PI-1 GRU (Pi-1 Gruis, plus a bit on Pi-2). Within the southern triangle of modern Grus (the Crane), the binoculard eye might be drawn to what appears to be a naked-eye double, the constellation's "two pieces of Pi." Just eight minutes of arc apart, the pair is slightly dominated by the more easterly of the two, sixth magnitude (5.62) Pi-2, a class F (F3) giant-subgiant (but see below) 130 light years away. The fainter, a sixth magnitude semi-regular variable that goes between approximate magnitudes 5.4 and 6.7 over a 150(?) day period is much farther, 532 light years (with an uncertainty of 66), so the two obviously have nothing to do with each other except that P1-2 provides a guide to Pi-1 and makes a fine comparison star for watching its variation. Despite its faintness, at the edge of naked-eye vision, Pi-1 Gru is quite the special star, as it's one of a trio of prototypes that in 1922 defined spectral class S. The others are the long-period Mira variables Chi Cygni (magnitude 3.5-14 over a 407 day period, 590 light years away) and R Andromedae (5.3-15.1, 409-days, too far for parallax). Cool class M giants are characterized by absorption bands of titanium oxide in their spectra, whereas S giants like Pi-1 Gru (subclass S5), which fall in the same temperature range of around 3000 Kelvin, display bands of zirconium oxide. S stars are advanced giants that are becoming carbon stars like R Leporis in which the carbon-oxygen ratio is more than reversed with freshly made carbon dredged up from below. They have all completed core helium fusion and are sloughing off their bloated outer layers to reveal their inner carbon/oxygen cores, which (after producing planetary nebulae) will become white dwarfs. When the rising carbon abundance roughly equals that of oxygen, the zirconium oxide tops the titanium oxide, the process enhanced by new zirconium (also created by nuclear processes) again dredged up from deep inside. The total luminosity of Pi-1 Gru has been measured at 15,850 Suns. The radius, found through interferometry, is spectacularly large, 370 times that of the Sun, 1.7 Astronomical Units, over 10 percent bigger than the orbit of Mars. From these, the temperature must be close to 3380 Kelvin. With an uncertain mass estimated from theory to be five or so times that of the Sun, as Pi-1 Gruis ages, it will most likely become a full-fledged Mira-type star. With a current mass-loss rate of around 5-millionths of a solar mass per year, the star is surrounded by a dusty molecular shell that extends outward for several stellar radii. All this action is watched by a sunlike class G (G0) dwarf that has kept pace at a separation of 2.5 seconds of arc over the better part of a century, so it clearly belongs to the larger star. Shining just 15 percent or so more brightly than the Sun, the little one orbits at a safe distance of greater than 400 AU and takes at least 3300 years to make a circuit. Unrelated, Pi-2 has a temperature estimated from its class to be 6900 Kelvin, distance and theory giving a luminosity of 7.0 Suns, the star actually a 1.6 solar mass dwarf with a speed relative to the Sun of some three times normal. (Thanks especially to S. Sacuto et al. in "Astronomy and Astrophysics," vol. 482, p. 561, 2008 and to R. Guandalini, ibid, 513, A4, 2010.)

Written byJim Kaler 10/31/14. Return to STARS.