Binary Star

Astrometric binaries are double star systems visible on astrometric photographs as single stars. They have a telltale wavy motion across the sky indicating that the visible star is revolving around the center of mass of the visible star and its invisible companion, and thus its motion over an interval of time is analyzed for gravitational orbital motion. This process is a slightly modified form of the method for visual binaries. Generally, the companion star is either too faint to be seen or too close to the primary star to be resolved as two stars. The largest ground telescopes and also the Hubble space telescope are used to try and "see" the fainter component which might turn out to be a brown dwarf or a planet. The star Sirius is a fine example of a visual binary, discovered first as an astrometric binary in 1844 by the German astronomer F. Bessel.