Jimmy Westlake: Algol, the Demon Star

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Ancient sky watchers believed that the stars were eternal and non-changing, but modern astronomers have discovered otherwise. Although it is true that most changes that occur in the stars are so slow that you wouldn't notice a difference during your lifetime, a few stars are noticeably non-constant to the unaided eye.

One such star is visible on crisp, autumn evenings in the constellation Perseus, the hero from Greek mythology who beheaded the horrible gorgon known as Medusa. Medusa had hissing snakes for hair and a gaze that would turn any onlooker into stone. Perseus avoided this fate by watching Medusa's harmless reflection in a mirror and then whacking her head off with his sword.

Medusa's evil eye is marked in the sky by the bright star Algol, a name that means "the Demon Star." Apparently, ancient stargazers must have realized that there was something unusual about this star. Every two days, 20 hours and 49 minutes, Algol fades to one-third of its usual brightness - for two hours - and then returns to its original luster. It's as if Medusa is winking her evil eye at us.

Horrifying and unexplainable to early sky watchers, Algol's periodic winking is no longer a mystery. Algol is actually a pair of stars that orbit one another so closely that their surfaces are almost in contact. They whirl around one another once every 2 days 20 hours and 49 minutes, and just by pure chance, their orbital plane lies almost exactly in our line of sight so that we view it edgewise. When the fainter star passes in front of the brighter star and blocks it from our view, we see an eclipse, and Algol's apparent brightness fades. When the fainter star moves on, Algol returns to her normal brightness.

Algol was the first eclipsing binary star to be discovered and continues to be the most famous. You can find Perseus and the Algol high in the northeastern sky on autumn evenings. Algol is the brightest star lying midway between the familiar W-shaped constellation of Cassiopeia and the equally familiar cluster of stars called the Pleiades, or the Seven Sisters.

If you would like to watch an eclipse of Algol, then jot down these dates and times: Algol will be in mid-eclipse at 4:22 a.m. Monday, at 1:11 a.m. Thanksgiving Day, at 10 p.m. Saturday and at 6:49 p.m. Nov. 28. Start watching a couple of hours before mid-eclipse and compare the brightness of Algol with the stars around it as it fades. Use a simple star chart if you need help locating Algol, then sit back and watch the eye of the Medusa wink.

Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published on the Web sites of CNN.com, NASA's "Astronomy Picture of the Day," Spaceweather.com, Space.com, Discover.com, and in Sky & Telescope, Astronomy, Night Sky, Discover, and WeatherWise magazines.

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