Object Analysis

Algol, designated Beta Persei, is a bright multiple star in the constellation Perseus. It is the prototype of "Algol variables," a class of eclipsing binary stars. Its variability was first recorded in 1667 by Geminiano Montanari, but it was known as the "Demon Star" in ancient times due to its "winking" eye behavior.

The system consists of a hot, bright blue-white main-sequence star and a cooler, fainter orange subgiant. Every 2.86 days, the fainter star passes in front of the brighter one, causing the total brightness of the system to dip from magnitude 2.1 to 3.4 for about 10 hours.

Algol presents the "Algol Paradox": the less massive star is more evolved (a subgiant) than the more massive star (main sequence). Standard stellar evolution says massive stars evolve faster. The solution is that the subgiant was originally more massive but transferred most of its mass to its partner as it expanded.