A binary system is an astronomy term referring to two objects in space, usually stars, which are so close that their gravitational forces attract one another into a mutual orbit. Astronomers find that more than 50% of stars are binary stars. A multiple system is like a binary system but consists of three or more objects ( star system ) .

See Binary star for the common case of a binary system composed of two stars.

List of a few nearby binary systems

Location	Satellite A	Satellite B
Kuiper Belt	Pluto	Charon
4.36 lightyears from the Sun	Alpha Centauri A	Alpha Centauri B

Brightness Fluctuation

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Sometimes, a binary system consists of a large dim star and a small bright one, causing fluctuations in brightness of the system. If the bright star is behind the dim one the system appears dim. If the bright star is in front of the dim one, the system has some intermediate brightness. If the stars are next to each other, the system appears very bright.

References

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• Astronomy: A Visual Guide by Mark A. Garlick

Celestial mechanics | Binary stars

Sistema binari (astronomia) | Binární systém | Planeta binário | Hệ sao đôi