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GAMMA ARA (Gamma Arae). One of the most southerly of ancientconstellations,Ara, the Altar, is seen bynortherners to hang low to the south beneath the tail ofScorpius, the Scorpion, where haze andhorizon obstructions block the view. Ara is not even seen aboveabout 40 degrees north latitude. Only from the southern hemisphereis the glory of the constellation and its setting truly revealed. The top four stars (all without proper names) are third magnitude,the top two (Alpha andBeta) vying for lead in brightness, withBeta getting the bare edge. While Gamma Arae, at magnitude 3.31,comes in fourth (just after Zeta Arae), it is by far the mostmagnificent of them. From its great distance of 1140 light years,this hot class B (B1) supergiant (with a temperature of 20,400)shines with the light of 32,600Suns. Fromluminosity, temperature, and evolutionary theory we estimate a massof 12 times that of the Sun and a radius 14.5 times solar. Thestar has just left the "mainsequence" of hydrogen-fusing dwarfs, and with a dead heliumcore is on its way to becoming a grand redsupergiant rather likeBetelgeuse orAntares. Only 16 or so million years old,it most likely fated to explode as a supernova. In the meantime itis losing mass through a variable two-component wind, one blowingat 750 kilometers per second, the other at double that speed. Gamma Arae is most noted, however, for its high minimum rotationspeed of 280 kilometers per second, quite unusual for a growingsupergiant. Given the diameter, the star makes a turn in under twoand a half days. (The B0 dwarf from which the star developed musthave been spinning very fast.) Gamma is listed as having twocompanions. Twelfth magnitude Gamma-C is merely a line of sightcoincidence, but Gamma-B seems truly to belong to Gamma proper. This 10th magnitude (10.3) class A7 dwarf (quite similar to Altairof Aquila) sits 18 seconds of arc away, which translates into areal separation of 6200 Astronomical Units. If there is noforeshortening, Gamma-B takes 135,000 years to make a full orbit(more if there is foreshortening), so while a real companion itmust be only loosely bound to Gamma-A. At that separation, fromGamma-A, Gamma B would shine roughly with the light of a quarterMoon, while from B gets illuminated by A with the light of 130 fullMoons.
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