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.