Algol- Beta Persei, also known as the "Demon Star", is 93 light years away in the constellation of Perseus.

Algol, was the first eclipsing binary to be discovered in 1669 and is the prototypical star of its class. Usually magnitude 2.1 at maximum, it dips to 3.4 every two days, 20 hours and 49 minutes. The entire eclipse lasts about 10 hours and is visible to the naked eye. There is also a secondary eclipse when the brighter star occults the fainter secondary. This secondary eclipse can only be detected photoelectrically.

The light curve of Algol demonstrates the geometry of the system. Eclipsing binaries are among the most important kinds of stars, as they provide us with information on stellar masses and dimensions.

But Algol is equally famed for the "Algol paradox." The less massive star is already a subgiant, and the star with much greater mass is still on the main-sequence. This seems paradoxical because the component stars of any binary are thought to have formed at approximately the same time and should have similar ages. So the more massive star, rather than the less massive one, should have evolved fastest and left the main sequence.

The paradox is resolved by the fact that in many binary stars, material can be exchanged between the two stars, disturbing the normal process of stellar evolution. The originally more massive star will reach the next stage in its evolution despite having lost much of its mass to its companion.

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