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.
Written by Jim Kaler. Return to STARS.