A binary star, also called a double star, is pair of stars in orbit around their common centre of gravity. A high proportion, perhaps one-half, of all stars in the Milky Way Galaxy are binaries or members of more complex multiple systems.
If the images of the two components of a binary star system can be separated by telescope, it is called a visual binary. Stars whose components are too close to each other to be distinguished visually can sometimes be identified as binaries by spectroscopic observation; as the members of these spectroscopic binaries move alternately toward the Earth and away from it, a Doppler effect of frequency change is observed in their spectral lines. Binary stars are sometimes detectable by changes in apparent brightness, as the darker (or dimmer) star occludes its brighter companion. Some stellar systems with so-called invisible companions are binaries; these companions can be detected through changes in the proper motion--that is, the rate of motion of the visible stars across the background of more distant stars.
Excerpt from the Encyclopedia Britannica without permission.