Algol, also known as the Demon Star, is a bright star in the Perseus constellation. It is one of the best known double stars that can only be seen in spectroscopes, the first star of its type discovered, and one of the first variable stars to be discovered before it explodes. 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.