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Measuring the radial velocity of binary stars

Animated image with two sections. The left side shows a graphic of two stars going around each other while the right shows a graph of speed of the smaller star and also how the spectrum shifts back and forth

A binary star system is one in which two stars orbit each other. This results in each star alternate moving towards or away from the Earth. This movement cause the lines in the spectrum to shift back and forth. Careful measurements of the movement of the spectral lines allows for the mass of the component stars to be measured. This same technique can detect planets around other stars but the shift in the spectral lines is minute compared to that caused by two stars orbiting each other.