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AL DHANAB (Gamma Gruis). The stars of modernGrus, the Crane, actually fall by name in order ofbrightness,Alpha at the top (1.74),followed byBeta (2.13), mid-thirdmagnitude Gamma (3.01),Delta (if youcombine Delta-1 and Delta-2), and not-quite-fourth magnitude (3.49)Epsilon, all rendering the constellation quite bright andrecognizable (except for northerners much above 40 degrees northlatitude, for which these southern stars become lost). Of those inthe Crane, only Alpha and Gamma carry proper names, neither ofwhich have anything to do with a bird, but with a fish. To theancient Arabians, the stars of the Crane were part of the tail ofancient Piscis Austrinus, the Southern Fish, Al Nair meaning "thebright one" (in the fish's tail), and the name of Gamma, Al Dhanab,referring to the fish's tail itself. The anglicized "Deneb" andits variant "Dhanab" literally mean "tail" out of Arabic. And manythere are, starting with the best-known,Deneb inCygnus. Then followDenebola inLeo,Deneb Kaitos inCetus,DenebAlgedi inCapricornus, the two Denebal Okabs (Borealis andAustralis) inAquila, and now the last of them, our al Dhanab, whichin the head of the Crane is quite misplaced (all of this showingthe difficulty with proper names and the ascendancy of theGreek letters and theFlamsteed numbers). Gamma Gruisitself is a solitary (no known companions) blue-white class B (B8)giant lying 203 light years away from us. From a warm surface at12,400 Kelvin it radiates at a rate 390 times that of the Sun(after allowing for considerable ultraviolet radiation). Theseparameters yield a radius 4.3 times that of theSun, and with the theory of stellar structureand evolution a mass four times solar and an age of 125 millionyears. Al Dhanab rotates with a minimum equatorial speed of 57kilometers per second, giving it a rotation period of under 3.8days. While that may see fast, it is rather slow compared withother stars of its class, suggesting that we may be seeing it withthe rotation axis more tipped toward us, but there is no way totell. As a beginning giant, Al Dhanab has reached the end of itshydrogen fusing life or, given the inevitable uncertainties, atleast will be doing so shortly. With an eventual quiet helium coreit will rapidly begin to evolve to a much brighterred giant. Before it loses itsouter hydrogen envelope and finally dies away as a carbon-oxygenwhite dwarf (from the futurefusion of helium in the core), thered-giant-to-be will first reacha luminosity some 20 times what it is today.
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