Algol, "The Demon Star" Reappears in the Night Sky

October is traditionally the month when thoughts turn to monsters, ghosts and spooky stories.  The night sky also has its fair share of monsters and frightening creatures.  One of the most famous is not a constellation itself, but is part of the constellation Perseus the Hero.

Perseus the hero is famous for slaying Medusa the Gorgon, who was so hideous that a single glance from her would turn any living creature to stone.  Perseus used the reflective surface of the inside of his shield to safely hunt the monster and cut off her head with a single stroke of his sword.  He then used Medusa's head to rescue the princess Andromeda from Cetus the Sea Monster (who is also in the fall sky).

The constellation Perseus depicts the hero holding Medusa's head.  The star Algol (ß Perseae) represents the eye of Medusa.  Algol has long been known as the Demon Star because it changes its brightness over a period of nights; like the eye of Medusa slowly opening and closing.  

We now Know that Algol is an eclipsing binary star.  An eclipsing binary is a pair of stars that orbit around each other in such a way that they alternate passing in front of each other, thus eclipsing.  In the case of Algol one star is a bright white star and the other is a dim red star.  When the red star passes in front of the white one the light from the white star is cut off or eclipsed. This means the brightness alternately drops and then brightens.  

Binary stars weren't observed until the advent of the telescope.  Yet a special binary star has been known to ancient times,  and it is perhaps fitting that one of the spookiest begins to appear in our sky as October rolls around.  

Eclipsing Binary star Algol in Perseus. Created with Stellarium.