Algol is the common name of the star Beta Persei. This star has the unusual property of having regular variations in brightness: its magnitude changes regularly between 2.3 and 3.5 over a period of 2 days, 20 hours and 49 minutes. Algol is a so-called eclipsing binary: it is actually two stars in close orbit about one another, and when (as seen from Earth's line of sight), the second, dimmer star passes in front of its companion once per orbit, the amount of light reaching earth is decreased, followed by an increase as the companion moves out of the way.

The variability of Algol was first recorded in 1670 by Montanari, but it is probable that this property was noticed long before this time; the name Algol means "demon star," (from Arabic al ghûl, "the ghoul") which was probably given due to its peculiar behavior. Astrologically, Algol was considered the most unfortunate star in the sky.

The Algol paradox was a problem of stellar evolution:

Components of a binary star form at the same time, and massive stars evolve much faster than the less massive ones. The more massive component of Algol is still in the main sequence. The less massive is a subgiant, showing 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. 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 actually be seen.)

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Algol is also a computer programming language; see Algol programming language.