Perseus constellation. Optical image of the centre of the constellation Perseus. North is at top. The bright star at centre is Mirfak (Alpha Persei). It is surrounded by several less bright stars, a grouping known as the Perseus OB association. O and B are the spectral classes of the majority of the stars, meaning they are among the hottest stars known. At lower right is the famous eclipsing binary star Algol (Beta Persei). This star changes in brightness between magnitudes 2.1- 3.4 with a period of just under three days. Algol is a very close double star, too close to be resolved from Earth. When the stars pass in front of each other, there is a drop in brightness.