Week of Oct. 25 - 31, 2009 by Dennis Mammana on Creators.com

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Week of Oct. 25 - 31, 2009

It's known as Algol, the Demon Star, and it shines diabolically in our sky each year as Halloween approaches.

To find it, go outdoors in the evening and look low toward the northeast. The accompanying sky map should help you locate it. After it becomes dark, you will easily spot the bright star Capella twinkling wildly over the northeastern horizon. Above lies the famous "W" of Cassiopeia and, just below, you'll find the stars of Perseus and its bright stars, Mirfak and Algol.

To the writers of classical times, Algol represented the head of the horrible monster Medusa, held by Perseus. Ancient and medieval astrologers considered it the most dangerous star in the heavens. What makes Algol intriguing to modern astronomers, however, is that it does not shine with a steady light, but exhibits a strange variability.

Every 2.85739 days, the star dims noticeably for about five hours, then gradually brightens again. Algol, also called Beta Persei, represents a class of stars known as eclipsing binaries — two stars orbiting a common center of gravity along nearly the plane of our line of sight. As the one star passes in front of the other, the light dims and we on Earth see an eclipse.

That even powerful telescopes can detect only one point of light in this position attests to the pair's great distance — 600 trillion miles, or about 100 light years.

Next week, Algol reaches its faintest when stargazers around the U.S. can be watching: on Nov. 5 at 5:54 a.m. EST, at 2:43 a.m. EST on Nov. 8, and again at 11:32 p.m. EST on Nov. 10.

If you keep an eye on Algol for a few hours after this time and compare its brightness to any nearby star whose light shines steadily, you'll be able to watch it emerge from eclipse. You won't need a telescope or binoculars to monitor its progress, though these certainly can help the view.

By plotting Algol's brightness over time, astronomers have learned that one of its stars is about three times larger than our sun while the other is only 20 percent larger. As these two stars orbit each other and the fainter one eclipses the brighter, we on Earth see the system's total light drop by about three times. In fact, extremely careful observations over the years have shown that there's actually a third star orbiting this pair every 1.86 years.

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