Algol [key], famous variable star in the constellation Perseus; Bayer designation β Persei; 1992 position R.A. 3h07.7m, Dec. +40°55′. Algol's variation in apparent magnitude, from 2.06 to 3.28, is due to the fact that it is an eclipsing binary star, with one component revolving about the other with a period of 2 days, 20 hr, 49 min. Because the plane of revolution is almost parallel to the line of sight, the star dims noticeably when the dimmer component passes in front of, or eclipses, the brighter component, and dims again very slightly when the brighter component eclipses the dimmer one (see eclipse); the primary minimum, when the brighter component is eclipsed, lasts about 10 hr. Algol is of spectral class B8 V and is about 105 light-years from the earth. The name Algol comes from the Arabic Ras al Ghul, which means “demon's head,” and the star is sometimes called the Demon Star.
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