Binary system

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For other uses, see Binary system (disambiguation).

A binary system is a system of two objects in space (usually stars, but also brown dwarfs, planets, neutron stars, black holes, galaxies, or asteroids) which are close enough that their gravitational movement causes them to circle around each other (orbit) around a shared mass. Some definitions require that this center of mass is not located within the interior of either object. A multiple system is like a binary system but consists of three or more objects.

Binary companion (minor planets)[edit]

Pluto and its moon Charon are often described as a binary system.

When binary minor planets are similar in size, they may be called "binary companions" instead of referring to the smaller body as a satellite.[1] Good examples of true binary companions are the 90 Antiope and the 79360 Sila–Nunam systems. Pluto and its largest moon Charon are sometimes described as a binary system because the barycenter (center of mass) of the two objects is not inside either of them.[2]

Close binary stars[edit]

Main article: Binary star

A binary star that has an orbital period of less than 30 years implies that the two system components are less than about 10 AU apart. Because of this proximity, most close binaries are spectroscopic binaries and/or eclipsing binaries. Mass transfer occurs at some stage in most close binaries, profoundly affecting the evolution of the component stars. If the two components are in a close binary and do not fill their Roche lobes, the system is considered a detached binary. In a semidetached binary, one star fills its Roche lobe and mass transfer occurs. In a contact binary, both stars fill their Roche lobes. The evolution of close binaries depends on the initial masses of the two stars and their separation. When the more-massive star evolves into a red giant first and fills its Roche lobe, material will spill through the inner Lagrangian point onto its companion, thereby affecting its companion's evolution. Mass transfer can also alter the separation and orbital period of the binary star.

In binaries that are initially widely separated, material escaping from the Roche lobe of the evolved red giant immerses the system in material, creating a common-envelope binary that contains the core of the red giant (a white dwarf) and the companion star. Friction causes the two components to approach, and thus the orbital period to shorten. The common envelope is ejected and a cataclysmic variable star is left, wherein the mass transfer from the companion to the white dwarf causes the periodic outbursts seen in novae, recurrent novae, dwarf novae, and symbiotic novae.

If one component of a close binary is massive enough, it may become a neutron star or black hole instead of a white dwarf. Such binary systems are observed (see X-ray binary), but often a supernova explosion will blow the system apart into separate single stars.

In popular culture[edit]

See also[edit]


  1. ^ "Satellites and Companions of Minor Planets". IAU / Minor Planet Center. 2009-09-17. Archived from the original on 2011-01-21. Retrieved 2011-01-08. 
  2. ^ Ian O'Neill (8 August 2014). "Can We Call Pluto and Charon a 'Binary Planet' Yet?". Discovery News. Retrieved 15 July 2015. 
  3. ^ "Defiance-The Votans". SciFiMovieZone. Retrieved 16 May 2014. 
  4. ^ "Map of The Twelve Colonies of Kobol". Quantum Mechanix. Retrieved 14 July 2014. 


  • Astronomy: A Visual Guide by Mark A. Garlick