GAMMA TRA (Gamma Trianguli Australis). If this star were not so bright -- within the top 150 stars -- it would probably be ignored. But at mid-third magnitude (2.89), it just barely makes the list at number 149, and thus earns some measure of respect, which it deserves if for no other reason than that it anchors the southwest apex of bright and obvious Triangulum Australe (the Southern Triangle), which is dominated by mid-second magnitude Atria. Lying 183 light years away from us, Gamma TrA seems at first to be just an ordinary class A (A1) dwarf rather like Sirius, just fainter as a result of its much greater distance. However, there are some big differences, showing that spectral class is but a beginning, that without other data we can be mightily fooled. With a luminosity of 220 times that of the Sun, Gamma TrA is more than eight times more luminous than the "Dog Star." Luminosity and a temperature of 10,060 Kelvin tell of a radius 4.9 times that of the Sun and a mass of between 3.2 and 3.4 times solar. The difference between it and Sirius is readily explained. Luminosity is terribly sensitive to mass. Very roughly, double the mass and a star becomes 10 times brighter as a result of increased compression and internal temperature. Gamma TrA is almost 60 percent more massive than Sirius. In addition, even while quietly fusing hydrogen in their cores, dwarf stars slowly brighten. Rather than being a dwarf, Gamma is really at or near the end of its stable hydrogen-fusing life, and is better described as a subgiant, one on its way to becoming a true giant star, while Sirius is not so far along its evolutionary path. Gamma is seen listed as a "chemically peculiar" star, one with odd chemical abundances as a result of separation of different kinds of atoms thanks to gravity and radiation. But there is no confirmation. Moreover, such "CP" stars are slow rotators, while Gamma TrA is spinning pretty fast, more than 200 kilometers per second at its equator, giving it a rotation period of under 1.2 days. It's also on lists as having a circumstellar debris disk, perhaps from some kind of planetary system, but the latest results remove it quite firmly from that category. The star's most interesting feature is that it is a guide to the very red carbon star X TrA, which lies 1.4 degrees to the southwest of it.
Written by Jim Kaler 9/14/07. Return to STARS.