Week of Nov. 1-7, 2015

It's known as Algol, the Demon Star, and we can see it shining diabolically in our sky each year around Halloween.

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 (now nearly on its side) 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. Al Ghul means "a mischief-maker," and the name appears in the Ghoul of the "Arabian Nights." The Hebrews knew Algol as Rosh ha Satan, Satan's Head, while the Chinese gave it the grisly name Tseih She, the Piled-up Corpses.

What makes Algol intriguing to modern astronomers, however, is that it shines not with a steady light, but exhibits a strange variability. Every 2.85739 days, the star dims noticeably for about five hours, remains fainter for a couple more hours, and then gradually brightens again.

Algol, also called Beta Persei, is the first-ever discovered example of 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. Even powerful telescopes can detect only one point of light here, attesting to the pair's great distance — 540 trillion miles, or about 90 light years.

This coming week, Algol reaches its faintest when stargazers can be watching: on Nov. 2 at 1:49 a.m. EST, Nov. 5 at 10:38 p.m. EST, and again on Nov. 8 at 7:27 p.m. EST — all while the bright moon is nowhere to be found. After that, its next favorable North American minima will come in during the last week of November.

If you keep an eye on Algol for a few hours before or after its minimum, and compare its brightness to any nearby star whose light shines steadily, you'll be able to watch it enter or emerge from eclipse. For example, the star Gamma Andromedae (Almach) to the west of Algol shines the same brightness as Algol does at its maximum; the star Epsilon Persei to the east of Algol is nearly as faint as Algol is at its minimum. 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.

Visit Dennis Mammana at www.dennismammana.com. To read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.