AIN (Epsilon Tauri). Star with planet. 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 one of three named stars 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 (Hyadum I), Delta-1 (Hyadum II), 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 150 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, which is consistent with other planet-holding stars (the planet described below). 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 2.7 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. The star is most unusual in that it is also orbited by a planet.

The Planet. Ain has the first planet ever to be found in an open cluster, and is also the most massive star (2.7 times that of the Sun) known to have one. The planet, which has a minimum mass 7.6 times that of Jupiter, orbits at an average distance of 1.93 Astronomical Units (290 million kilometers), giving it a long orbital period of 595 days (1.63 years). An eccentricity of 15 percent takes the planet from as far as 2.22 AU away to as close as 1.64 AU. Ain thus joins Pollux as a visually bright planet-holding giant. Imagine the view! From the planet, not only would the sky be bright with the cluster's many stars, but Ain itself would appear over three degrees across, nearly 7 times the angular size of the Sun. Ain thus joins Pollux as a visually bright planet-holding giant.
Written by Jim Kaler. Return to STARS.