Algol is a star in the constellation Perseus. For centuries humans have known something was up with it. In Arabic, it was called the “demon star”.  In the pseudoscience of astrology, it is associated with violence, and historical generals tried to avoid having battles while Algol was at a minimum. It increases and decreases in brightness a noticeable amount over a period of just under three days, making it a variable star. If you have a run of good clear nights, you can see this cycle yourself with the unaided eye.

Its variability was first written about in the seventeenth century, but it took over one hundred years for someone to propose a mechanism for its observed change in brightness in 1783. This explanation was almost right, which is impressive for back then. British astronomer John Goodricke proposed that there was something dark that regularly passed in front of the star, making it appear dimmer. The correct answer wasn’t far off, but it would take yet another century to prove it. 

In 1881, American astronomer Charles Pickering discovered that there was another star orbiting Algol, which was dimmer than the main star and which he called an eclipsing binary star. The orbit of the fainter star caused the regular apparent change in brightness. This was discovered purely by analyzing the Doppler shift of the two stars and noticing that they weren’t what they were supposed to be. This 1881 work wasn’t the end, however, because the doppler data revealed some things that couldn’t be explained by just two stars. 

In 1957, a third star in the system was discovered, and in 2019, NASA’s TESS spacecraft narrowed the lightcurve period down to the minute. For the nerds: two days, twenty hours, 49 minutes.

To find Algol, look for the “W” asterism of Cassiopeia in the northwest. To the west of Cassiopeia is Perseus, where Algol is. Perseus has a line of stars with two more below and to the left. Algol is one of these lower two. Algol is also halfway between the bright star Aldebaran in Taurus and below Capella in Auriga.

More Information

Algol is the Demon Star (EarthSky)

John Goodricke (Sky Over Berlin via Internet Archive)

“Dimensions of the Fixed Stars, with Especial Reference to Binaries and Variables of the Algol Type,” Edward C. Pickering, 1880 May 25, Proceedings of the American Academy of Arts and Sciences

“A Spectroscopic Investigation of Algol,” Alan S. Meltzer, 1957 March, Astrophysical Journal

“Two centuries of study of Algol systems,” Alan H. Batten, 1989 June, Space Science Reviews