Algol, designated Beta Persei (β Persei, abbreviated Beta Per, β Per), known colloquially as the Demon Star, is a bright multiple star in the constellation of Perseus. The first and best known eclipsing binary and also one of the first (non-nova) variable stars to be discovered. A three-star system - Beta Persei Aa1, Aa2, and Ab - in which the large and bright primary β Persei Aa1 is regularly eclipsed by the dimmer β Persei Aa2. Thus, Algol's magnitude is usually near-constant at 2.1, but regularly dips to 3.4 every 2.86 days (2 days, 20 hours and 49 minutes) during the roughly 10-hour-long partial eclipses. There is also a secondary eclipse (the "second minimum") when the brighter star occults the fainter secondary. This secondary eclipse can only be detected photoelectrically.