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ALGEDI
Though only the third brightest star in the constellation Capricornus, the "Water Goat," Algedi is still the Alpha star, most likely because of its western-most position within the classical figure. Its name, from Arabic meaning "the Kid," refers to the whole constellation of Capricornus, which is one of the three "wet" constellations of the zodiac, the other two Aquarius and Pisces. The star's claim to naked eye fame does not lie in its brilliance but in its duplicity. Even a casual examination shows that it consists of two fourth magnitude stars, one (close to third magnitude) notably brighter than the other, the stars separated by about five minutes of arc, 1/6 the angular diameter of the full Moon. It is not entirely clear if the name refers to the pair or to just the brighter of the two. The doubling is a remarkable illusion, however, as the two stars have very different distances, making them an "optical" or line-of-sight double. They are not otherwise associated at all. The fainter, called Alpha-1 because of its more westerly position, is 690 light years away, while the brighter, Alpha-2, at a distance of 109 light years, is over 6 times closer. Such coincidences among brighter naked eye stars are unusual. Much odder is that the stars themselves each fall into similar and relatively rare categories, both evolved, dying yellow stars similar in temperature to the Sun. Alpha-2, the closer, is a giant star 35 times more luminous than the Sun, while Alpha-1 is a supergiant 14 times brighter yet, only seeming the fainter because of its greater distance. Alpha-2 is also the larger of the two, having a radius 35 times larger than our Sun, making it rather small as supergiants go. Both stars are probably living from the fusion of helium into carbon in their cores. Alpha-2, Algedi-the- giant, is deficient in metals, its iron abundance somewhere between a tenth and a half that of the Sun, indicating it comes from an older set of stars, whereas Alpha-1, Algedi-the-supergiant, has a normal, solar, chemical composition.