AL DHANAB (Gamma Gruis). The stars of modern Grus, the Crane, actually fall by name in order of brightness, Alpha at the top (1.74), followed by Beta (2.13), mid-third magnitude Gamma (3.01), Delta (if you combine Delta-1 and Delta-2), and not-quite-fourth magnitude (3.49) Epsilon, all rendering the constellation quite bright and recognizable (except for northerners much above 40 degrees north latitude, for which these southern stars become lost). Of those in the Crane, only Alpha and Gamma carry proper names, neither of which have anything to do with a bird, but with a fish. To the ancient Arabians, the stars of the Crane were part of the tail of ancient Piscis Austrinus, the Southern Fish, Al Nair meaning "the bright one" (in the fish's tail), and the name of Gamma, Al Dhanab, referring to the fish's tail itself. The anglicized "Deneb" and its variant "Dhanab" literally mean "tail" out of Arabic. And many there are, starting with the best-known, Deneb in Cygnus. Then follow Denebola in Leo, Deneb Kaitos in Cetus, Deneb Algedi in Capricornus, the two Deneb al Okabs (Borealis and Australis) in Aquila, and now the last of them, our al Dhanab, which in the head of the Crane is quite misplaced (all of this showing the difficulty with proper names and the ascendancy of the Greek letters and the Flamsteed numbers). Gamma Gruis itself is a solitary (no known companions) blue-white class B (B8) giant lying 203 light years away from us. From a warm surface at 12,400 Kelvin it radiates at a rate 390 times that of the Sun (after allowing for considerable ultraviolet radiation). These parameters yield a radius 4.3 times that of the Sun, and with the theory of stellar structure and evolution a mass four times solar and an age of 125 million years. Al Dhanab rotates with a minimum equatorial speed of 57 kilometers per second, giving it a rotation period of under 3.8 days. While that may see fast, it is rather slow compared with other stars of its class, suggesting that we may be seeing it with the rotation axis more tipped toward us, but there is no way to tell. As a beginning giant, Al Dhanab has reached the end of its hydrogen fusing life or, given the inevitable uncertainties, at least will be doing so shortly. With an eventual quiet helium core it will rapidly begin to evolve to a much brighter red giant. Before it loses its outer hydrogen envelope and finally dies away as a carbon-oxygen white dwarf (from the future fusion of helium in the core), the red-giant-to-be will first reach a luminosity some 20 times what it is today.
Written by Jim Kaler. Return to STARS.