The Demon Star, also known as Algol, is a binary star system located in the constellation Perseus. It is composed of two stars, Algol A and Algol B, which orbit each other every 2.87 days. Algol is one of the brightest stars in the night sky and has been observed since ancient times. It is often referred to as the 'Demon Star' due to its variable brightness, which is caused by the eclipsing of Algol A by Algol B.