 Algol ( Beta Per , β Persei , β Per ) , known colloquially as the Demon Star , is a bright star in the constellation Perseus . It is one of the best known eclipsing binaries , the first such star to be discovered , and also one of the first ( non @-@ nova ) variable stars to be discovered . Algol is a semiregular variable star , with a period of 143 days, 20 hours, 49 minutes, 24 seconds, and a brightness that varies from magnitude 2. Unterscheidung zwischen den verschiedenen Klassen von Demonstheulen ist nicht immer einheitlich