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GAMMA PSA (Gamma Piscis Austrini).PiscisAustrinus, the Southern Fish, is part of the sky's "WetQuarter," in which we findzodiacalCapricornus (the Water Goat),Aquarius (the Water Bearer, who is often depicted aspouring water into the Fish's mouth), andPisces proper. If it were not for first magnitudeFomalhaut, though (from Arabic, meaning the "fish's mouth"), nobodywould have bothered making aconstellationof the dim stars that surround it, the brightest being fourthmagnitude Epsilon PsA well to the west of Fomalhaut. Fourthmagnitude (4.46, almost fifth) Gamma Psa, though ranking sixth inthe constellation, is however among the easier to find, as it isthe southwestern of an unrelated pair just south of Fomalhaut thatare just three-fourths of a degree apart, the upper one Delta PsA. Piscis Austrinus is one of those constellations where, except forFomalhaut as Alpha, theGreek letters wereassigned byBayer according to position. Gamma PsA is listed as a white class A (likeVega, A0, which it is)giant (which it isn't, as seenbelow). From a distance of 216 light years (give or take 7), andwith a temperature estimated at 10,600 Kelvin (a bit high for theclass), the star shines with a rather decent luminosity of 81 timesthat of the Sun, which leads to a radius of 2.7 solar and a mass of2.9 Suns. Theory clearly shows that the star is really a dwarfabout halfway through its allotted 400-million year hydrogen-fusinglifetime (dwarf-giant distinctions in classification among suchstars difficult). The rotation speed of 55 kilometers per secondleads to a rotation period of under 2.4 days. The relativelyleisurely spin is consistent with the star's atmosphere apparentlyrich in some heavy elements (strontium, chromium, europium), theresult of radiative lofting in a fairly quiet environment (thoughno detailed study has ever been done). Four seconds of arc awaylies a probable 8th magnitudecompanion, Gamma PsA B, thatover the years has shown a small amount of motion that is likelythe result of slow orbit, the two stars pretty well trackingtogether through space. If a real binary member, then it has theabsolute brightness of a cool class F (F8 or 9) dwarf, which wouldhave a temperature around 6100 Kelvin, a luminosity of 1.9 Suns,and a mass perhaps 25 percent greater than solar. Separated by atleast 260 Astronomical Units, the pair must take at least 2200years to make a full orbit. Analysis of the actual positionssuggests a consistent 3000 years. It's certain, though, thatnearby Delta PsA has nothing to do with Gamma, as it's 62 lightyears closer. From each, the other would shine at second magnitudeand be nice parts of their constellation patterns -- if there isanybody there to see them, which seems highly unlikely. (Manythanks to Bill Hartkopf for analysis of the binary companion'smotion.) Written byJim Kaler 10/29/10. Return toSTARS.
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