visual binary

visual binary

A binary star, such as Sirius or Gamma Centauri, whose components are sufficiently far apart to be seen separately, either with the naked eye or with a telescope. In most cases the components differ in brightness: the brighter star is designated the primary and the other is the companion. Observations over a period of time reveal the apparent orbit of the companion relative to the primary, from which the true orbit is derived by adjusting for the inclination of the orbital plane. If the distance is known the total mass of the binary can be obtained from Kepler's third law:

M ₁ + M ₂ = a ³/p ³P ²

a is the semimajor axis of the true orbit, p is the trigonometric parallax (both in arc seconds), and P is the orbital period in years; the total mass is obtained in terms of the solar mass. It is sometimes possible to measure the absolute orbit of each component against the background of more distant stars. Then, if a ₁, a ₂ are the semimajor axes of these orbits, the ratio of masses is given by

a ₁ /a ₂ = M ₂ /M ₁

and by applying both equations the individual masses can be obtained. If the distance is unknown, both it and the total mass can be found using the method of dynamical parallax.

Visual binaries generally have long orbital periods and in some cases the period is too long for any orbital motion to be detected. Such pairs are called common proper motion stars because, unlike optical double stars, they appear to stay together as they move across the sky.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006