Don't be surprised if you're looking at the constellation Perseus one night and notice one of the stars winking at you. That would be Algol, the Demonic Star. Every two days, 20 hours and 49 minutes, Algol fades to a third of its usual brightness - for two hours - and then quickly returns to its original brightness. It's as if Algol is winking. It takes Algol less than ten hours to go from its normal brightness to its minimum brightness and then back to normal, so the star's entire performance can be observed in a single night. It is a perfect example of an "eclipsing binary" star, brightening and dimming as regularly as a clock. Algol is one of the most popular and well-known variable stars in the sky because it can be observed with the naked eye and has a relatively short period of less than three days. This means that if you go out every night and do a careful check, sooner or later you will see Algol winking at you. Located in the constellation Perseus, the Hero, Algol has been known since ancient times as "The Demon Star". Her name comes from the Arabic word al-ghul, meaning "female demon". Its name is due to the position of Algol which marks the eye of Medusa, a demon from ancient Greek mythology. According to the myth, Medusa had hissing snakes instead of hair and a gaze capable of turning anyone who looked at it to stone. Perseus avoided this fate by observing Medusa's harmless reflection in a mirror and then striking her head with his sword. Horrific and inexplicable to early skygazers, Algol's periodic blinking is no longer a mystery. History credits Geminiano Montanari, a mathematics professor at the Universities of Bologna and Padua in Italy, with the discovery of Algol's variability. From 1667 to...... middle of document ......iod of 4.13 days.• DI Herculis is an Algol-type binary star in the constellation Hercules. The system has an overall magnitude of about 8.5 and consists of two young blue stars of spectral types B5 and B4. The magnitude varies between 8.4 and 9.1 during an eclipse over the last 10.55 days. • Beta Aurigae is a binary star system in the constellation Auriga. The combined apparent visual magnitude of the system is 1.9, making it the second brightest member of the constellation after Capella. The combined apparent magnitude varies over a period of 3.96 days between 1.89 and 1.94, as every 47.5 hours one of the stars partially eclipses the other. While this is just a short list of Algol binaries, you can choose from many hundreds to look at. The latest edition of the General Catalog of Variable Stars lists 3,554. This is 9% of all variable stars.