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.