Tuesday, October 30, 2007


Keeping an eye on Comet Holmes means keeping an eye on Algol too. The comet is in the constellation Perseus. Perseus is the wing-footed hero who saved beautiful Andromeda from the sea monster Cetus. He had first slain the evil snaky-haired Medusa, who was so ugly that anyone who looked directly at her turned to stone. Perseus chopped off her head while looking at her reflection in his shield. He then carried the head around in a bag, using it for his own advantage to turn his enemies into stone -- including Cetus. (And the lovers lived happily ever after.)

But look at the eye of Medusa on this old star map. This is the Ghoul Star, Algol (al-gol, the ghoul), supposed to be the most demonic and malevolent star in the sky, a perfect star for Halloween. Ghoulish because it changes brightness dramatically every three days, by nearly a magnitude and a half.

Algol is the sky's best example of an eclipsing binary, two stars bound in a gravitational dance that appear to our eye as one -- a hot blue-white star several times larger than the Sun, and an even larger but cooler yellow star that is not as bright as the primary. The stars circle their common center of gravity every three days, in a plane orientated in such a way that from our perspective the two stars eclipse each other. When the brighter primary passes in front of the less bright secondary, the combined brightness takes a small dip. When the bigger, less bright secondary totally eclipses the brighter primary, the combined brightness take a sharp dip. The Medusa's ghoulish eye winks every three days. You can find out when to catch the wink here.

The map of Perseus is from the atlas of the German astronomer Johannes Hevelius, published posthumously by his wife Elizabeth, also an astronomer, in 1690. It is reversed right-to-left from what you see in the sky, as if you were looking at the stars from outside of the celestial sphere. Over the next few weeks, Comet Holmes will move toward Mirfak, the brightest star in the constellation, shown here in the right middle of Perseus' back.