lgol, also known as Beta Persei in the constellation Perseus, is the prototype of a class of
interacting binary stars which differ from cataclysmic
. It was the first eclipsing binary ever found. Viewed from Earth, Algol varies its
brightness over a period of 2.9 days, with the changes caused by a a pair of stars periodically
passing in front of each other and so cutting out the other's light. The larger of the two stars is
losing material to its small companion, the gas forming a tenuous disc around the smaller star.
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