BETA TRA (Beta Trianguli Australis). While not a dramatic star,
third magnitude (2.85) Beta TrA, the number two star of Triangulum Australe, the Southern Triangle, is still so
bright -- well in the top 150 -- that it surely deserves
recognition, especially since TrA itself is a nice, bright
constellation. The star also shows us the various difficulties we
can encounter with our celestial neighbors. At a distance of just
40 light years, this class F (F2) so-called giant is one of the
closer stars to the Earth. The star's proximity and a rather
rapid motion almost perpendicular to the line of sight give it
a rather high motion across the sky of nearly half a second of
arc per year. If it really were a giant, it would
be third closest of its kind, after Pollux and Arcturus. However, it isn't. With a
temperature of 7220 Kelvin, Beta shines with the modest light of
8.5 Suns, from which we derive a radius of
1.9 times solar and a mass of 1.65 solar. Luminosity, temperature,
and theory clearly show the star to be an ordinary hydrogen-fusing
dwarf that is about halfway through its billion-year dwarf
lifetime. A fairly quick projected equatorial rotation speed of 92
kilometers per second gives it a rotation period of just under a
day. The rotation and surface convection together produce a
surrounding corona and a measurable magnetic field. The chemical
composition relative to the Sun is "all over the map," with iron
and oxygen apparently enhanced, while other elements (carbon,
silicon, sulphur, calcium) are reduced, which is nothing unusual
given stars' various birthplaces. Beta TrA is listed as a "wide"
double star with a 13th
magnitude "companion" almost three minutes of arc away. The color
of the companion, though, suggests a class K dwarf, which would be
much brighter, though it could be a white dwarf. Most likely,
though, the companion is just another line-of-sight coincidence.
Written by Jim Kaler 8/10/07. Return to STARS.