Home PageAstronomy ArticlesStars PageAstronomy Links

New: Join EarthSky on Facebook!

Watching Algol, Perseus' Winking Demon Star

The star Algol in the constellation Perseus is said to represent the evil winking eye in the head of the Medusa monster being held by Perseus, the son of Zeus and Danae. Like clockwork, Algol, perhaps the most famous variable star in all the sky, winks to about one-third its normal brightness every 2 days 20 hours and 49 minutes. The Ghoul star has drawn attention to itself since time immemorial, but the mystery of this star has been only unraveled in the late 19th century.
Algol is actually a binary star - two stars revolving around a common center of mass. The plane of these two stars' mutual orbit is seen almost edge-on from Earth, so when the dimmer star passes in front of the brighter star, Algol dims dramatically. But when the brighter star passes in front of the dimmer star, the slight dimming is not really perceptible to the unaided eye. This special kind of binary star is called an eclipsing binary. This page well illustrates the geometry of the Algol binary system and its accompanying light curve.

Finding Algol and Comparison Stars

This map shows Algol in Perseus, and two common comparison stars, Almach in the constellation Andromeda and Epsilon in Perseus. When Algol is at its brightest (which is most often the case), it's equal in brightness to Almach (Gamma Andromedae). When Algol is at its dimmest, it's fainter than Epsilon Persei.
Thanks to Sky & Telescope magazine, here is another chart of Algol and the comparison stars. Moreover, Sky & Telescope provides the times for Algol's mimimum brightness in your sky.
One way to verify that you're looking at Algol and Almach (sometimes spelled Almaac) is to locate the open star cluster M34 through binoculars. M34 resides in between these two stars, though not quite on line with them. Depending on the width of your binocular field, M34 and Algol may or may not fit within the same field of view. Epsilon Persei almost aligns with Algol and Almach.
By the way, if you have a telescope, be sure to look at Almach. This star rates as one of the most beautiful and colorful binary stars in all the heavens. It definitely rivals the star Albireo in Cygnus the Swan.

Algol Not a Visual Binary

Algol is not a visual binary - a binary whose component stars can be individually seen with the unaided eye or an optical aid. The stars revolve around each other much too closely. However, Algol is a spectroscopic binary, meaning that the star's dual nature is resolvable with a spectroscope, but not a telescope. Although the star's dimming and brightening has probably been seen since the Stone Age, an understanding of the star's dual nature had to wait for relatively modern times.

copyright 2008 by Bruce McClure

Minimal for Algol, courtesy of Sky & Telescope magazine

Autumn Double Star Tour

October 2008 Feature * December 2008 Feature