Abinary system is a system of twoastronomical bodies of the same kind that are comparable in size. Definitions vary, but typically require thecenter of mass to be located outside of either object. (Seeanimated examples.)
The most common kinds of binary system arebinary stars andbinary asteroids, butbrown dwarfs,planets,neutron stars,black holes andgalaxies can also form binaries.
Amultiple system is similar but consists of three or more objects, for exampletriple stars andtriple asteroids (a more common term than 'trinary').
In a binary system, the brighter or more massive object is referred to asprimary, and the other thesecondary.
Binary stars are also classified based on orbit.Wide binaries are objects with orbits that keep them apart from one another.[1] 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 distinguished through binoculars or a small telescope.[2]
Eclipsing binaries are where the objects' orbits are at an angle that when one passes in front of the other it causes aneclipse, 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 isoptical 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.[3][4]
When binary minor planets are similar in size, they may be called "binary companions" instead of referring to the smaller body as a satellite.[5] Good examples of true binary companions are the90 Antiope and the79360 Sila–Nunam systems.Pluto and its largest moonCharon are sometimes described as a binary system because thebarycenter (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.[6]Orcus and its moonVanth also orbit around a barycenter not inside either of them.The Sun and Jupiter orbit a point outside of either, but are not considered a binary because they are different kinds of objects.
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