OBJECTS IN THE SKY – PERSEUS AND ALGOL

The star Algol appears in the constellation Perseus. Stars in constellations generally have technical names with Greek-letter prefixes (alpha, beta, gamma, etc.) in decreasing order of brightness, or magnitude. Thus Algol is officially known as Beta Persei, or the second-brightest star in Perseus. In the case of Algol, there is a problem. It isn’t always the second-brightest star. Algol is a variable star, decreasing and increasing in brightness with a very regular period of just a bit less than 3 days. This was a phenomenon that was readily observable by the ancients, but they had no explanation for it. They called it the Demon Star, Gorgona, Gorgonea Prima, or the Demon’s Head, Ra’s al-Ghūl (in Arabic, “رأس ال”). The name stuck, and it was recorded by the astronomer Claudius Ptolemy of Alexandria in one of his written works in the second century A.D., the Tetrabiblos.

 

In ancient Greek mythology, the god Zeus sired Perseus, who was a mighty warrior, king and founder of the Persians. As a youth, Perseus was sent by Polydectes (more about him later) to do away with the Gorgon Medusa (hence the star’s name Gorgona), who had snakes for hair. Her appearance was so terrifying that mortals who looked upon her were turned to stone. Instead of looking directly at her, he walked (or flew, by some accounts) backward toward her, gauging his progress by looking at her reflection in a mirror of polished bronze given him by Athena. He decapitated Medusa with the sword of Hermes and thus had her head to use as a weapon.

 

Algol has varying brightness because it is a triple star, that is, three stars that orbit a common center. The three stars (Beta Persei Aa1, Aa2 and Ab) are in an orbit that is almost edge-on to Earth and thus pass in front of each other, causing eclipses. This produces the dimming and brightening, which ancient astronomers explained as the “winking eye” of Medusa. For simulations of this eclipsing, please refer to:

 

https://www.constellation-guide.com/wp-content/uploads/2019/04/Algol-orbit.gif
and
https://en.es-static.us/upl/2016/10/eclipsing_binary_star_animation.gif

 

The rest of the story: On Perseus’ return to his homeland Greece after slaying Medusa, he found his mortal mother Danae still suffering from unwanted romantic advances by Polydectes. Perseus showed him the head of Medusa, thus turning Polydectes into stone and ending the latter’s harassment of Perseus’ mother.

 

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