Post common envelope binary
= Post common envelope binary
{wiki=Post_common_envelope_binary}
A post-common envelope binary is a type of binary star system that evolves from an earlier stage known as a common envelope phase. In a binary star system, two stars orbit around a common center of mass. When one of the stars expands significantly—often as it evolves off the main sequence—it can engorge its companion within its outer layers, creating a common envelope of gas that surrounds both stars.