Algol, also known as Beta Persei in the constellation Perseus, is the prototype of a class of interacting binary stars which differ from cataclysmic binaries. It was the first eclipsing binary ever found. Viewed from Earth, Algol varies its brightness over a period of 2.9 days, with the changes caused by a a pair of stars periodically passing in front of each other and so cutting out the other's light. The larger of the two stars is losing material to its small companion, the gas forming a tenuous disc around the smaller star. Click the link to the right for an animated version.