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 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 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.