IOTA CMA (Iota Canis Majoris). Sirius, the brightest star of Canis Major, the Larger Dog (one of Orion's hunting companions), shines so brightly at nearly magnitude -2, that one hardly pays attention to its surroundings, let alone fourth magnitude (4.37) Iota Canis Majoris, which weakly glows a bit short of three degrees to the east of its far more brilliant neighbor. Sirius, though, is bright mostly because it's so close, just 8.6 light years away. Iota, on the other hand, lies at a distance of 2500 light years (give or take 250), nearly 300 times farther, and is obviously by far the more magnificent star. Correction for a bit of interstellar dust-dimming would raise it to magnitude 4.02, but still short of ready viewing (the star in rather a clear path).
Iota CMa Fourth magnitude Iota Canis Majoris lies at center, and is rather overwhelmed by brilliant Sirius (toward upper right). Sirius, however, is almost 300 times closer. Comparable in apparent brightness, Muliphein (Gamma Canis Majoris) is up and to the left. The prominent, young (250 million year old) open cluster Messier 41, 2250 light years away (about the same distance as Iota), is near the lower right corner.
A blue-white class B (B3) bright giant (re-classed of late as a lesser supergiant (which seems more appropriate) with a high temperature of 16,750 Kelvin, Iota CMa shines (with ample correction for ultraviolet light) at a luminosity of 46,000 times that of the Sun! Temperature and luminosity then lead to a radius of 26 solar. Given a projected equatorial rotation velocity of 54 kilometers per second, the star completes a rotation in under 24 days. And given too that class B stars tend strongly toward far more rapid rotation speeds, it's probable that the rotation pole more or less points at us. Theory gives it a mass of about 14 times that of the Sun, shows that it has recently given up core hydrogen fusion, is around 11 million years old, and deserves its supergiant-bright giant status. Iota CMa was long mistaken as a Beta Cephei fast variable, but that is clearly a mistake, which goes along with the supergiant class, as such variables are of lesser luminosity. Sadly, it's also been rejected as a member of the Collinder 121 expanding association of massive stars (named after them embedded open cluster), which includes such well-known members as Delta CMa (Wezen) and Omicron-1 CMa. "Associations" are loose assemblies of stars that were born more or less at the same time from huge clouds of interstellar gas. Though they are falling apart, the stars live such short lives that they do not get very far from their birthplaces, and thus hang together. Before long (on an astronomical timescale), Iota CMa will explode as a wondrous supernova 100,000 times brighter than it appears now.
Written by Jim Kaler 2/26/10. Return to STARS.