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Use Cassiopeia to find Perseus and the Ghoul Star

If you can locate the M- or W-shaped constellation Cassiopeia, you can find Perseus.  Then notice Algol, the Ghoul Star!

Tonight for October 22, 2015

Via Wikimedia and Caravaggio

The constellation Perseus as seen from mid-northern latitudes on an October evening. Perseus lords over the northeast sky, above the bright star Capella (lower left of Perseus) and the Pleiades star cluster (lower right of Perseus).

The constellation Perseus as seen from mid-northern latitudes on an October evening. Perseus lords over the northeast sky, above the bright star Capella (lower left of Perseus) and the Pleiades star cluster (lower right of Perseus).

EarthSky lunar calendars make great gifts for astronomy-minded friends and family.

Tonight … use the more prominent W-shaped constellation Cassiopeia the Queen to find the less obvious constellation Perseus and the Ghoul Star. As Halloween 2015 approaches, try looking for the star Beta Persei, otherwise known as Algol in the constellation Perseus. This star’s proper name comes from the Arabic for head of the ghoul, or head of the demon.

Find the Ghoul Star of Perseus. Why did early stargazers name this star for a ghoul or demon? Beta Persei, otherwise known as the Ghoul Star, is known to vary in brightness over a regular time interval. The cycle lasts 2 days, 20 hours and 49 minutes. All the while, the star remains visible to the eye.

Algol’s brightness variations are not due to some special quality of a single star. In fact, this is a multiple star system, where one star regularly passes in front of another as seen from our earthly perspective.

Algol is the Demon Star

When the dimmer of the two stars passes in front of the brighter, Algol shines at minimum brightness. The forecast calls for Algol to reach minimum brightness late tonight, on October 30 – at 11:52 p.m. CDT in the central U.S (that’s October 31 at 4:52 Universal Time).

Early stargazers had no way to know why Algol varied in brightness. But it’s likely that they did notice this star’s brightness change. Throughout parts of the ancient world, Algol was associated with demons or monsters. The Greeks and Romans identified the star with the Head of Medusa, a fearful monster with snakes in place of hair.

Bottom line: See the Ghoul Star, Beta Persei, around Halloween.

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Read more on Algol: The Demon Star