Illustration showing 3 types of binary starsystems. A binary star is a pair of two stars, held together by the force of gravity. The brighter star is the primary star and the other is the secondary star. At top, detached binary: There is no physical contact between the stars, and neither has filled it's Roche lobe. Middle, a Semi-detached binary: One of the stars has filled its Roche lobe and is an egg shape due to gravitational distortion of a very close neutron star or a black hole. There is a mass transfer stream present between the two stars. At bottom, Contact binary: In this case, both stars have filled their Roche lobe and have a common envelope (CE). CE is an initiated unstable mass transfer from the primary star to its companion star.