Since ancient times, star watchers have observed that some stars change their brightness over time, in a cyclical pattern. Their brightness increases and decreases, over and over again. Some take years to complete one cycle, some take months or days, and others complete a cycle within a few hours. Some do this according to an exact, predictable schedule, while others seem to do it almost randomly.The star called Algol, meaning "Demon Star" in the constellation Perseus, mystified star watchers for thousands of years. It was thought that a demon caused it to do strange things. It was a moderately bright star (magnitude 2.1) that mysteriously started getting dimmer approximately every 2 days and 20 hours. It would stay near its dimmest (magnitude 3.4, higher number means dimmer) for about 2 hours, with the entire fading and brightening process lasting about 7 hours. Then it would return to its "normal" brightness. Approximately 2 days and 20 hours after it started dimming the whole cycle would begin again. What caused this?
Modern astronomical tools have made it possible to solve the mystery. Algol is, in fact, part of a binary star system, two stars in each other’s gravitational field rotating around each other (See July’s Star Facts article Star Twins for further explanation). The two stars complete one full rotation around each other in about, you guessed it, 2 days and 20 hours. Algol’s partner is a larger, but dimmer, star. When the partner rotates in front of Algol, from our line of sight it blocks out most of Algol, so we see mostly the partner and only part of Algol. During this blocking phase the total light we see decreases, until the partner star moves out of the way and Algol returns to its full brightness.
Stars that change their brightness in a cyclical way are generally called "variable stars". These are fascinating celestial objects for amateur astronomers to observe. Algol belongs to one category of variables called "eclipsing variables". These are double stars (or in some cases triple stars) that eclipse each other periodically from our line of sight, causing their brightness to vary. Future articles will discuss other categories of variable stars.
Moral of the story: most demons can be defeated with facts and knowledge.
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