Week of October 29-November 4, 2017

Ghosts and goblins will be out in force as Halloween arrives this week, and in the heavens, a Demon Star will be diabolically shining. Its name is Algol.

To find it at this time of year, go outdoors during the early evening and look low toward the northeast. The accompanying sky map should help you locate it. After the sky becomes dark, you will easily spot the bright star Capella twinkling wildly over the northeastern horizon. Above it lies the famous "W" of Cassiopeia (which is now nearly on its side), and just below, you'll find Perseus and its bright stars Mirfak and Algol.

To the writers of classical times, Algol represented the head of the horrible monster Medusa being held by Perseus. Ancient and medieval astrologers considered it the most dangerous star in the heavens. The Hebrews knew Algol as Rosh haSatan, or Satan's Head, while the Chinese gave it the gruesome title Tseih She, the Piled-up Corpses.

What modern astronomers find fascinating about Algol is that it does not shine with a steady light; it 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 discovered example of a class of stars known as eclipsing binaries: two stars orbiting a common center of gravity nearly along the plane of our line of sight. As the one star passes in front of the other, the combined light dims as we on Earth see an eclipse. Even powerful telescopes can detect only one point of light here, attesting to the pair's great distance of 540 trillion miles, or about 90 light-years.

This coming week, Algol reaches its faintest when stargazers can be watching: on Oct. 30 at around 4:50 a.m. PDT; Nov. 2 at around 1:38 a.m. PDT (4:38 a.m. EDT); Nov. 4 at about 10:27 p.m. PDT (12:27 a.m. EST on Nov. 5); and Nov. 7 at around 6:16 p.m. PST (9:16 p.m. EST). Moonlight will be bright during the early part of this period, so you may find binoculars will help.

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, above Algol, the star Almach shines with the same brightness as Algol does at its maximum; below Algol, the star Epsilon Persei is nearly as faint as Algol at its minimum.

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.