PI PUP (Pi Puppis). Almost seeming to be a part of Canis Major, almost connected to the lower triangle
that makes the Greater Dog, Pi Puppis shines at us at bright third
magnitude (2.70) from the constellation Puppis, the Stern. If it were farther north, the star
probably would have acquired a proper name. Though way down the Greek letter list, Pi Pup is third brightest
in the constellation, in part because the Greek letters are
distributed among the three parts of Argo: Vela, the Sails; Carina the
Keel; and Puppis the Stern. In Puppis, they do not begin until Zeta. The star itself is a glory far beyond
its recognition in the sky and in the literature. This class K
(K3) supergiant
lies some 925 light years away and shines with the
light of 19,200 Suns. Rather orange in
color, it contrasts smartly with the surrounding hot blue stars of
class B. It is far enough from us that the dimming effects of
interstellar dust become somewhat evident. Were none in the way,
Pi Pup would appear some 30 percent brighter and shine at magnitude
2. The luminosity and temperature of 4000 Kelvin tell of a star
with a vast radius 290 times that of the Sun, or 1.35 Astronomical
Units, 35 percent larger than the orbit of Earth. Well evolved
from hydrogen fusion, the star is now probably fusing helium in its
core. With a mass of between 13 and 14 solar masses, it will most
likely
explode. A bit over a minute of arc away lies a class B
(B9.5) 2.5 solar mass companion. At a minimum separation of 20,000
Astronomical Units, the stars must take at least 700,000 years to
orbit each other. Most significantly, Pi Pup is a part of a
barely-recognized cluster called Collinder 135. Readily visible to
the naked eye as a small clump of fifth and sixth magnitude stars
around Pi (and many more below naked-eye vision), the reality of
the cluster was discounted until the Hipparcos satellite revealed
similar distances to its stars. The distance used here in fact
comes from the average of the those of the cluster stars. The
distance of Pi itself is measured at near 1000 light years, the
difference certainly the result of simple errors associated with
stars this far away. The brighter stars of the cluster are all
blue class B2 dwarfs. Pi was the most massive of the group, and
therefore was the first to turn into a supergiant. Only 15 million
years ago Pi was a much hotter class B0 star. The others will
follow Pi's lead in another 30 or so million years.