Editing Contact binary (section)

{{Short description|Binary star system whose component stars are very close}}
{{About|contact binary stars|comets and minor planets|Contact binary (small Solar System body)}}
[[File:An artist’s impression of the hottest and most massive touching double star.jpg|thumb|upright=1.1|right|Artist's rendering of the massive contact binary star [[VFTS 352]], located in the [[Large Magellanic Cloud]]]]

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.<ref name=DarlingBinary/><ref name=Thompson2006/> The term "contact binary" was introduced by astronomer [[Gerard Kuiper]] in 1941.<ref name=Kuiper1941/> Almost all known contact binary systems are [[Binary star#Eclipsing binaries|eclipsing binaries]];<ref name=Tassoul2000/> eclipsing contact binaries are known as [[W Ursae Majoris variables]], after their archetype star, [[W Ursae Majoris]].<ref name=Mullaney2005/>

In a contact binary, both stars have filled their [[Roche lobe]]s, 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 temperature]]s 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.<ref name=Csizmadia2004/>

Contact binaries are not to be confused with [[common envelope]]s. 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.<ref name=Ivanova2013/>