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.