PI HYA (Pi Hydrae). Casual stargazers may look to the prominent head of Hydra, the Water Serpent, then perhaps down to Hydra's heart, bright (mid-second magnitude) Alphard, the "Solitary One," which shines in lonely splendor to the south- southwest of Regulus. The view might then extend to the southeast to admire Corvus (the Crow) and, if under a very dark sky, Crater (the Cup), both of which ride upon Hydra's back (Crater to the west). Nobody thinks much about Hydra's tail, in part because the constellation is so huge, wrapping a third the way around the sky, that it can't easily be viewed in one take. The tail's underappreciated end is represented by rather prominent third magnitude (3.27) Pi Hydrae, which lies about an hour to the west of southern Libra and about ten degrees due north of Menkent (Theta Centauri), and is the eastern-most star in the constellation carrying a Greek letter. Shining at a distance of 101 light years (give or take a half), Pi Hya is also a classic orange class K (K1) giant rather like a distant Arcturus, though with a higher metal content (from scattered measures, the iron abundance about 90 percent solar). With a well-determined temperature of 4660 Kelvin (from which we can derive the amount of invisible infrared radiation), we get a luminosity 98 times that of the Sun, which then (with temperature) gives a radius of 15 times solar, eight percent less than studies that use the star as a calibrator for finding stellar radii from interferometry. Rotation measures are inconsistent, the rotation period perhaps a year, maybe more, possibly less. Though estimation is difficult for such stars, theory best shows Pi to be a helium-fusing "clump star" (the term meaning that there are lots of stars in the same position in a graph of luminosity vs. temperature) with a mass 2.5 times that of the Sun. On a finer scale, it could have just arrived in that situation with an age of 600 or so million years, or it could be another 150 million years older and just leaving it. In either case, Pi Hya started life as a cool-side class B9 hydrogen-fusing dwarf. The star's velocity relative to the Sun of 39 kilometers per second, a bit more than twice normal, is consistent with it being a modestly "cyanogen- weak" star, with a deficiency of carbon or nitrogen or both, the star passing by us from a somewhat different region of the Galaxy than the one in which we find our Sun.
Written by Jim Kaler 4/29/11. Return to STARS.