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ASELLUS BOREALIS (Gamma Cancri). The lore of the sky is notmonolithic, but cuts amazingly across time and cultures. There isno single set of stories, as witnessCancer, the Crab, one of the dimmer figures of theZodiac that is sandwiched betweenGemini andLeo. Its main feature is a box of four stars thatencloses a marvelous naked-eye cluster, Messier 44, which is betterknown 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 anythingwhatsoever to do with a crab. (Don't confuse them with the three"Donkeys" ofBootes near the handle oftheBig Dipper,Asellus Primus, Secundus, and Tertius, orTheta, Iota, and Kappa Boo, which likewise have nothing to do withthe Herdsman.) To the brighter of the two, fourth magnitude (3.94)Asellus Australis (number 3 in the constellation afterBeta andIotaCancri), Bayer assigned theGreekletter Delta, while the fainter, fifth magnitude Borealis(4.66, ranking fifth), oddly got Gamma Cnc. The two Donkeys ofcourse really have nothing to do with each other. Gamma Cnc(Borealis) is a class A (A1) subgiant (but see below) 181 lightyears away (second Hipparcos reduction, 14 percent higher thanthe original), while Delta (Australis) is a K0giant 131 light years off. Gammais pretty well neglected, with a temperature of 9400 Kelvin that isestimated only from its class. Its luminosity comes in at 38 timesthat of theSun, its radius 2.3 solar, itsmass 2.3 solar. A swift projected equatorial rotation speed of 79kilometers per second leads to a rotation period under 1.5 days. Theory reveals not just mass, but that the star is really a dwarfthat is about a quarter of the way through its hydrogen fusinglifetime of 780 million years. The star has two "companions," tenth magnitudeGamma Cnc B two minutes of arc away and twelfth magnitude Aa at oneminute away. Both just lie along the line of sight (with Gamma Ba real double, about which nothing is known). The chemicalabundances of Asellus Borealis are normal, the result of mixing ofgases caused by rapid rotation, and while the star is occasionallyocculted by theMoon, its disk remainsunresolved. Written byJim Kaler 5/01/09; updated 7/24/09. Return toSTARS.
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