Astro Bob: Bewitched by the Demon Star

Watch the binary star Algol in Perseus brighten as it emerges from eclipse tonight, Jan. 25.

Algol model
In this artist's view, we see that Algol is a binary star — two suns in close orbit about their common center of gravity. Every 2.86 days, the larger, fainter star deeply eclipses the smaller, brighter one. The change in the system's brightness during and after eclipse is easily visible without optical aid.
Contributed / Pablo Carlos Budassi

Algol in the constellation Perseus is one of the sky's most remarkable stars. It knows how to wink. Which is another way of saying that its light is variable, and the changes are easily visible with the naked eye. Many variable stars pulsate, inflating and deflating like balloons, which causes them to brighten and fade. Not Algol. It's an eclipsing binary — two stars in very close orbit about their common center of gravity. Algol A is the smaller sun, but it's hotter and brighter than Algol B, a cooler and fainter orange giant.

Graph showing Algol eclipses
This graphic shows how the combined light of Algol dips steeply during eclipse, when Algol B passes in front of Algol A. A small, secondary eclipse (the tiny dip) occurs when Algol B swings around the backside of Algol A.
Contributed / Public domain with additions by Bob King

Like clockwork every 2.9 days, Algol B passes in front of and partially eclipses its shinier companion. Eclipses last about 10 hours, from the time Algol B first encroaches on Algol A, to recovery, when the two stars separate. Minimum light, when Algol appears faintest, occurs at mid-eclipse and lasts about 2 hours. Thirty hours after the main eclipse, the bright star partially eclipses the fainter companion, and the system dims again but this time by only a fraction of a magnitude.

To watch an entire eclipse means a long night under the stars. This isn't exactly realistic in winter when the cold chases you inside after 20 minutes. Instead, why not watch Algol fade to minimum or return to maximum as it exits eclipse? Why not indeed.

Algol bright and dim
You can see Algol's changing brightness in this panel of before and after-eclipse photos. The outlined constellation is Perseus the Hero.
Contributed / Bob King

Tuesday, Jan. 25, skywatchers across the eastern two-thirds of the the Americas you'll have a chance to see Algol re-brighten as it comes out of eclipse. The star will be faintest at magnitude 3.4 through 5:30 p.m. CST (6:30 p.m. EST, 4:30 p.m. MST) and then slowly brighten and return to peak light around 9:30 p.m. CST. At maximum, it shines at magnitude 2.1, as bright as the stars in the Big Dipper.

I suggest you step out for a few minutes to make an observation around 6 p.m. CST (7 p.m. EST), when Algol will still be close to minimum light. Then go out for a second peek around 7:30 p.m. and a third at 9:30 or later, when it returns to full brilliance. Or you can check just twice — once at 6 and again after 9:30. The change in brightness should be obvious.


Algol finder map
Algol stands nearly overhead in late January as soon as it gets dark, around 6:30-7 p.m. local time. To find the star, start with the Pleiades cluster (Seven Sisters) and look not quite two fists to their upper right. Algol also makes a nice, symmetrical triangle with the bright stars Aldebaran in Taurus and Capella in Auriga.
Contributed / Stellarium

On the map I've marked the magnitudes of several comparison stars near Algol that you can use to gauge its changing brightness. For instance, at minimum it will resemble the star directly above it. When it returns to normal, it will match neighboring Almach in the adjacent constellation of Andromeda.

Algol, eye of Medusa
Algol is associated with the mythical Gorgon Medusa, who was slayed by Perseus. The sight of her face and hair of writhing snakes was said to turn a person to stone.
Contributed / Urania's Mirror

The name Algol derives from the Arabic ra 's al-ghul or "head of the ogre or ghoul." In English we know it as the Demon Star. Its association with ghouls may originate from ancient observations of its light trickery, but there's no definitive evidence for this. We do know that Italian astronomer Geminiano Montanari was the first to notice its variability in 1667. But it wasn't until 1783 that British amateur astronomer John Goodricke recognized that those variations followed a precise cycle.

With clear skies and 20 below in the forecast I'll be dashing in and out with my eye on the Demon Star tonight and hope you will, too. If you're clouded out, here are some additional opportunities (all times are CST). You can also check the Minima of Algol website for additional dates. Another way to see Algol eclipses is to look at the star pretty much anytime and then compare its light to the times of minimum (below):

February 11: Minimum at 9:31 p.m. Fading during evening twilight
February 14: Minimum 6:21 p.m. Brightening after nightfall
March 3: Minimum 11:17 p.m. Fading after nightfall
March 6: Minimum 8:06 p.m. Brightening through the night

"Astro" Bob King is a freelance writer for the Duluth News Tribune.

"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune. You can reach him at
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