Brightest Stars

Algol is the Demon Star

Star chart for Perseus and Algol the Demon Star
Find Algol the Demon Star in the constellation Perseus on autumn evenings. Perseus lies below the easy-to-recognize W-shaped constellation Cassiopeia.

Why the Demon Star?

What’s the scariest star in the sky? If you were one of the early stargazers, you might have chosen Algol in the constellation Perseus. Early astronomers nicknamed Algol the Demon Star. Shivers!

When you look at Algol, it doesn’t appear any scarier that any other star. In skylore it’s associated with a mythical scary monster – the Gorgon, Medusa – who had snakes for hair. Legend said that her appearance was so terrifying that if anyone even looked at her they would turn to stone.

The star Algol takes its name from an Arabic word meaning the Demon’s Head or, literally the Ghoul. It represents the terrifying snaky head of the Medusa monster.

In the mythology of the skies, Perseus – a great hero often depicted mounted on Pegasus the Flying Horse – slayed Medusa. Then, he used Medusa’s head to his advantage, showing it to Cetus the sea monster to turn him into stone. Perhaps the ancients associated this star’s variable brightness with the evil, winking eye of the Medusa.

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Painting of the Gorgon Medusa with snakes in place of hair.
The Gorgon Medusa had snakes in place of hair. Eek! Image via Wikimedia/ Caravaggio.

Algol is a variable star

Winking? Yes. Algol is a known variable star, which waxes and wanes in brightness.

The early stargazers surely knew about its changing brightness. This probably led them to name the strangely behaving star in a sky full of steadily shining stars for a mythological demon.

There are many variable stars known throughout the heavens, but Algol might well be the most famous of them all. The Demon Star brightens and dims with clockwork regularity, completing one cycle in two days, 20 hours, and 49 minutes. Plus, you can view its entire cycle with your eye alone.

Algol’s variation is easy to observe. At its brightest, Algol shines about three times more brightly than at its faintest. When it reaches maximum brilliance, Algol matches the brightness of the nearby second-magnitude star Almach. At minimum, Algol’s light output fades to that of the star Epsilon Persei.

Modern-day astronomy has unlocked the secret of Algol’s mood swings. It’s an eclipsing binary star. This kind of binary is composed of two stars, with each star revolving around the other. From Earth, we see the orbital plane of this binary star almost exactly edge-on. Therefore, when the dimmer of the two stars swings in front of the brighter star, we see Algol at minimum brightness.

Large and small stars rotate around each other with graph of brightness.
Animation of an eclipsing binary star. Image via Wikimedia Commons.

How to find Algol

The Demon Star easy to find. Our sky chart shows the northeastern sky for autumn evenings, especially around Halloween.

The conspicuous W or M-shaped constellation Cassiopeia enables you to star-hop to Perseus. Look below Cassiopeia toward the horizon to spot the dangling icicle shape of Perseus. Off to the right of the icicle is Algol. At mid-northern latitudes, the Demon Star appears for at least part of the night all year round. But it’s best seen in the evening sky from autumn to spring. It’s visible in the northeast sky in autumn, shines high overhead in winter, then swings to the northwest sky by spring.

Antique star chart etching with Greek hero with sword in one hand and Medusa's head in the other.
Perseus and Medusa from Uranographia by Johannes Hevelius. Image via Wikimedia Commons.

Bottom line: Algol has the nickname the Demon Star because it represents the head of Medusa. This variable star probably intrigued the ancients with its inconstant behavior.

October 29, 2021
Brightest Stars

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Bruce McClure

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