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.