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A binary system is a system of two astronomical bodies which are close enough that their gravitational attraction causes them to orbit each other around a barycenter (also see animated examples). More restrictive definitions require that this common center of mass is not located within the interior of either object, in order to exclude the typical planet–satellite systems and planetary systems.
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The most common binary systems are binary stars and binary asteroid, but brown dwarfs, planets, neutron stars, black holes and galaxies can also form binaries.
A multiple system is like a binary system but consists of three or more objects such as for trinary stars and trinary asteroids.
Classification
In a binary system the brightest object is referred to as primary, and the other the secondary.
They are also classified based on orbit. Wide binaries are objects with orbits that keep them a part from one another. They evolve separately and have very little effects on each other. Close binaries are close to each other and are able to transfer mass from one another. They may also exert a gravitational force on each other.
They can also be classified based on how we observe them. Visual binaries are two stars separated enough that they can be viewed through a telescope or binoculars.
Eclipsing binaries are where the object's 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 that they appear to be a binary system but are not.
Binary companion (minor planets)
When binary minor planets are similar in size, they may be called "binary companions" instead of referring to the smaller body as a satellite. 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.
Close binary stars
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.