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.