A binary star is a pair of stars bound by gravity and orbiting a common center of mass. Something over half the stars in the galaxy belong to systems of two or more, making solitary stars like the Sun less typical than they appear. Binaries are scientifically precious because their orbits reveal stellar masses directly, a quantity almost impossible to measure otherwise. When the two are close enough to exchange material, the results include novae, Type Ia supernovae, X-ray binaries, and eventually merging compact objects.