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SEAT (Pi Aquarii). Shining at fifth magnitude (nominally 4.66),the faintest and most northerly star of one of the most famousasterisms of the sky (the four-star Y-shaped "Water Jar" ofAquarius), PiAquarii is oddly one of two that carry proper names. The brightestof the set of four (Zeta Aquarii, 3.7,the one in the middle of the "Y") lacks one as does number three inbrightness (Eta, 4.02). That leavesnumber two (Sadachbia, Gamma Aqr,3.84) and finally "Seat," a name (such as it is) made up in theseventeenth century that refers back to the meaning of Sadachbia asa "lucky star of hidden things" (which comes from the Arabictradition and has nothing to do with a water bearer). Though thefaintest of the quartet, Pi Aquarii (best to drop the proper name)distinguishes itself by being the farthest (338 light years), andat the hot end of class B (B1) by far the hottest, various measuresaveraging 26,500 Kelvin. Like many of its kind, this hot hydrogen-fusing dwarf is a "B-emission" ("Be") star, one with a surroundinghot equatorial disk of its own making in part as a result of itsvery fast rotation, in this case at least 270 kilometers persecond. It has even been ranked as a "shell star," one with athick disk set more or less edge-on to the line of sight. (Fastrotation makes the star highly oblate, which makes the temperatureproblematic, as the flattened poles become hotter than the extendedequator.) Be stars are unstable, Pi Aquarii varying betweenmagnitudes 4.5 and 4.8 over an interval of several decades.

Pi Aquarii, a "B-emission" star that possesses an unstable extendedequatorial disk, varies by several tenths of a magnitude (visualmagnitude, "V") over an observed interval of 40 years. Most, ifnot all, of the variations are the result of the disk and a fierce,variable wind. From a paper in theAstrophysical Journal,vol. 573, p. 812, 2002, by K. S. Bjorkman, A. S. Miroshnichenko, D.McDavid, and T. M. Pogrosheva (including data from D. McDavid,Publ. Astro. Soc. Pacific, vol. 11, p.494, 1999).

The tilt of the disk's axis to the line of sight is estimated to bearound 60 degrees, giving a true rotation speed of 300 km/s. Fromdistance and temperature (to account for a LOT of ultravioletlight), we find the star's luminosity to lie between 15,000 and17,500 times that of theSun (depending onhow much we allow for absorption by intervening dust, which fallsbetween 0.15 and 0.32 magnitudes). Luminosity and temperature thengive a mass between 12 and 13 solar masses, a radius of about 6times solar, and a rotation period of just a day. Now in themiddle of its dwarf lifetime of between 12 and 14 million years, PiAquarii is a clear candidate to explode someday as asupernova. Variations in thespectrum reveal a companion thatorbits with a period of 84.1 days, its mass estimated between 2 and3 times that of the Sun, which (given the mass of Pi Aqr proper)yields an average separation of about 0.9 Astronomical Units. PiAquarii has one the highest mass loss rates found among Be stars,just over two billionths of a solar mass per year, 100,000 timesthat of the Sun, the wind flowing at a speed of up to 1500kilometers per second, it and the unstable disk responsible for thebrightness variations. Some Be stars, notablyGamma Cassiopeiae andDelta Scorpii are known to go into extremeoutburst states. So keep your eye on the Pi.

Written byJim Kaler. Return toSTARS.

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