Algol is actually a three-star system ( Beta Persei A , B , and C ) in which the large and bright primary Beta Persei A is regularly eclipsed by the dimmer Beta Persei B. Thus , Algol 's magnitude is usually near-constant at 2.1 , but regularly dips to 3.4 every two days , 20 hours and 49 minutes during the roughly 10-hour long partial eclipses .
Algol is made of three stars ( Beta Persei A , B and C ) and the brightest is Persei A . These stars partly eclipse each other every 2 days , 20 hours and 49 minutes for 10 hours .