Spectroscopic Binary Animation

The following animation illustrates the basic idea of a spectroscopic binary for an idealized situation where the two stars have identical spectra, there is only one line in the spectrum, and the stars are in circular orbits around their common center of mass.

Single-Line and Double-Line Binaries
If both lines (generally, sets of lines in the more realistic case) in the above example can be seen, we say that this is a double-line spectroscopic binary. However, imagine that one of the stars is too faint for the lines from it to be seen. We would still see the lines from the other star periodically shifted up and down in frequency by the Doppler effect. This is called a single-line spectroscopic binary. In both cases the periodic shifting of spectral lines allows us to conclude that we are observing a binary star system.

Doppler-Shifted Lines

The observer sees two lines (except when the stars are moving perpendicular to her line of sight) because one star is moving away and one star is moving toward the observer. These give red-shifted and blue-shifted spectral lines, respectively. The arrow below the spectrum indicates the location of the unshifted line. Note that the colors in the above animation are used only to distinguish one star from another; if the stars were actually different colors, they would not have the same spectrum.

More Realistic Binaries

The above animation illustrates the basic principle, but real spectroscopic binaries are more complicated. The orbits may not be circular and may be tilted relative to the line of sight, and the two stars will generally have different spectra with each of the two spectra having many lines, not just one. Furthermore, the lines of one star may be faint or even not visible relative to the other star (see the box). A more realistic depiction of the spectra and velocity curves for a binary star are illustrated in this Java animation by Terry Herter of Cornell University.