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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.[1][2] The term "contact binary" was introduced by astronomer Gerard Kuiper in 1941.[3] Almost all known contact binary systems are eclipsing binaries;[4] eclipsing contact binaries are known as W Ursae Majoris variables, after their type star, W Ursae Majoris.[5]

In a contact binary, both stars have filled their Roche lobes, allowing the more massive primary component to transfer both mass and luminosity to the secondary member. As a result, the components in a contact binary often have similar effective temperatures and luminosities, regardless of their respective masses. The rate of energy transfer between the components is dependent on their mass ratio and luminosity ratio. In cases where the stars are in geometric contact but the thermal contact is poor, there can exist wide differences between their respective temperatures.[6]

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.[7]
See also

Contact binary (small Solar System body), two asteroids gravitating toward each other until they touch.
HR 5171, a yellow hypergiant and yellow supergiant contact binary with a third star orbiting further out.
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.
Thorne–Żytkow object, a type of star wherein a red giant or supergiant contains a neutron star at its core.
VFTS 352, a massive contact binary in the Tarantula Nebula.


Darling, David. "binary star". Retrieved 2019-05-06.
Thompson, Michael J. (2006). An Introduction to Astrophysical Fluid Dynamics. London: Imperial College Press. pp. 51–53. ISBN 1-86094-615-1.
Kuiper, Gerard P. (1941). "On the Interpretation of β Lyrae and Other Close Binaries". Astrophysical Journal. 93: 133. Bibcode:1941ApJ....93..133K. doi:10.1086/144252.
Tassoul, Jean Louis; et al. (2000). Stellar Rotation. Cambridge, UK, New York: Cambridge University Press. p. 231. ISBN 0-521-77218-4.
Mullaney, James (2005). Double and Multiple Stars and how to Observe Them. New York, London: Springer. p. 19. ISBN 1-85233-751-6.
Csizmadia, Sz.; Klagyivik, P. (November 2004). "On the properties of contact binary stars". Astronomy and Astrophysics. 426: 1001–1005.arXiv:astro-ph/0408049. Bibcode:2004A&A...426.1001C. doi:10.1051/0004-6361:20040430.
Ivanova, N.; et al. (2013). "Common envelope evolution: where we stand and how we can move forward". The Astronomy and Astrophysics Review. 21: 59.arXiv:1209.4302. Bibcode:2013A&ARv..21...59I. doi:10.1007/s00159-013-0059-2.


Accretion Molecular cloud Bok globule Young stellar object
Protostar Pre-main-sequence Herbig Ae/Be T Tauri FU Orionis Herbig–Haro object Hayashi track Henyey track


Main sequence Red-giant branch Horizontal branch
Red clump Asymptotic giant branch
super-AGB Blue loop Protoplanetary nebula Planetary nebula PG1159 Dredge-up OH/IR Instability strip Luminous blue variable Blue straggler Stellar population Supernova Superluminous supernova / Hypernova

Spectral classification

Early Late Main sequence
O B A F G K M Brown dwarf WR OB Subdwarf
O B Subgiant Giant
Blue Red Yellow Bright giant Supergiant
Blue Red Yellow Hypergiant
Yellow Carbon
S CN CH White dwarf Chemically peculiar
Am Ap/Bp HgMn Helium-weak Barium Extreme helium Lambda Boötis Lead Technetium Be
Shell B[e]


White dwarf
Helium planet Black dwarf Neutron
Radio-quiet Pulsar
Binary X-ray Magnetar Stellar black hole X-ray binary


Blue dwarf Green Black dwarf Exotic
Boson Electroweak Strange Preon Planck Dark Dark-energy Quark Q Black Gravastar Frozen Quasi-star Thorne–Żytkow object Iron Blitzar

Stellar nucleosynthesis

Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova
Symbiotic Remnant Luminous red nova


Core Convection zone
Microturbulence Oscillations Radiation zone Atmosphere
Photosphere Starspot Chromosphere Stellar corona Stellar wind
Bubble Bipolar outflow Accretion disk Asteroseismology
Helioseismology Eddington luminosity Kelvin–Helmholtz mechanism


Designation Dynamics Effective temperature Luminosity Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation Starlight Variable Photometric system Color index Hertzsprung–Russell diagram Color–color diagram

Star systems

Contact Common envelope Eclipsing Symbiotic Multiple Cluster
Open Globular Super Planetary system


Solar System Sunlight Pole star Circumpolar Constellation Asterism Magnitude
Apparent Extinction Photographic Radial velocity Proper motion Parallax Photometric-standard


Proper names
Arabic Chinese Extremes Most massive Highest temperature Lowest temperature Largest volume Smallest volume Brightest
Historical Most luminous Nearest
Nearest bright With exoplanets Brown dwarfs White dwarfs Milky Way novae Supernovae
Candidates Remnants Planetary nebulae Timeline of stellar astronomy

Related articles

Substellar object
Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Guest Gravity Intergalactic Planet-hosting stars Tidal disruption event

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