THETA AND LAMBDA CMA (Theta and Lambda Canis Majoris), a two for one special. Outwardly ordinary, Theta and Lambda Canis Majoris are really a mapreader's special, as they are Canis Major's named extremes: Theta CMa is the most northerly star carrying a Greek letter, while Lambda CMa is the most southerly (just beating out Kappa CMa for the title by a mere four minutes of arc, 0.07 degree). Both fourth magnitude, Theta (mag 4.07) and Lambda (4.48) also present an admirable study in physical contrast, making it fun to place them together and showing more directly the nature of stellar diversity. The northern one (Theta) is an orange class K (K4) giant 261 light years away (give or take 13), while the southern (Lambda) is a blue-white class B (B4) hydrogen fusing dwarf at a distance of 424 light years (plus or minus 9). With a surface temperature of 4000 Kelvin (needed to account for a fair bit of infrared light), Theta shines with the light of 334 Suns, which with temperature gives it a radius of 38 times solar, 0.18 Astronomical Units, or 45 percent the size of Mercury's orbit. Theory then tells that Theta CMa carries a mass of about 1.7 times that of the Sun and suggests that it is a "red giant" with a dead helium core brightening in preparation for the ignition of helium, which it will fuse to carbon and oxygen, the star's age just under two billion years. After ejecting its outer envelope, it will die as a modest white dwarf with a mass of about 60 percent that of the Sun.

Lambda CMa, on the other hand, glows at a much higher temperature of 16,700 Kelvin, from which we derive a lot of ultraviolet light at the other end of the spectrum, which together with distance gives it a considerably higher luminosity of 940 Suns, the result of a much higher mass of 5.5 times solar. (That is, up to a point, higher mass among dwarfs can beat out even the severe brightening effects witnessed among lower mass giants.) With a radius of just 3.7 times that of the Sun, Lambda CMa contrasts with Theta through its fairly recent birth. Its hydrogen fusing dwarfhood lasts a total of just 63 million years (massive stars burning out by far the more quickly). As a swollen lower mass giant, Theta CMa would be rotating slowly (though with an unknown period), while Lambda is a fast spinner typical (if not on the low side) of its breed. With an equatorial rotation speed of at least 128 kilometers per second, it completes a rotation in under 1.4 days (the axial tilt unknown). With a much higher initial mass, Lambda will expire as a white dwarf with a mass close to that of the present Sun. They do share something of high speed relative to the Sun, but even here they differ, Lambda going at 35 kilometers per second (two to three times normal), while Theta rockets along at 109 km/s, suggesting that it belongs to an older population, as witnessed also by as lower iron content of 60 percent that of the Sun. Both seem to be single, with no companions sharing their rides.

Written by Jim Kaler 3/8/13. Return to STARS.