Around 8 o'clock on these New England evenings, the rotating sky brings a relatively inconspicuous star in Perseus almost exactly overhead.  Algol is a blue-white star about three and a half times as massive as our sun.

These characteristics do not mark Algol as unusual, nor is it particularly odd that what we call Algol is actually three stars orbiting each other.  What is unusual is that our solar system happens to lie in the orbital plane of two of those stars.  Every two days, 20 hours, 49 minutes, the smaller, dimmer Algol B passes in front of Algol A.  During that eclipse the star blinks and an inconspicuous second-magnitude star becomes a more inconspicuous third-magnitude star.  The dimming is gradual, but for several hours, Algol goes from the second-brightest to the fifth-brightest star in Perseus and the constellation appears to change form.

Algol's periodic winking may be the reason why it became known in several cultures as Demon Star or Devil's Star.  Like many stars, its modern name was derived from its early Arabic name -- Ra's al-Ghul, meaning head of the devil -- but the first surviving written note of its variability was made much later by a 17th century Italian astronomer.  More than a century after that, an amateur astronomer in England made careful observations of Algol's winkings and charted their surprising regularity.  When he described his observations to the Royal Society in 1783 he received a standing ovation.  Algol was the first star identified as an eclipsing binary.

Many eclipsing binaries have been discovered in the centuries since, but Algol remains the demon king of inconstant stars.


An overview of the constellation Perseus, with a sky map, is on the web at <www.universetoday.com/22391/perseus/<http://www.universetoday.com/22391/perseus/>>.


“. . . therefore as a stranger give it welcome.
There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy.”
               - Hamlet, Act 1, Scene V




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