A binary star is a double star system in which two stars orbit each other around a central point of gravity.

Before the early 1800s, binary stars were thought to be an optical illusion. An observer looking into the sky might see two stars that appeared to be side by side, but it was assumed they were really very far apart, and just happened to be in the same direction of view. For some stars this is indeed true, but for many others it is not.

English astronomer William Herschel made this first identifications of true binary stars. Like James Bradley, Herschel had been trying to measure the parallax of a star; that is, its apparent shift in position resulting from the motion of the earth around the sun. A successful parallax measurement would finally make it possible to determine how far away a star was...