PI PEG (Pi Pegasi), both PI-1 AND PI-2. Appearing right next to
each other, separated by but 10 minutes of arc (1/6 degree), the
two Pi's of Pegasus can hardly be
taken separately. Though they may remind us of Mizar and Alcor,
appearing to be something of a naked-eye double star, they are
unrelated, yet present a pretty sight (use binoculars) that is
closer to the stars of Lacerta, the
Lizard, than to the prominent Square than makes the bulk of Pegasus. The numbers
relate not to brightness rank, but to position, just-barely-sixth
magnitude (5.59) Pi-1 to the west of brighter fourth magnitude
(4.29) Pi-2. While just an "optical" line-of-sight double (not
related by gravity), the two are still fairly close together, Pi-1
283 light years away, Pi-2 252 light years, making them just 31
light years apart. Given the errors in the measured parallaxes, they could
actually be at the same distance, which would separate them by but
0.22 light years, or 47,000 Astronomical Units. They are, however,
moving in quite different directions at different speeds,
separating at some 25 kilometers per second, so there is no
question about their being unrelated; they are just visiting each
other. Rather oddly, Pi-1 and Pi-2 Peg are similar stars with
similar ages, respectively class G (G6) and F (F5) dying giants, though in different
stages of "gianthood." Pi-1, the fainter, has a temperature of
4790 Kelvin, a luminosity 51 times that of the Sun, a radius 10 times solar (not so much for
a "giant"), and a mass double solar. It is a "clump star" some 600
million years old that is quietly fusing helium into hydrogen in
its deep core ("clump" referring to the large numbers of similar
stars that lie plotted on a graph of temperature vs. luminosity).
Slowly rotating with a velocity of at least 6 kilometers per
second, it may take as much as 89 days to make a full spin. Nearby
lie four 10th through 12th magnitude stars that are also accidental
line-ups and not real companions. Pi-2 is much more interesting.
A warmer temperature of 6320 Kelvin and a luminosity of 92 solar
(radius 8 solar) reveal a larger mass of 2.5 solar and tell of a
relatively rare kind of star with a quiet helium core that is in
transition to becoming a true helium-burner. Pi-2 is also a rare
class F "shell star," one with a circumstellar disk of surrounding
matter that has an unusually large rotation speed for a giant, 145
kilometers per second, which gives a rotation period less than 1.8
days. Rotations slow as stars expand (conservation of angular
momentum, the same reason skaters speed up as they bring in their
arms), but Pi-2 is not far enough along for much braking to have
taken place. About 590 million years ago it was a rapidly spinning
class B8 star, and is therefore probably the descendent of a disk-
encased B-emission star (similar to, say, Zeta Tauri). Consistently, the star
reveals a magnetically heated outer chromosphere (produced in part
by rotation). If nothing else, the pair shows us how coincidences
abound in astronomy and why measurements are needed to ferret out
the relational truth. And though unrelated, the stars' brief
visitation makes them prominent in each others' skies. If at their
stated distances, Pi-1 is first magnitude (0.8) as seen from Pi-2,
while Pi-2 glows at magnitude zero (-0.2) from Pi-1.