The Demon Star: Algol, The Demon Star

Don’t be surprised if you’re observing the constellation Perseus one night and notice one of the stars wink at you. That would be Algol, the Demon Star.

Every two days, 20 hours and 49 minutes, Algol fades to one-third of its usual brightness - for two hours - and then quickly returns to its original luster. It's as if Algol is winking. Algol takes less than ten hours to go from its normal brightness down to its minimum light and then back to normal, so the star's whole performance can be watched in a single night. It is a perfect example of an "eclipsing binary" star, which brightens and dims almost as regular as clockwork.

Algol is one of the most popular and well known variable stars in the sky because it can be observed with the unaided eye and it has a relatively short period of less than three days. This means you if go outside every night and make a careful check of it, sooner or later you will see Algol wink at you.

Located in the constellation of Perseus, the Hero, Algol has been known as "The Demon Star" since ancient times. Its name comes from the Arabic word al-ghul, which means "female demon." Its name is due to Algol's position marking the eye of the Medusa, a demon in ancient Greek mythology. According to the myth, Medusa had hissing snakes for hair and a gaze that would turn any onlooker into stone. Perseus avoided this fate by watching Medusa's harmless reflection in a mirror and then whacking her head off with his sword.

Horrifying and unexplainable to early sky watchers, Algol's periodic winking is no longer a mystery. History credits Geminiano Montanari, mathematics professor at the Universities of Bologna and Padua in Italy, with the discovery of Algol’s variability. From 1667 to...

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...iod of 4.13 days.

• DI Herculis is an Algol-type binary star in the constellation of Hercules. The system has an ensemble magnitude of about 8.5 and consists of two young blue stars of spectral type B5 and B4. The magnitude varies between 8.4 and 9.1 during an eclipse last 10.55 days.

• Beta Aurigae is a binary star system in the constellation of 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.

Although this is just a short list of Algol binaries, you can choose from many hundreds to observe. The latest edition of the General Catalogue of Variable Stars lists 3,554 of them. That’s 9 percent of all variable stars.