In astronomy, a contact binary is a

binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

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. The term "contact binary" was introduced by astronomer Gerard Kuiper in 1941. Almost all known contact binary systems are eclipsing binaries; eclipsing contact binaries are known as

W Ursae Majoris variables A W Ursae Majoris variable, also known as a low mass contact binary, is a type of eclipsing binary variable star. These stars are close binaries of spectral types F, G, or K that share a common envelope of material and are thus in contact with one ...

, after their type star,

W Ursae Majoris W Ursae Majoris (W UMa) is the variable star designation for a binary star system in the northern constellation of Ursa Major. It has an apparent visual magnitude of about 7.9, which is too faint to be seen with the naked eye. However, i ...

. 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 temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

s and luminosities, regardless of their respective masses. The rate of energy transfer between the components is dependent on their mass ratio or absolute mass number and luminosity ratio. In cases where the stars are in geometric contact but the thermal contact is overall poor, there can exist wide differences between respective temperatures. And if contact binary system breaks down, neither of the two stars can actually form a new two-star system with the other star, temporarily attracting and soon losing their common center of mass, and with it their gravitational connection. 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.

References

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See also

References