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The star Beta Persei, the first eclipsing binary to be discovered. Algol was found to be variable in 1669 by the Italian astronomer and mathematician Geminiano Montanari (1633–87), but the period was first determined by J. Goodricke in 1782–3. Algol varies from magnitude 2.1 to 3.4 in a period of 2.867 3043 days, although the period has varied slightly. The variable period and the emission lines that are sometimes detectable in its spectrum are evidence for mass transfer, indicating that the system is a semidetached binary. The eclipsing pair, which consists of a B8 dwarf and a K2 subgiant, is accompanied by a third component (Algol C), magnitude 4.7, having an orbital period of 1.862 years. The system is a faint X-ray source and also emits radio bursts. Algol lies 90 l.y. away.

Subjects: Astronomy and Astrophysics.

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