A binary star is a star system consisting of two stars orbiting around their common centre of mass. The brighter star is called the primary and the other is its companion star or secondary.
Double Stars may include binary systems; however a double star is used to describe two stars which are apparently close together as observed from Earth. The stars may not interact with each other and may be separated by a considerable distance.
Binary star systems are important to astrophysicists because their orbits allow the masses of two stars to be calculated. This allows other parameters such as radius and density to be estimated. These calculations allow an empirical mass-luminosity relationship to be created which allows the masses of single stars to be estimated.
If a binary star happens to orbit in a plane along our line of sight, its components will mutually eclipse and transit each other; these pairs are called eclipsing binaries or photometric binaries.
Most binary stars orbit at large distances and there is relatively little gravitational pull from one to the other. In this situation the stars are said to be detached.
It is possible for two binary stars to be close enough together that the gravitational pull of one is enough to "pull" matter from the other. The Roche Lobe is the region of space around a star in a binary system within which orbiting material is gravitationally bound to that star. If the star expands past its Roche lobe, then the material outside of the lobe will fall into the other star through a process known as Roche Lobe overflow (RLOF). These are called Semidetached binary stars.
If the stars are very close to each other than their outer atmosphere will "merge" and both stars will fill their Roche lobes. The uppermost part of the stellar atmospheres forms a common envelope that surrounds both stars. These are contact binaries.
The image above right shows the most famous binary star Sirius A and B. This image was taken by the Hubble Space Telescope and was intentionality over exposed so that the much dimmer Sirius B can be seen in the lower left hand corner. While Sirius A and B can be observed with amateur telescopes, the difference in magnitude between the two means that it is very difficult.
|Mizar & Alcor||13h 23.9m||+54° 56'||11.8'|
|Albireo||19h 30.7m||+27° 58'||34"|
|Epsilon Lyrae||18h 44.3m||+39° 40'||3.5'|
|Almach||02h 03.9m||+42° 20'||9.8"|
|Ras Algethi||17h 14.6m||+14° 23'||4.6"|
|Trapezium||05h 35.3m||-05° 23'||25"|
|Castor||07h 34.6m||+31° 53'||3"|
|Izar||14h 45.0m||+27° 04'||2.8"|
You can find a list of 600 multiple star systems here.
My website and its content are free to use without the clutter of adverts, tracking cookies, marketing messages or anything else like that. If you enjoyed reading this article, or it helped you in some way, all I ask in return is you leave a comment below or share this page with your friends. Thank you.