A binary star in which two stellar cores are immersed in an extremely large, common envelope of gas. All close binaries that contain white dwarfs, neutron stars, or black holes have gone through such a process. These range from the relatively common cataclysmic variables that feature a white dwarf to the more exotic objects such as X-ray binaries containing neutron stars or presumed black holes. In addition, some merged objects such as the FK Comae Berenices stars could be products of the same process. The envelope arises when one member of the binary becomes a red giant and fills its Roche lobe. Mass transfer leaves the core of the giant and the relatively denser companion deep inside a common envelope. Friction then causes the cores to spiral towards each other. The angular momentum and energy they lose in the process is transferred to the envelope, which then spins up and sheds mass in response. The phase comes to an end either when the whole envelope has been lost, leaving an exposed compact binary, or the two cores merge within the rapidly spinning envelope.