroscopic binary, the two stars differ greatly in luminosity so only the spectrum of the brighter component can be discerned. However, the lines are seen to change wavelength in a periodic way when measured in relation to a standard comparison spectrum.
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