Algol, << AL gol, >> also called Beta Persei, is the second brightest star in the constellation Perseus. It is about 100 light-years from the earth (see Light-year ). Its name derives from an Arabic word meaning ghoul, likely due to the star’s unusual periodic brightening and dimming. Algol appears to be a single star, but it is actually a type of double star called an eclipsing binary. In such a binary, a pair of stars revolve around each other so that one periodically blocks the light of the other. This action reduces the brightness of the double star as viewed from the earth.
Algol’s variations in brightness can be seen with the unaided eye. About every three days, Algol loses two-thirds of its brightness in a period of five hours. In another five hours, it returns to its normal apparent magnitude of 2.3 (see Star (Brightness of stars) ). Algol was one of the first eclipsing variables to be discovered.
The distance between the centers of the two stars is about 7 million miles (11 million kilometers). One star has a surface temperature of about 22,000 °F (12,000 °C). The second, fainter star is only one-third as hot. A third, more distant star belongs to the Algol system. It orbits the other two stars, but it does not take part in the eclipses.
