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.