Algol (Beta Persei), known colloquially as the Demon Star, is a bright star in the constellation Perseus and 92.8 light years from Earth. Algol is one of the best known eclipsing binaries  and the first such star to be discovered - it is actually a three-star system  in which the large and bright primary Beta Persei A is regularly eclipsed by the dimmer Beta Persei B. As an eclipsing binary, Algol in reality is two stars in close orbit around one another. The dimmer star (Algol B) passes in front of the brighter star (Algol A) once per orbit, and the amount of light reaching Earth is temporarily decreased. The larger of the two stars is losing material to its small companion, the gas forming a tenuous disc around the smaller star.




Every 69 hours, Algol's yellow component crosses in front of its blue companion star - reduces the star's brightness for four hours.

In Star Trek: The Next Generation, the Algolians were a humanoid species who were members of the United Federation of Planets.  In the episode "Ménage à Troi",  an Algolian musician played music in the background at the biennial Trade Agreements Conference held on Betazed  and the song he played was later used as a distress call when Commander Riker, Counselor Troi, and Ambassador Troi were kidnapped by Ferengi.

Studies of Algol led to the Algol paradox in the theory of stellar evolution: although components of a binary star form at the same time, and massive stars evolve much faster than the less massive ones, it was observed that the more massive component Algol A is still in the main sequence, while the less massive Algol B is a subgiant star at a later evolutionary stage. The paradox can be solved by mass transfer: when the more massive star became a subgiant, it filled its Roche lobe, and most of the mass was transferred to the other star, which is still in the main sequence.





The larger of the two stars is losing material to its
small companion, the gas forming a tenuous disc
around the smaller star.