By Deborah Byrd
EarthSky Communications

Friday, Sept. 11, 2009
You can locate the constellation Perseus in the northeast in late evening this week.

The brightest star in Perseus is Alpha Persei, whose proper name is Mirfak, pronounced MEER-fak.

Meanwhile, the best-known star in this constellation is Beta Persei, or Algol, pronounced AL-gul. The name Algol comes from the Arabic for “head of the ghoul” or “head of the demon.” Algol is a very interesting star. It’s known to vary in brightness in a way that’s extremely regular. The cycle lasts exactly 2 days, 20 hours and 49 minutes.

For a few hours during the cycle, Algol’s brightness falls far below normal, then returns to normal. All the while, the star remains visible to the eye.

Algol’s brightness variation is not due to some special quality of a single star. Instead, this is a multiple star system, where one star regularly passes in front of another as seen from our earthly perspective. Thus Algol is what’s called an “eclipsing variable star.” Thousands of these stars are known, but Algol is perhaps the most famous of this class because its periodic dip in brightness can be seen with the eye alone, and because the cycle is relatively short. Algol is at a minimum brightness tonight, at 6:27 p.m. Central Daylight Time.

After tonight, it will brighten. Its next minimum will come on Sept. 14. The ancient stargazers had no knowledge of multiple star systems, but it’s thought they did notice this star’s brightness change. It’s thought that the brightness change is why, throughout parts of the ancient world, the star Algol was associated with demons or monsters. The Greeks and Romans identified the star with the Head of Medusa, a fearful monster with snakes in place of hair. The star has also been called The Ghoul Star.

More online at www.EarthSky.org.