According to Arabic legend a ghoul is an evil demon that lurks in graveyards and feeds off the dead, a chilling thing to see, eyeing you from the shadows! Yet Arabic astronomers named a star after it because it winks at you from the depths of the night.
Known as Algol to us, it lies in the constellation of Perseus and carries just as much malevolence in Greek mythology as Arabic since it represents the eye of Medusa whose head was cut off by Perseus.
Medusa was a beautiful maiden with lovely long hair who got ravished by a sea god while she was praying in a temple. Athena, the Goddess of the temple was understandably outraged and sought revenge on all predatory men by turning Medusa into a hideous monster with writhing snakes instead of hair. Any man who looked at her in the eye was immediately turned to stone.
Perseus, however, outwitted the women by viewing Medusa in a reflection from his polished shield and struck off her head.
He then went off to rescue the beautiful Andromeda who was chained to a rock and used the head of Medusa to turn to stone any other men who approached his woman.
In the night sky, Perseus still holds the head of Medusa which seems to wink at us through the star Algol.
Science, of course, is scornful of such misogynistic stories and treats Algol as an interesting example of an eclipsing binary. It consists of two stars in orbit round each other, and although they cannot be resolved in any telescope, we know that the dimmer one is moving in front of the brighter one because the total brightness of the pair drops and the star seems to wink.
The time of orbit is exactly 2 days 20 hours 48 minutes and 56 seconds. The total eclipse lasts about 10 hours but the noticeable dimming of the star lasts only 2 hours around the moment of maximum occultation.
To best appreciate the dimming of Algol you should compare it with neighbouring stars. At its brightest (mag 2.1), Algol is as bright as Gamma (γ) Andromedae (mag 2.1); at its dimmest (mag 3.4), it’s dimmer than Epsilon (ε) Persei (mag 2.9). Mirfak (mag 1.8) remains always the brightest star in the constellation.
The scale of magnitude, of course, runs backwards: Mag 0 is the brightest of stars visible to the naked eye and Mag 6 is the dimmest. A shift from 2.1 to 3.4 represents a dimming of about 70%.
This blog first appeared as a story for the Cairo BCA Magazine of December, 2009, to promote star gazing in the Sinai with an 8″ Meade LX90 Telescope.