A system of two stars that orbit a common center of mass, appearing as a single star when visible to the unaided eye. The orbital periods of binary stars range from several hours to several centuries. By some estimates, at least half of the stars in the Milky Way galaxy are members of binary star systems. Also called double star
♦ Binary stars are divided into four main classes based on how their dual nature is detected. A visual binary
can be resolved telescopically into its two components. Only one star of an astrometric binary
is visible, but the unseen component can be identified from its gravitational effect on the visible star, causing it to oscillate slightly, or wobble, against the background of more distant stars. The two components of a spectroscopic binary
are identified based on their varying orbital velocities toward or away from Earth as revealed by periodic Doppler shifts in their spectral lines. In an eclipsing binary
, the two components orbit each other in such a way that they periodically obscure or eclipse each other as viewed from Earth, causing changes in their observed brightness. Eclipsing binaries are also considered a kind of variable star.
♦ Two stars that lie very close to each other along an observer's line of sight but that are not associated with each other in a gravitational system are known as optical binaries
. Although they appear close to each other in the sky, such stars are actually very distant from each other in space. See also multiple star, variable star.