This week offers northern hemisphere observers a good opportunity to see a variable star in action. Algol is located in the constellation of Perseus and is one of only a few stars that can be seen to change brightness with the naked eye. This extraordinary characteristic led our ancestors to associate it with the severed head of the gorgon Medusa being held in Perseus’s hand. The name Algol derives from an Arabic word that translates into ghoul or demon. Algol’s brightness changes because there is a smaller, dimmer star in orbit around it. Every 2.87 days, that smaller star passes in front of its larger, brighter companion, blocking out some of its light. From beginning to end, these eclipses last for about 10 hours. The chart shows the view looking east at 20.55 GMT on 8 October. At this time, Algol will be in mid-eclipse and its brightness will be less than half its normal value. Over the next five hours, it will return to full brightness.
