Final answer:
Algol, known as the 'demon star' in the constellation Perseus, is an eclipsing binary system whose brightness changes regularly as one star passes in front of the other.
Step-by-step explanation:
Algol, also known as Beta Persei, is a famous star located in the constellation of Perseus. Known as the 'demon star', Algol is a bright star that exhibits changes in its brightness due to it being an eclipsing binary system. This means that Algol is not a single star, but a pair of stars that orbit a common center of mass, and as one star passes in front of the other, it causes the brightness we observe to decrease periodically. Algol's brightness fades to one-third its regular brightness at regular intervals of approximately 2 days, 20 hours, 49 minutes. This phenomenon is interpreted as the dimmer star of the pairing eclipsing the brighter one, causing the apparent fluctuation in brightness.