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AIN

(Epsilon Tauri). Looking down with its vee-shaped head, Taurus glares at us, the great celestial Bull's eyes formed by the bright orange giant Aldebaran and by just-barely-fourth magnitude (3.53) Ain, the name coming from an Arabic phrase that literally means "the Bull's eye." Ain, which received the Epsilon designation from Bayer, is the only named star in Taurus's Hyades cluster (as opposed to the more compact Pleiades, nine of whose stars are named.) The Hyades, an older cluster whose stars were born not quite half a billion years ago, has no bright massive stars left, as these die young. It does, however, have four relatively bright dying giant stars, of which class G (G9.5) Ain is the brightest. The others are Gamma, Delta-1, and Theta-1; the class K giant Aldebaran, merely in the line of sight, does not belong to the Hyades. At a measured distance of 155 light years, just a hair farther than the cluster's average of 151 light years, Ain shines at us with a luminosity 90 times that of the Sun from a yellow-orange 4925 Kelvin surface. The star is close enough to have had its angular diameter measured, this and a calculation of radius from luminosity and temperature agreeing at 13 times the size of the Sun. Like the other Hyades stars, Ain is somewhat richer -- about 40 percent -- in metals than the Sun. The more massive a star at birth, the shorter life it will live. More-massive stars are hotter inside, and convert their internal hydrogen fuel to helium much faster than do less-massive stars. The age of a cluster can therefore be found from the most- massive stars that the cluster still has. Ain and the three other giants, which are now dying and fusing helium into carbon in their deep cores, have masses somewhat over double that of the Sun, Ain falling at 2.3 solar masses. From these measures we find the cluster to have an age of 650 million years. Ain has a faint 11th magnitude companion that lies about 3 minutes of arc away. It is not known if the little star is a physical companion or just a line-of-sight coincidence. If the two are truly joined, then they are at least 8600 Astronomical Units apart and take at least half a million years to orbit each other. From Ain, the companion would shine with the light of Venus in our sky, while from the companion, Ain proper would shed the light of the near-full Moon.