Beta Persei, better known as Algol, is the premier eclipsing binary star of the northern sky. This is because it can be observed with the unaided eye and it has a relatively short period of less than three days. You can check on it whenever you step outdoors on nights when Perseus is in view.

Algol’s name derives from the Arabic Al Ra’s al Ghul, “The Demon’s Head”, and in Greek mythology it represents the head of Medusa held by Perseus. To ancient and medieval astrologers Algol was the most dangerous and unfortunate star in the heavens, which seems to suggest that its strange variability might have been noticed in antiquity. However, this remains unsupported by any other real evidence.

Algol’s variations in light were first recognized by the Italian astronomer Geminiano Montanari in 1667, and the periodicity of the variations was first measured by the English amateur astronomer John Goodricke in 1783. Goodricke also suggested, correctly, that the variations were caused by eclipses.

Algol is an eclipsing binary, consisting of two stars that regularly pass in front of each other as seen from the Earth. The stars are too close together to be distinguished individually in a telescope, but analysis of the light from Algol reveals that the brightest of the pair, Algol A, is a hot star one hundred times as luminous as the Sun. Its companion, Algol B, is a larger but fainter orange star that covers about 80-percent of Algol A during the eclipses. There is also a third star in the system, Algol C, but it does not participate in the eclipses.

Algol fades and rebrightens like clockwork every 2 days 20 hours 49 minutes. Its changes are very plain to the naked eye – in the middle of an eclipse it shines dimly at magnitude +3.4 instead of its usual +2.1. The fading lasts for about ten hours as the companion passes in front of the primary along our line of sight. As the companion moves off the primary, Algol returns to peak brightness. A second, minor dip in brightness occurs as the companion passes behind the primary, but the fading is so slight that it is undetectable to the eye.

Algol's Light Curve
Light curve of the eclipsing variable star Algol (Beta Persei). The star stays at its faintest value for two hours centered on the time of mid-eclipse.

To track Algol, compare it periodically with stars of known brightness in its immediate surroundings. Good comparison stars are nearby Gamma Andromedae (magnitude +2.2) to Algol’s west, and Epsilon Persei (magnitude +2.9) to its east. You should end up with a light curve looking like the one shown above. For your convenience, use this comparison star chart for estimating Algol’s magnitude.

Minima of Algol (May 2010)

• May 2 – 06:35 UT
• May 5 – 03:24 UT
• May 8 – 00:13 UT
• May 10 – 21:02 UT
• May 13 – 17:51 UT
• May 16 – 14:40 UT
• May 19 – 11:29 UT
• May 22 – 08:18 UT
• May 25 – 05:07 UT
• May 28 – 01:56 UT
• May 30 – 22:45 UT

These are the dates and times, in Universal Time, when Algol should be at its dimmest, magnitude +3.4 instead of +2.1.