Perseus constellation. Optical image of Perseus, is a large northern constellation. In Greek myth, Perseus rescues Andromeda, the Princess, who had been chained to a rock and left for Cetus, the Sea Monster, by her parents, Cepheus, the King, and Cassiopeia, the Queen. Perseus later marries Andromeda. Perseus contains the famous variable star Algol (Beta Persei). This star varies between magnitude 2.1 and 3.4 as it is an eclipsing binary, a system of two stars in a mutual orbit which eclipse each other as seen from Earth, blocking the eclipsed star's light for a short time.