SIGMA PUP (Sigma Puppis). Find the bright triangle south of Sirius in Canis
Major, which holds the last of the first magnitude stars, Adhara, the star lying at the triangle's
southwest apex. Bisect the triangle toward the south and you run
into Pi Puppis (of Puppis, the Stern of Argo),
which in turn lies just to the north-northwest of Sigma Pup. At a
very noticeable third magnitude (3.25), Sigma Pup is representative
of the many more or less anonymous and under-studied stars of the
vast triple constellation (the trio
completed with Vela, the Sails, and Carina, the Keel, which holds the sky's
second brightest star, Canopus). A
rather luminous cool orange class K (K5) giant, Sigma Pup lies at distance
of 194 (plus or minus just 6) light years. After accounting for
invisible infrared radiation,
it shines with the luminosity of 347 times that of the Sun, which leads to a radius of 37 times
solar, smaller than indicated by published indirect estimates of
angular diameter (which give 44 solar). Like many stars of its
kind, the mass as derived from theory is indeterminate, but
probably lies in the neighborhood of twice solar, giving it an age
of around 1.4 billion years. Starting life as a white class A
star, it gave up core hydrogen fusion some 300 million years ago.
After shedding its outer envelope, it will
die as a modest white
dwarf of about six-tenths of a solar mass. While seeming as
just another class K giant, Sigma Pup has three things to recommend
it. First, it is likely to be on the rise as giant in which core
helium fusion has ceased and fused to a mix of carbon and oxygen.
Second, it is at least double
and may well be triple. Very close in to Sigma proper is a
spectroscopically and interferometrically observed companion in a 257.8-day, somewhat
elliptical, path. Measures suggest an orbit with a mean stellar
separation of half an Astronomical Unit, which through
Kepler's laws leads to an absurdly low sum of masses, the star
clearly requiring more work. At a current separation of 74 seconds
of arc lies a 9th visual companion, Sigma Pup B. If real, from its
brightness it is a G3 dwarf very similar to our own Sun that would
lie at least 1300 AU away and have a period of more than 27,000
years. At first, Sig B seems to be changing its position relative
to Sigma A somewhat too quickly, having shifted away by four
seconds of arc over a 181 year period, leading to a conclusion that
it is just a line-of-sight coincidence. But then we look at the
Sigma A's motion. It's a high-velocity star zipping along relative
to the Sun at a speed of 103 kilometers per second, some six times
normal. It's moved across the sky by some 35 seconds of arc in
that 181 year interval, while Sigma B has moved just four,
suggesting that the stars may indeed be traveling together. Sigma
Pup thus epitomizes a general rule, that if you pick any star at
random, you will find unique, if not curious, things about it, no
two of the shining stellar lights the same.
Written by Jim Kaler 3/16/12. Return to STARS.