GAMMA ARA (Gamma Arae). One of the most southerly of ancient constellations, Ara, the Altar, is seen by northerners to hang low to the south beneath the tail of Scorpius, the Scorpion, where haze and horizon obstructions block the view. Ara is not even seen above about 40 degrees north latitude. Only from the southern hemisphere is 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 and Beta) vying for lead in brightness, with Beta getting the bare edge. While Gamma Arae, at magnitude 3.31, comes in fourth (just after Zeta Arae), it is by far the most magnificent 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 32,600 Suns. From luminosity, temperature, and evolutionary theory we estimate a mass of 12 times that of the Sun and a radius 14.5 times solar. The star has just left the "main sequence" of hydrogen-fusing dwarfs, and with a dead helium core is on its way to becoming a grand red supergiant rather like Betelgeuse or Antares. Only 16 or so million years old, it most likely fated to explode as a supernova. In the meantime it is losing mass through a variable two-component wind, one blowing at 750 kilometers per second, the other at double that speed. Gamma Arae is most noted, however, for its high minimum rotation speed of 280 kilometers per second, quite unusual for a growing supergiant. Given the diameter, the star makes a turn in under two and a half days. (The B0 dwarf from which the star developed must have been spinning very fast.) Gamma is listed as having two companions. Twelfth magnitude Gamma-C is merely a line of sight coincidence, but Gamma-B seems truly to belong to Gamma proper. This 10th magnitude (10.3) class A7 dwarf (quite similar to Altair of Aquila) sits 18 seconds of arc away, which translates into a real separation of 6200 Astronomical Units. If there is no foreshortening, Gamma-B takes 135,000 years to make a full orbit (more if there is foreshortening), so while a real companion it must be only loosely bound to Gamma-A. At that separation, from Gamma-A, Gamma B would shine roughly with the light of a quarter Moon, while from B gets illuminated by A with the light of 130 full Moons.
Written by Jim Kaler. Return to STARS.