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October 31, 2016

Ghosts and goblins and ghouls prowl the streets tonight. And they also prowl the sky. In fact, four stars in the constellation Perseus represent the Gorgons — the mythological sisters whose heads were covered with snakes. One look at them turned the viewer to stone.

The most prominent of these stars really does do something strange. For a couple of hours every three days or so, it fades dramatically. Ancient skywatchers must have found that a bit frightening, because they gave the star a spooky name: Ras Al-gul — “the demon’s head.” Today, it’s known by a shorter version of the name: Algol.

The star doesn’t really do anything ghoulish, though. Instead, its dimming spells are the result of eclipses.

Algol consists of three stars. Two of them are quite close together, so they orbit each other in a bit less than three days. We see them eclipse each other during each orbit.

One of the stars is a couple of hundred times brighter than the Sun, while the other is much fainter. But the fainter star is also the bigger of the two. So when it passes in front of the brighter star, the system drops to about a third of its normal brightness. Skywatchers in ancient times knew nothing about the cause of the star’s fading act, though — only that it was a ghoulish thing for a star to do.

This “demon” star is low in the northeast as night falls, not far to the lower right of Mirphak, Perseus’s brightest star. It’s a perfect companion for a Halloween night.


Script by Damond Benningfield

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