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((short description|Two astronomical bodies which orbit each other))((For|binary numbers|Binary number))((other uses))
((Refimprove|date=February 2015))

A '''binary system''' is a system of two [[astronomical bodies]] of the same kind that are comparable in size. Definitions vary, but typically require the [[barycenter|center of mass]] to be located outside of either object. (See [[Barycenter#Gallery|animated examples]].)

The most common kinds of binary system are [[binary star]]s and [[binary asteroid]]s, but [[brown dwarf]]s, [[planet]]s, [[neutron star]]s, [[black hole]]s and [[galaxies]] can also form binaries.

A ''multiple system'' is similar but consists of three or more objects, for example [[trinary star]]s and [[trinary asteroid]]s.

== Classification ==
In a binary system, the brighter or more massive object is referred to as '''[[primary (astronomy)|primary]]''', and the other the '''secondary'''. 

Binary stars are also classified based on orbit. '''Wide binaries''' are objects with orbits that keep them apart from one another. They evolve separately and have very little effect on each other. '''Close binaries''' are close to each other and are able to transfer mass from one another. They can also be classified based on how we observe them. '''Visual binaries''' are two stars separated enough that they can be disinguished through binoculars or a small telescope.

'''Eclipsing binaries''' are where the objects' orbits are at an angle that when one passes in front of the other it causes an [[eclipse]], as seen from Earth.
 
'''Astrometric binaries''' are objects that seem to move around nothing as their companion object cannot be identified, it can only be inferred. The companion object may not be bright enough or may be hidden in the glare from the primary object.

A related classification though not a binary system is '''optical binary''', which refers to objects that are so close together in the sky that they appear to be a binary system, but are not. Such objects merely appear to be close together, but lie at different distances from the Solar System.<ref>((cite web |url=http://www.space.com/22509-binary-stars.html |title=Binary Star Systems: Classification and Evolution |date=17 January 2018 |publisher=[[Space.com]]))</ref><ref>((cite book |publisher= [[Thüringer Landessternwarte Tautenburg]] |title=Analysis of Spectroscopic Binaries |first=Holger |last=Lehmann))</ref>

== Binary companion (minor planets) ==
[[File:Pluto-Charon_system-new.gif|thumb|right|The dwarf planet [[Pluto]] and its moon [[Charon (moon)|Charon]] are often described as a binary system in the [[Solar System]], which orbit the [[Sun]].]]
((main|Binary asteroid))

When binary minor planets are similar in size, they may be called "[[Binary asteroid|binary companions]]" instead of referring to the smaller body as a satellite.<ref name="IAU-list">((cite web |date=2009-09-17 |title=Satellites and Companions of Minor Planets |publisher=IAU / [[Minor Planet Center]] |url=http://www.cbat.eps.harvard.edu/minorsats.html |access-date=2011-01-08 |url-status=dead |archive-url=https://web.archive.org/web/20110121153650/http://www.cbat.eps.harvard.edu/minorsats.html |archive-date=2011-01-21 ))</ref> Good examples of true binary companions are the [[90 Antiope]] and the [[79360 Sila–Nunam]] systems. [[Pluto]] and its largest moon [[Charon (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, but Charon is small enough compared to Pluto that it is usually classified as a moon.<ref>((cite web |url=http://news.discovery.com/space/can-we-call-pluto-and-charon-a-binary-planet-yet-140808.htm |title=Can We Call Pluto and Charon a 'Binary Planet' Yet? |author=Ian O'Neill |website=Discovery News |date=8 August 2014 |access-date=15 July 2015))</ref>
The Sun and Jupiter orbit a point outside of either, but are not considered a binary because they are different kinds of objects.

==See also==
((Portal|Astronomy))
* [[Binary asteroid]]
* [[Binary star]]
* [[Contact binary]]
* [[Contact binary (small Solar System body)]]
* [[Double planet]]
* [[Rotational Brownian motion (astronomy)|Rotational Brownian motion]]

==References==
((reflist))

==External links==
*[https://www.nasa.gov/home/hqnews/2012/dec/HQ_12-425_Wide_Binary_Stars.html NASA Astrobiology Institute Shows How Wide Binary Stars Form - NASA]

==Bibliography==
*''Astronomy: A Visual Guide'' by Mark A. Garlick

[[Category:Celestial mechanics]]
[[Category:Binary systems| Binary system]]
[[Category:Astronomical dynamical systems]]

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