In astronomy, a contact binary is a binary star system whose component stars are so close that they touch each other or have merged to share their gaseous envelopes. A binary system whose stars share an envelope may also be called an overcontact binary. Almost all known contact binary systems are eclipsing binaries; eclipsing contact binaries are known as W Ursae Majoris variables, after their type star, W Ursae Majoris.
Contact binaries are not to be confused with common envelopes. Whereas the configuration of two touching stars in a contact binary has a typical lifetime of millions to billions of years, the common envelope is a dynamically unstable phase in binary evolution that either expels the stellar envelope or merges the binary in a timescale of months to years.
- Contact binary (small Solar System body), two asteroids gravitating toward each other until they touch.
- Thorne–Żytkow object, a type of star wherein a red giant or supergiant contains a neutron star at its core.
- VFTS 352, a contact binary in the Tarantula Nebula.
- KIC 9832227, a contact binary and a previous candidate for stellar merger.
- Luminous red nova, e.g. V1309 Scorpii (2008), may result from the merger of a contact binary.
- Darling, David. "binary star". www.daviddarling.info. Retrieved 2019-05-06.
- contact binary Archived 2010-08-17 at the Wayback Machine, David Darling, The Internet Encyclopedia of Science. Accessed on line November 4, 2007.
- overcontact binary Archived 2011-10-17 at the Wayback Machine, David Darling, The Internet Encyclopedia of Science. Accessed on line November 4, 2007.
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