Algol

From South Dublin Astronomical Society

Algol is probably the most famous of all variable stars visible from these latitudes. It changes it’s brightness over a 10 hour period once every 2.86739 days or 2 days 20 hours 48 minutes and 50 seconds, to you and me! When the cycle starts, the star dims from magnitude 2.1 to 3.4 and back up to 2.1 again over the course of 10 hours. It then remains at magnitude 2.1 for 2.86739 days before the cycle repeats.

Algol is actually two stars, a blue dwarf from which we see most of the light and a fainter yellow star that orbits it. The blue star is much brighter and this is the star we see for most of the time. However, when the yellow star passes in front of the blue star, it blocks more light than it shines, thus Algol appears to dim for a short while. To see a demonstration of this, check out this website: http://www.cosmion.net/software/ebs/