Beta PsA

BETA PSA (Beta Piscis Austrini). Piscis Austrinus, the Southern Fish, is best known for its bright, planet-holding luminary, first magnitude Fomalhaut, and not much else. Well, there is nutty Pi PsA, but that is another story. Most constellations have Beta stars that are reasonably notable, but not this one. Within the Southern Fish we drop from Fomalhaut's magnitude 1.17 to fourth magnitude, beginning with Epsilon at 4.18, then going to Delta (4.23) before arriving at Beta PsA at 4.29, ranking not second (as might be expected), but fourth (then followed by Iota at 4.34 and finally Gamma at 4.50). There was no way that Bayer could discriminate these subtle differences with the naked eye, so he seems to have used position, with Beta, Gamma, Delta, and Epsilon (all in letter order) named clockwise from the south in a ragged circle that includes obvious Fomalhaut. Poor Beta is about as neglected as can be. There are a mere 54 references to it in the professional literature over the past century; Fomalhaut has 10 times as many. And there is something "fishy" (sorry) about what is found. First, though, what is securely known. Beta PsA is, like Fomalhaut, a white class A hydrogen-fusing dwarf, but at A0 a bit warmer than its bright class A4 neighbor. The apparent faintness derives from a much greater distance of 142 light years, more than five times Fomalhaut's. From that and temperature (to allow for some ultraviolet radiation), we find a luminosity of 33 times that of the Sun, a radius just over double solar, a rotation period of under three days (from a projected rotation velocity of 36 km/s), and a mass of 2.3 times solar. The fairly youthful star is a quarter to a third through its 560 million year core-hydrogen-fusing lifetime. Now for the awaited fishy stuff. There is one reference to Beta PsA having a surrounding disk that would make it a class A version of a "B-emission" star like Zeta Tauri. But there is no confirmation, and it probably is not. There are a number of references to a "chemically peculiar" nature with magnetic spots (like Alpha-2 Canum Venaticorum). But there are no data to back the contention up. And Beta PsA has a "companion." Or not. At a current separation of 35 seconds of arc lies a seventh magnitude neighbor. Between 1826, when it was first observed, to 2002, the thing moved 5.3 seconds of arc farther away from its originally- observed position, which seems far too great given an orbit with a radius of 1500 Astronomical Units, so it's probably just a line-of- sight coincidence. Our star is thus most likely all alone, apparently unloved by either a companion or, for that matter, by astronomers.

Written by Jim Kaler 12/11/09. Return to STARS.