ALGOL, THE DEVIL'S STAR

 Algol is a star visible in the constellation of Perseus, only 90 light-years from Earth. The name Algol comes from the Arabic word ra's al-ghul, which can be translated as "demon's head". In Greek mythology, however, the star corresponded to the severed head of Medusa held by the hero Perseus. Algol was therefore always seen as a bringer of misfortune, due to its strange behavior.

Algol is in fact a variable star that changes its brightness cyclically over a period of 2 days and 20 hours. At its maximum the star has a magnitude of +2.1, while at its minimum it has a magnitude of +3.4. Today, thanks to our astronomical knowledge, we know what causes this peculiar behavior, while in ancient times this variation in brightness was unexplained and frightened the inhabitants of various populations. The ancient Egyptians, for example, studied the star, which they called Fury, to try to predict inauspicious days.

Algol is actually a triple system of stars. The central binary consists of a 0.8 solar mass orange giant and a 2.5 solar mass main sequence star, separated by only 0.06 AU. Surrounding the main system is a third star slightly larger and more massive than the Sun at a distance of 2.7 AU.

From Earth, by a perspective coincidence, we see the secondary star pass exactly in front of the primary, obscuring it. This causes the brightness of the star to decrease for a period of 10 hours, the time it takes for the secondary to pass in front of the primary. Once the transit is complete, the brightness of the system returns to its normal value. 

The third component of the system, although orbiting the other two, does not cause a dip in brightness because from Earth we do not see it pass in front of the others.

Credit: John Chumack.