SEAT (Pi Aquarii). Shining at fifth magnitude (nominally 4.66), the faintest and most northerly star of one of the most famous asterisms of the sky (the four-star Y-shaped "Water Jar" of Aquarius), Pi Aquarii is oddly one of two that carry proper names. The brightest of the set of four (Zeta Aquarii, 3.7, the one in the middle of the "Y") lacks one as does number three in brightness (Eta, 4.02). That leaves number two (Sadachbia, Gamma Aqr, 3.84) and finally "Seat," a name (such as it is) made up in the seventeenth century that refers back to the meaning of Sadachbia as a "lucky star of hidden things" (which comes from the Arabic tradition and has nothing to do with a water bearer). Though the faintest of the quartet, Pi Aquarii (best to drop the proper name) distinguishes itself by being the farthest (338 light years), and at the hot end of class B (B1) by far the hottest, various measures averaging 26,500 Kelvin. Like many of its kind, this hot hydrogen- fusing dwarf is a "B-emission" ("Be") star, one with a surrounding hot equatorial disk of its own making in part as a result of its very fast rotation, in this case at least 270 kilometers per second. It has even been ranked as a "shell star," one with a thick disk set more or less edge-on to the line of sight. (Fast rotation makes the star highly oblate, which makes the temperature problematic, as the flattened poles become hotter than the extended equator.) Be stars are unstable, Pi Aquarii varying between magnitudes 4.5 and 4.8 over an interval of several decades.
Pi Aqr Pi Aquarii, a "B-emission" star that possesses an unstable extended equatorial disk, varies by several tenths of a magnitude (visual magnitude, "V") over an observed interval of 40 years. Most, if not all, of the variations are the result of the disk and a fierce, variable wind. From a paper in the Astrophysical 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 be around 60 degrees, giving a true rotation speed of 300 km/s. From distance and temperature (to account for a LOT of ultraviolet light), we find the star's luminosity to lie between 15,000 and 17,500 times that of the Sun (depending on how much we allow for absorption by intervening dust, which falls between 0.15 and 0.32 magnitudes). Luminosity and temperature then give a mass between 12 and 13 solar masses, a radius of about 6 times solar, and a rotation period of just a day. Now in the middle of its dwarf lifetime of between 12 and 14 million years, Pi Aquarii is a clear candidate to explode someday as a supernova. Variations in the spectrum reveal a companion that orbits with a period of 84.1 days, its mass estimated between 2 and 3 times that of the Sun, which (given the mass of Pi Aqr proper) yields an average separation of about 0.9 Astronomical Units. Pi Aquarii has one the highest mass loss rates found among Be stars, just over two billionths of a solar mass per year, 100,000 times that of the Sun, the wind flowing at a speed of up to 1500 kilometers per second, it and the unstable disk responsible for the brightness variations. Some Be stars, notably Gamma Cassiopeiae and Delta Scorpii are known to go into extreme outburst states. So keep your eye on the Pi.
Written by Jim Kaler. Return to STARS.