Algol (Beta Persei), 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 actually a three-star system (Beta Persei A, B and C) in which the large and bright primary Beta Persei A is regularly eclipsed by the dimmer Beta Persei B. Thus, Algol's magnitude is usually near-constant at 2.1, but regularly dips to 3.4 every 2 days, 20 hours and 49 minutes during the roughly 10-hour long partial eclipses. There is also a secondary eclipse when the brighter star occults the fainter secondary.

In Star Trek. the Algolians are a humanoid species native to the Alpha or Beta Quadrant. They are members of the United Federation of Planets who sit on the Federation Archaeology Council and attended the symposium on the USS Enterprise-D in 2367. (TNG: "Qpid"). An Algolian musician played music in the background at the biennial Trade Agreements Conference held on Betazed in 2366. The song he played was later used as a distress call when Commander Riker, Counselor Troi, and Ambassador Troi were kidnapped by DaiMon Tog. (TNG: "Ménage à Troi")

As an eclipsing binary, Algol in reality is two stars in close orbit around one another. Because the orbital plane coincidentally contains the Earth's line of sight, 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.

To be more precise, however, Algol happens to be a triple star system: the eclipsing binary pair is separated by only 0.062 AU, while the third star (Algol C) is at an average distance of 2.69 AU from the pair and the mutual orbital period is 681 days (1.86 years). The total mass of the system is about 5.8 solar masses. Algol is 92.8 light years from Earth.

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. In some binaries similar to Algol, a gas flow can be seen.