29 CMA (29 Canis Majoris). Among the most brilliant of constellations, Canis Major (Orion's Big Dog) holds the brightest star of the sky, Sirius, as well as the faintest of first magnitude, Adhara. In the southern part of the constellation, to the east of Adhara, Canis Major is also host to two rare naked eye class O stars, Tau CMa, a nominal fourth magnitude O9 supergiant that inhabits the open cluster NGC 2362 and, less than half a degree to the north, 29 CMa, which is listed as a hotter fifth magnitude (4.98) O7 supergiant (but see below). Little is certain about the star. It's been suggested to be a runaway from the aforementioned cluster, but at a distance of 1900 light years it's much closer, Tau an uncertain 3000 light years away, the cluster 4500 or so. But 29 CMa could statistically be as close as 1650 or as far as 2260, and there are uncertainties associated with the other distances, so it's certainly possible. At first our star looked so simple, an eclipsing binary of equal components with a period of 4.39341 days that clip each other to produce a variation of a few tenths of a magnitude, which gives it the variable star name UW CMa. But then it all falls apart. There is another sixth magnitude (5.98) component at a separation of 0.2 or so seconds of arc, 29 Ab, which makes the brighter eclipsing double 29 Aa and lowers its magnitude to 5.53. And there may be yet another member, tenth magnitude 29 CMa E at a full second of arc separation. (Faint and more distant 29 B, C, and D seem to be just line-of-sight coincidences.) So the picture after evaluation of the magnitudes, including 0.58 magnitudes of dimming by interstellar dust, is that of an inner pair of close O7 "supergiants" orbited by a B0 dwarf, the inner triple in turn orbited by a B7 dwarf. O stars first. The temperature, needed to evaluate a whopping amount of ultraviolet radiation, falls between 33,200 and 38,000 Kelvin; we'll adopt a weighted mean of 36,400. Each of the O stars then has a luminosity of around 35,400 Suns and a radius of 4.7 solar. Projected equatorial rotation velocities for one of the O dwarfs is 128 kilometers per second, 176 for the other, which yields rotation periods under 1.8 and 1.3 days, faster than required for synchronous rotation. Theory shows the masses of each to be around 20 Suns and that the stars are young dwarfs just starting their hydrogen-fusing lifetime of 8 million years. However, the stars fall into a difficult fix. They are either too hot for their luminosities or too faint for their temperatures, so something is wrong, which is no surprise. In any case, Kepler's laws suggest that the two O7 stars of 29 CMa Aa orbit with a separation of just 0.14 Astronomical Units. They may in fact transfer some mass, one to the other, as one is given as O7e, for emission of radiation, suggesting free-flowing gas. Ab, the B0 dwarf, orbits these at maybe 120 AU, and from a probable mass of 13 Suns takes 175 years to go around. 29 E, the outer B7 dwarf, should come in at around 3 Suns. At a distance from the inner triple of some 600 AU, it would take 2500 years to make a full circuit. Or not, as none of the orbits are really known. All is uncertain, not surprising for stars of this sort. On a minor note, 29 Uma might be the exciting star of a local diffuse nebula, S 310. Or not. One thing is for sure, however, that the inner O stars will blow up as supernovae. Unless one is ejected by the explosion, they might wind up as an exceedingly rare double neutron star. The B0 star, which may well be ejected, should explode too. 29 CMa E, which will almost certainly be ejected, can watch the fun.

Written byJim Kaler 4/24/15. Return to STARS.