ALGEDI (Alpha Capricorni). 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 6.6 minutes of arc, 1/5 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.
Alpha Cap Capricornus's Double or Nothing! Separated by 6.6 minutes of arc, the naked eye "double star" Algedi (toward the upper right) is actually an "optical double" that consists of two unrelated line- of-sight stars, Alpha-1 Capricorni at right, Alpha-2 at left. On the other hand, Dabih (Beta Cap), toward the bottom, is a real, though wide, binary, the pair far enough apart (3.4 minutes of arc) to produce an elongated image, with Beta- 1 (the fainter of the two) a bit down and to the right. The two Alpha stars point down and to the left toward Nu Cap.
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 of class G (G3 for Alpha-1, G8 for Alpha-2), and at 5000 Kelvin rather similar in temperature to the Sun. Alpha-2, the closer, is a giant star 43 times more luminous than the Sun, while Alpha-1 is a supergiant 930 times brighter than the Sun (21 times brighter than Alpha-2), only seeming the fainter because of its greater distance. Alpha-1, the supergiant, is the larger of the two, having a radius 40 times that of our Sun (5 times larger than Alpha-2), making it rather small as supergiants go. It is, as a supergiant (though a lesser one), also the more massive, containing 5 solar masses, double that of Alpha-2. Both stars have quit hydrogen fusion in their cores, and are preparing to fuse their internal helium to carbon, if they have not done so already. 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.
Written by Jim Kaler. Return to STARS.