Contact binary

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.

[1][2] The term "contact binary" was introduced by astronomer Gerard Kuiper in 1941.

[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.

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.