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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 what is known as an 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.

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