Beta Persei is Algol, one of the most notable of variable stars.

Perseus is represented here holding the head of the Gorgon Medusa, whose head he has just severed. One look from the Gorgon's evil eye and the victim was dead.

Algol, ‘The Demon’, represents the sparkling eye of Medusa. The name indicates that the ancients also knew that the star shone with a variable magnitude.

Algol is the prototype of an important type of variable known as the eclipsing variable.
     Two close stars orbit each other, with the visual magnitude changing as they block out each other's light. In Algol's case, these eclipses happen every 2.9 days, with the star's visual magnitude fluctuating between 2.2 and 3.5, a very noticeable difference. This minimum lasts about ten hours before the maximum magnitude is again reached.

Algol is quickly found by first locating Mirfak (alpha Persei). Once Mirfak is located, move opposite to the Pole Star (due south) ten degrees: binoculars.