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Every 2.9 days the star Algol in Perseus the Hero figuratively blinks its eye. If your timing is right, and you look at it say, early in the evening, Algol looks its normal self -- a star similarly bright as those in the Big Dipper. But go out a couple hours later when it's partially eclipsed by its close companion star and the change is very noticeable to the naked eye. In that short space of time Algol will have dimmed and look like a faint star.
The Algol system is 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. When Algol B partially eclipses Algol B the overall brightness of the system drops. The entire eclipse start to finish lasts about 10 hours, but the juicy part, when the drop in light is most dramatic, takes about 2 hours.
On clear nights I always check to see what Algol's up to. Most of the time the star shines at maximum (normal brightness). But sometimes it's fainter, so I watch to see whether it's moving deeper into eclipse or on its way out. Of course, this is totally unnecessary because anyone can find out exactly when Algol is expected to dip to minimum by checking out the Minima of Algol calculator at skyandtelescope.org/observing/the-minima-of-algol .
Astronomers measure star brightness using the magnitude scale. All the really bright stars we see are magnitude 1 or brighter. Stars in the Big Dipper are around magnitude 2 as is the North Star, Polaris. Outside of eclipse, Algol shines at magnitude 2.1. But at minimum, when its fainter companion mostly covers Algol A, the star dims to 3.4.
A difference of more than a magnitude is very easy to notice, which is why watching eclipses of Algol is easy and fun. In the next week we'll have two opportunities to catch one. Eclipses are visible anywhere on Earth where Algol is above the horizon. Perseus is a winter constellation, so it stands high in the sky for northern hemisphere skywatchers as soon as it gets dark out.
Our first opportunity happens on Monday night, Jan. 27 when Algol dims to minimum at 9:17 p.m. CST. The best way to see the dramatic light change is to look at the star as soon as it gets dark around 6:30 p.m. When you return for a second look around 9:15 p.m. I think you'll be amazed at how much dimmer it will appear.
The second chance occurs on Thursday night, Jan. 30 with the star at minimum at 6:06 p.m. just as the sky's getting dark. Look for Algol around 6:30 p.m. again and you'll notice it's quite dim. Then when you return for another peek around 9 p.m., it will be mostly back to its familiar, bright self.
On the map I've included magnitudes for several stars so you can more easily follow Algol's changes during eclipse. An excellent comparison star is Gamma Andromedae (Gamma on the map). It's magnitude 2.1 and a perfect match for Algol at normal light. Just above Algol a 3.8-magnitude star serves as a good comparison during eclipse.
The name Algol derives from the Arabic ra's al-ghul or "head of the ogre or ghoul." Skywatchers know it best by its nickname, the Demon Star. Its association with ghouls may originate from ancient observations of its variability, but there's no definitive evidence for this. 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.
You can join the long list of observers who've seen Algol wink -- including some who've winked back -- by donning coat and gloves and taking a peek for yourself.