Many stars have great names. But one of the best has to be Ra’s al-Ghul — better known as Algol, “the head of the ghoul” or the demon star. It represents Medusa, a monster from Greek mythology. Its head was covered in snakes, and one glance at her instantly turned the viewer to stone.

The name probably was bestowed because Algol does something that skywatchers in ancient times found a bit scary: It periodically gets a lot fainter. The heavens were thought to be eternal and unchanging, so such a star was considered a big problem.

The cause of its behavior isn’t scary at all, though.

Algol includes two stars that are locked in a tight orbit around each other — they’re just a few million miles apart. One of the stars is big and bright — almost 200 times brighter than the Sun. The other star is much smaller and fainter. Every two days, 20 hours, and 49 minutes, the fainter star passes in front of the brighter one. That causes the system to drop to just a third of its normal brightness. An eclipse lasts about 10 hours, with about four hours at minimum brightness. After that, the fainter star moves out of the way, and Algol roars back to full brilliance.

Algol’s next eclipse will take place tomorrow night. The star is high in the east-northeast as night falls, in Perseus the hero — the character who killed Medusa. If you can find Algol, check it out tonight, then again late tomorrow night — to see the action of a “ghoulish” star.

Script by Damond Benningfield

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