During October and November, the constellation of Perseus rises in the evening sky to the northeast after sunset. One of most interesting stars in the fall night sky is Algol, the second brightest star after Mirfak in Perseus.
Algol is also known as the Demon Star.
In Perseus, Algol represents the head of the Gorgon Medusa, the horrific mythical monster that turns people into stone just by looking at her.
It lies about 93 light years from Earth.
Our eye sees one star, but like many stars, Algol is a binary star. The primary star is about three times the diameter of the Sun. It’s companion is a cooler star. The two stars are very close to each other - less than the Earth is from the Sun - so they orbit quickly in 2.867 days.
The orbit of Algol’s companion is aligned to our line of sight so the cooler companion blocks the brighter star during each revolution. Then the brighter star blocks the light from the fainter star.
When there is no eclipse, the star has a magnitude brightness of 2.2. At its peak eclipse it dims to 3.3.
Algol is known as an eclipsing variable star. There are many such variables in the night skies but Algol is the most famous as it completes one cycle in 2 days 20 hours and 49 minutes with clockwork regularity.
All this happens over 10 hours, from maximum brightness to minimum brightness. An observer on earth can watch the whole process in one night.
So if you have time, go out and watch Algol a star tens of trillions of miles away change in brightness.