Algol

Algol is a multiple-star system in the constellation Perseus, best known for an eclipsing inner binary whose changing brightness defines Algol-type variable stars.

Algol-type variable star: An eclipsing binary whose regular brightness changes resemble those of Algol. Algol is the prototype that gives this class its name.
Binary star: A system of two stars bound together by their mutual gravity. Algol’s main eclipsing pair is a binary within a larger system.
Johannes Hevelius: A seventeenth-century Polish astronomer known for detailed observations and star catalogues. Hevelius recorded Algol’s brightness variation in early systematic observations.
Light curve: A graph showing how an astronomical object's measured brightness changes over time. Algol’s light curve displays the primary and secondary eclipses used to model its orbit.
Intrinsic variable star: A star whose brightness changes because its own physical properties vary. Algol dims mainly when one star eclipses another, rather than through intrinsic pulsation.
Eclipsing binary: A binary star system whose orbit carries one star in front of the other from Earth’s viewpoint. Mutual eclipses of Algol’s inner pair cause its prominent brightness dips.
Triple star system: A gravitationally bound system containing three stars. Algol’s eclipsing binary has a more distant third stellar companion.
John Goodricke: An eighteenth-century English astronomer who explained Algol’s periodic dimming as an eclipse. Goodricke proposed that an orbiting body periodically obscures Algol’s light.
Binary star mass: The masses of stars in a gravitationally bound pair, inferred from orbital observations. Algol’s orbit and spectrum let astronomers estimate its component masses.
Pulsating variable star: A variable star whose radius and temperature change periodically, altering its brightness. Unlike pulsators, Algol’s main brightness changes arise from eclipses.

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