Beta Persei, better known as Algol or the Demon Star, is actually three stars orbiting in a trinary star system, in which, viewed from Earth, one star eclipses a second star a little less than once every three days. Because only these two stars are visible from Earth, Algol is classified as an eclipsing binary star system.
Seen from Earth, Algol is located within the constellation Perseus, and is the winking eye of the Gorgon Medusa. Ancient observers from multiple cultures tended to assign it grim and frightening traits: Algol comes from the Arabic for "ghoul," but the ancient Jews knew it as "Satan's Head" and the ancient Chinese called it "piled up corpses." Astrology has long associated Algol with misfortune.
Algol's tendency to "blink" or "wink" at Earth observers was first officially observed in the 17th century by Italian astronomer Geminiano Montanari (unwritten observations probably preceded this by a substantial time). It took over a century for a British astronomer to propose an explanation for the blinking effect. The individual in question, John Goodricke, turned out to be essentially right about the process involved, if not the actual reason. Goodricke suggested that there must be some regularly orbiting, large dark object which passed between Algol and the Earth, causing temporary drops in the brightness of the star in a manner analogous to solar eclipses.
Goodricke was not correct, but it took over a century for astronomers to come up with another explanation. This time, it was a late 19th-century American scientist, Edward Pickering, who first suggested that the blinking was instead caused because the "demon star" Algol was actually two stars. Pickering turned out to be correct: Algol blinks every several days because one of the stars passes in front of the other. This causes a temporary drop in light received from Algol because the light of the first star is blocked by the pass of the second.
Modern astronomy has subsequently identified three specific stars in the Algol system: Algol A and Algol B, which orbit each other closely and cause the "winking" effect through their regular eclipses, as well as Algol C, which orbits the pair at a distance similar to that of the Asteroid Belt in our own solar system. Today, Algol is about 93 light-years distant from the Earth, but the stars have been much closer in the past, before drifting apart into their current positions. Astronomers have estimated that Algol was within ten light-years of Earth several million years ago, closer than all stars but our nearest neighbours, such as Alpha Centauri and Barnard's Star.