OMEGA CMA (Omega Canis Majoris). This fourth magnitude (4.2) star sits in southern Canis Major (Orion's Larger Dog) well below Sirius just barely to the east of the lower and prominent triangle formed by Delta (Wezen), Eta (Aludra), and Epsilon (Adhara) CMa, the latter at the bottom end of the first magnitude stars. Omega's relative faintness against its brilliant surroundings precludes any sort of proper name, and it wound up with the last letter of the Greek alphabet instead. Most basic star maps don't even bother to include it. Nevertheless, it's well worth keeping an eye on for its occasional outbursts, the star rather suddenly jumping in brightness by a quite-visible half a magnitude or so. It's not the star proper doing it, but the rise and fall of an erratic circumstellar disk. But we are ahead of ourselves.
Omega CMa The visual variations of Omega Canis Majoris are monitored for seven and a half years, between October 5, 1995, and March 1, 2003 (the lower axis given as Julian Day number, a running count of days since January 1, 4713 BCE of the Julian Calendar). The observations are bracketed by major outbursts. From a paper by S. Stefl et al. in Astronomy and Astrophysics, vol. 402, p. 253, 2003.
Omega Canis Majoris is a hot (at a more-or-less estimated 21,750 Kelvin) class B (B2) subgiant-dwarf 910 light years away (give or take 25) that radiates emissions from hydrogen, making it a "Be star," many of which (Gamma Cas, Zeta Tauri, Delta Scorpii) are dotted across the sky (the Pleiades full of them). Allowance for a lot of ultraviolet light and 0.2 magnitudes (about 20 percent) absorption by interstellar dust gives it a very impressive luminosity 11,800 times that of the Sun and a radius of 7.7 solar. A projected rotation velocity of 112 kilometers per second yields a rotation period under 3.4 days. Be stars, however, are as a class very fast rotators, some spinning three or more times as fast, which strongly suggests that the rotation pole is rather tilted toward us. Temperature, luminosity, and the broad brush of theory give an impressive mass of 10.5 times solar, which should place it above the limit at which it will someday blow up as a supernova. But it has a ways to go, since as of now it's just reaching the end of its hydrogen- fusing lifetime (making "subgiant-dwarf" an apt description) with an age a bit under 20 million years. Though they are related to rapid rotation, nobody really knows what causes these disks that rather commonly develop around class B stars. Magnetic fields, winds, multi-periodic pulsations have all been invoked, but no theory really works. And that is just for the more or less steady cases. More confusing are the stars whose disks seem go undergo erratic outbursts (including formation and disappearance), Omega CMa doing it every several years, with variations visible to the naked eye (and with smaller outbursts in between). So under a dark sky keep watch over the Greater Dog, as it holds some treasures, including a fascinating star that sits at the alphabet's end, one that is trying to tell us something -- we just yet do not know what.
Written by Jim Kaler 3/05/10. Return to STARS.