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.