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). 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.