BETA ARA (Beta Arae). Beta Arae, an orange class K (K3) supergiant (one on the faint side) at a distance of just about 600 light years, shows us how easily it is to be fooled. A minor point is that in this constellation, Ara (the Altar), just south of Scorpius, the third magnitude Beta star (2.85) is brighter than the Alpha star (none of those in the constellation actually "named"). That is a bit of an overstatement, however, as the two stars are so close in magnitude. Separated by a mere ten percent, they so close that the human eye cannot quite detect the difference. More to the point, ground-based observations of Beta and Gamma Arae, the latter lying not quite a degree to the south of Beta, suggested that the two were moving through space together and might actually be an ultra-wide double star. Much improved data taken with the Hipparcos satellite (which measured precise distances and motions for over 100,000 stars) showed the supposition to be quite untrue, as Gamma lies 1140 light years away, almost twice as far as Beta's 603 light years, and has different motions as well (rather too bad, as it would have made for a fine story had they been a real pair). By itself, Beta still has a few things that recommend it. A class K (K3) supergiant (or bright giant), it is quite luminous. At a temperature of 4582 Kelvin, it shines 4600 times more brightly than the Sun, making it 92 times the solar radius, 0.43 Astronomical Units (nearly half the distance between Earth and Sun). From these figures and the theory of evolution (the star is alone, no companion to give its mass), the mass must be between 6.2 and 7.3 times that of the Sun (there is some ambiguity), meaning that it began its life at least as hot as a class B3 star less than about 50 million years ago. Gamma, its once-thought "companion" is an even more massive class B supergiant that is behind Beta in its evolution, making the two an even-more impossible pair, as more massive stars die first. As stars develop from dwarfs to giants, they must rotate more slowly, and for sure Beta does. Spinning at least 5.4 kilometers per second at it equator, it takes (at most) 2.33 years to make a full rotation. More interesting perhaps is that Beta Arae is a rather unusual "super metal-rich" star, with an iron content over three times that of the Sun.
Written by Jim Kaler. Return to STARS.