Spectroscopic binaries are pairs which are too close to each other as seen from the earth to be resolved into two stars. However, when the light from the star is analyzed with a spectrograph, which spreads the light into a continuous spectrum of colors with dark absorption lines superimposed, the spectral lines are alternately shortened or lengthened indicating Doppler motion, a to-andfro motion as seen from Earth. This shift in the wavelengths results from the periodic motion, in the line of sight, of the visible star revolving around the center of mass of the system. When only the brighter component has sufficient light to show on the spectrogram the system is known as a single-lined spectroscopic binary. When the spectra of the fainter component is also recorded the name double-line spectroscopic binary is used.
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