ASELLUS BOREALIS (Gamma Cancri). The lore of the sky is not
monolithic, but cuts amazingly across time and cultures. There is
no single set of stories, as witness Cancer, the Crab, one of the dimmer figures of the Zodiac that is sandwiched between
Gemini and Leo. Its main feature is a box of four stars that
encloses a marvelous naked-eye cluster, Messier 44, which is better
known popularly by two names, the "Beehive" and (from Latin) the "Praesepe," or manger,
the concept going back to ancient Greece, predating Christianity.
As such, it is flanked (to the east) by two Asses, or Donkeys,
Asellus Borealis and Australis (the northern and southern Donkeys),
neither (and with the Praesepe, none) of which have anything
whatsoever to do with a crab. (Don't confuse them with the three
"Donkeys" of Bootes near the handle of
the Big Dipper, Asellus Primus, Secundus, and Tertius, or
Theta, Iota, and Kappa Boo, which likewise have nothing to do with
the Herdsman.) To the brighter of the two, fourth magnitude (3.94)
Asellus Australis (number 3 in the constellation after Beta and Iota
Cancri), Bayer assigned the Greek
letter Delta, while the fainter, fifth magnitude Borealis
(4.66, ranking fifth), oddly got Gamma Cnc. The two Donkeys of
course really have nothing to do with each other. Gamma Cnc
(Borealis) is a class A (A1) subgiant (but see below) 181 light
years away (second Hipparcos reduction, 14 percent higher than
the original), while Delta (Australis) is a K0 giant 131 light years off. Gamma
is pretty well neglected, with a temperature of 9400 Kelvin that is
estimated only from its class. Its luminosity comes in at 38 times
that of the Sun, its radius 2.3 solar, its
mass 2.3 solar. A swift projected equatorial rotation speed of 79
kilometers per second leads to a rotation period under 1.5 days.
Theory reveals not just mass, but that the star is really a dwarf
that is about a quarter of the way through its hydrogen fusing
lifetime of 780 million years. The star has two "companions," tenth magnitude
Gamma Cnc B two minutes of arc away and twelfth magnitude Aa at one
minute away. Both just lie along the line of sight (with Gamma B
a real double, about which nothing is known). The chemical
abundances of Asellus Borealis are normal, the result of mixing of
gases caused by rapid rotation, and while the star is occasionally
occulted by the Moon, its disk remains
unresolved.
Written by Jim Kaler 5/01/09; updated 7/24/09.
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