Contact binary

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In astronomy, the term contact binary refers to two astronomical bodies that are so close that they touch each other. Normally, the term may refer either to stars or to asteroids.

In the study of stars, a contact binary is a type of binary star in which both components of the binary fill their Roche lobes. The uppermost part of the stellar atmospheres forms a 'common envelope' that surrounds both stars. As the friction of the envelope brakes the orbital motion, the stars eventually coalesce, unless something unusual happens. An example of a contact binary star is W Ursae Majoris.

In the study of asteroids, a contact binary is caused when two asteroids gravitate toward each other until they touch, forming an oddly-shaped single body. Asteroids suspected of being contact binaries include the unusually elongated 624 Hektor and the bilobated 216 Kleopatra and 4769 Castalia. 25143 Itokawa, recently photographed by the Hayabusa probe, also appears to be a contact binary which has resulted in an elongated, bent body.Template:Star-stub