SIGMA CMA (Sigma Canis Majoris). Canis Major, Orion's larger hunting dog, contains so many sparkling bright blue, blue-white, and white stars (including Sirius, the brightest star in the sky) that lesser lights that would brighten most other constellations are rather ignored. Lying almost on the line between Wezen and Adhara (Delta and Epsilon CMa, the latter the faintest of the first magnitude stars), third magnitude (3.47) Sigma Canis Majoris opposes the Big Dog's blue and white star rule by being rather red. Long defined as a class K (K7) red supergiant, the more modern view takes it into class M (M1.5) rather like Betelgeuse and Antares, which fits better with a temperature of 3750 Kelvin (so we will adopt the M1.5 class here). At a hefty measured distance of 1220 light years (with a 22% uncertainty), and after an uncertain correction of 0.2 magnitudes for interstellar dust absorption, this magnificent "forgotten star" shines at us with a luminosity 23,300 times that of the Sun! From that and temperature, we find a radius of 360 times solar, 1.7 Astronomical Units, big enough that were it to replace our Sun it would go somewhat past the orbit of Mars. While Sigma CMa is often listed as a member of the Collinder 121 association, which contains Delta and Omicron-1 CMa (the latter also a supergiant), its motion and shorter distance shows that it is not. As are many, if not most, stars of its kind, Sigma is a slight irregular variable, erratically changing its brightness by about 10 percent. Luminosity, temperature, and theory reveal a star of 12 solar masses, near or just above the limit at which stars no longer form white dwarfs, but explode as supernovae. Starting life as a class B0.5 blue-white dwarf around 17 million years ago, Sigma gave up hydrogen fusion only 300,000 to 1.5 million years ago (depending on its current exact state), its wind carrying away nearly half a solar mass. No matter what the final state, it does not have too much time left to it. Sigma CMa may or may not be "watched" by a 14th magnitude companion that lies only 10 seconds of arc away (which would make it a class K2 dwarf, place it at a distance of at least 3700 Astronomical Units, and give it an orbital period of at lest 66,000 years). Nearly a second-of-arc's- worth of relative motion over a quarter century, however, strongly suggests that the two have nothing to do with each other, and lie simply along the same line of sight.
Written by Jim Kaler 2/29/08. Return to STARS.