A star in the constellation Perseus, also known as Beta Persei. Algol is a prototype of eclipsing binaries, close double stars that periodically pass in front of each other as they move along their orbits, temporarily blocking the light from the other star. Every 2 days and 21 hours, Algol, which is normally a 2nd magnitude star, dips to one third of its usual brightness and remains there for ten hours as its dimmer companion partially eclipses it, blocking some of its light.