Inastronomy, acontact binary is abinary star system whose component stars are so close that they touch each other or have merged to share their gaseous envelopes. Abinary system whose stars share an envelope may also be called anovercontact binary.[1][2] The term "contact binary" was introduced by astronomerGerard Kuiper in 1941.[3] Almost all known contact binary systems areeclipsing binaries;[4] eclipsing contact binaries are known asW Ursae Majoris variables, after their archetype star,W Ursae Majoris.[5]
In a contact binary, both stars have filled theirRoche 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 similareffective 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 withcommon 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]