Gamma Piscis Austrini

GAMMA PSA (Gamma Piscis Austrini). Piscis Austrinus, the Southern Fish, is part of the sky's "Wet Quarter," in which we find zodiacal Capricornus (the Water Goat), Aquarius (the Water Bearer, who is often depicted as pouring water into the Fish's mouth), and Pisces proper. If it were not for first magnitude Fomalhaut, though (from Arabic, meaning the "fish's mouth"), nobody would have bothered making a constellation of the dim stars that surround it, the brightest being fourth magnitude Epsilon PsA well to the west of Fomalhaut. Fourth magnitude (4.46, almost fifth) Gamma Psa, though ranking sixth in the constellation, is however among the easier to find, as it is the southwestern of an unrelated pair just south of Fomalhaut that are just three-fourths of a degree apart, the upper one Delta PsA. Piscis Austrinus is one of those constellations where, except for Fomalhaut as Alpha, the Greek letters were assigned by Bayer according to position. Gamma PsA is listed as a white class A (like Vega, A0, which it is) giant (which it isn't, as seen below). From a distance of 216 light years (give or take 7), and with a temperature estimated at 10,600 Kelvin (a bit high for the class), the star shines with a rather decent luminosity of 81 times that of the Sun, which leads to a radius of 2.7 solar and a mass of 2.9 Suns. Theory clearly shows that the star is really a dwarf about halfway through its allotted 400-million year hydrogen-fusing lifetime (dwarf-giant distinctions in classification among such stars difficult). The rotation speed of 55 kilometers per second leads to a rotation period of under 2.4 days. The relatively leisurely spin is consistent with the star's atmosphere apparently rich in some heavy elements (strontium, chromium, europium), the result of radiative lofting in a fairly quiet environment (though no detailed study has ever been done). Four seconds of arc away lies a probable 8th magnitude companion, Gamma PsA B, that over the years has shown a small amount of motion that is likely the result of slow orbit, the two stars pretty well tracking together through space. If a real binary member, then it has the absolute brightness of a cool class F (F8 or 9) dwarf, which would have a temperature around 6100 Kelvin, a luminosity of 1.9 Suns, and a mass perhaps 25 percent greater than solar. Separated by at least 260 Astronomical Units, the pair must take at least 2200 years to make a full orbit. Analysis of the actual positions suggests a consistent 3000 years. It's certain, though, that nearby Delta PsA has nothing to do with Gamma, as it's 62 light years closer. From each, the other would shine at second magnitude and be nice parts of their constellation patterns -- if there is anybody there to see them, which seems highly unlikely. (Many thanks to Bill Hartkopf for analysis of the binary companion's motion.)

Written by Jim Kaler 10/29/10. Return to STARS.