See binary stars.
A binary star that cannot be resolved visually or photographically, but whose duplexity is established by periodic Doppler shifts of lines in its spectrum. All close binaries (including cataclysmic and interacting binaries) fall into this category, as do most eclipsing binaries. The information obtainable about a spectroscopic system depends on whether a radial velocity curve can be obtained for one or both components. With a single-line binary, it is possible to obtain only an expression known as the mass function, which is a combination of the masses of each star and the inclination of the orbit. With a composite-spectrum binary or double-lined binary, the relative masses may be derived from the mass ratio. If the orbital inclination can be found (normally only in eclipsing systems), the dimensions and masses of the stars can be calculated. With interacting binaries, there are limitations to the latter procedure because of their complex structure.
spectroscopic binary:
The top diagrams show the positions of two stars, A and B, as they orbit their centre of mass, C. As a result of the Doppler effect, the wavelengths of light from each star are displaced towards the blue or red end of the spectrum, depending on whether the star is approaching or receding from the Earth. Any spectral line in the spectrum of each star is therefore seen to oscillate back and forth across its normal wavelength position, as shown in the row of spectra.
A binary star in which two stars are so close together that they cannot be seen separately, but their individual light spectra can be distinguished by a spectroscope.
As the two stars revolve around their mutual centre of mass, they alternately approach and recede from the observer, resulting in a periodic Doppler shift (see doppler effect) in the lines of their spectra. In about one case in six, the component stars are sufficiently similar in brightness for the spectra of both to appear, giving a double-line spectroscopic binary. The line-of-sight velocity of the brighter star, or of each star in a double-line spectroscopic binary, is measured from the Doppler shift at various stages of the orbital period. Analysis of these velocity curves gives a lower limit of the combined mass of a single-line binary, or of the mass of each component of a double-line binary.
See star pair.