Astrometry and Astrometric Binaries

Not all binary stars are visible as two separate stars. In fact, in most binary systems that are known only one of the stars can be seen clearly. The two most important ways in which an unseen companion star can be inferred are by the gravitational influence of the unseen star on the visible one and by effects in the observed spectrum.

A binary inferred from the perturbation of its motion by an unseen companion is called an astrometric binary and the precise measurement of stellar motion required to find such perturbations is called astrometry. Astrometric binaries are the subject of this module. Binary systems deduced from effects on the spectrum are called spectroscopic binaries; they are the subject of a separate module.