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Surely one of the most remarkable stars of the sky and appropriately one of the most famed, Algol is the second magnitude Beta star of Perseus, the great mythological hero who rescued Andromeda from Cetus the Sea Monster. The Arabic name, "al Ghul" (related to our word "ghoul") means "the demon," from a longer phrase that refers to the demon's head. In Greek mythology, Algol represents the Medusa's head with which Perseus turned Cetus to stone, the star considered an "unlucky" one for centuries. To the eye, the class B star appears rather normal, its slightly bluish white light radiating from a surface with a temperature of about 12,000 K. Like the Sun, it is a "main sequence" star fusing hydrogen in its core, though it is three to four times more massive. From its distance of 93 light years, we calculate a visual luminosity 98 times that of the Sun, raised to 200 times if we factor in the invisible ultraviolet light radiated by the hot surface. Steady observation, however, reveals a surprise. As regular as clockwork, every 2.87 days, the brightness of the star plummets from mid second magnitude to dim third, to less than a third normal, the whole event (including recovery) taking only a few hours. Though the variation was discovered in 1667, it was probably known long before that and is probably the reason for the star's bad reputation. The cause of the sudden drop is a stellar eclipse. Algol is a close double star whose components orbit each other every 2.87 days. The companion to the visually observed star is a much dimmer but also larger orange (class K) giant star that each orbit simply gets in the way of the brighter smaller blue- white star. The eclipse is only partial, some of the light of the principal component still shining brightly through. Algol is famed first as a prototype of the class of eclipsing variable stars, of which thousands are known. They are among the most important kinds of stars known, as they provide us with information on stellar masses and dimensions. But Algol is equally famed for the "Algol paradox." The higher the mass of a star, the shorter its lifetime, as its fuel is used so much faster. The companion to Algol is the dying giant star. Yet it is the LESS massive of the two. The only explanation is that the dim companion has lost mass. The two stars are so close together, separated by less than a tenth the distance between the Earth and the Sun, and the giant has grown so large, that the bright star produces tides in the large one. Matter then flows in the reverse direction, from the large one to the small bright one, the effect directly observed. The star is no demon at all, but a true friend, teaching us how stars interact and die, the effects of which you can see from your own backyard with no telescope at all.