Visual binary

An example of a visual binary: Theta1 Orionis C1 (lower) and C2 (upper), as imaged by VLT/GRAVITY.

A visual binary is a gravitationally bound binary star system that can be visually resolved as two separate stars, as opposed to a singular point source with fluctuating brightness. Like all binaries, a visual binary consists of two stars –usually– of different brightnesses, with the brighter star considered the primary and the fainter star considered the companion. If the primary is too bright, relative to the companion, this can cause a glare making it difficult to resolve the two components. However, it is possible to resolve the system if observations of the brighter star show it to wobble about a centre of mass. In general, a visual binary can be resolved into two stars with a telescope if their centers are separated by a value greater than or equal to one arcsecond, but with modern professional telescopes, interferometry, or space-based equipment, stars can be resolved at closer distances.

For a visual binary system, measurements taken need to specify, in arc-seconds, the apparent angular separation on the sky and the position angle of the companion star relative to the primary star. Taken over a period of time, the apparent relative orbit of the visual binary system will appear on the celestial sphere. The study of visual binaries reveals useful stellar characteristics: masses, densities, surface temperatures, luminosity, and rotation rates. These stars are estimated, via Kepler's third law, to have periods ranging from a few years to thousands of years.