W Canis Majoris (W CMa) is a carbon star in the constellation Canis Major. A cool star, it has a surface temperature of around 2,900 K and a radius 234 times that of the Sun, with a bolometric absolute magnitude of −4.13 and distance estimated at 443 or 445 parsecs (1,444–1,450 light-years) based on bolometric magnitude or radius. The Gaia Data Release 2 parallax of 1.8049±0.1454 milliarcseconds implies a distance of about 555 parsecs. It is a variable star, whose brightness ranges from magnitude 6.35 to 7.90. At its brightest, it might be very faintly visible to the naked eye of an observer with ideal observing conditions.

Variability

In 1901, it was announced that Williamina Fleming had discovered the star, then called BD −11° 1805, is a variable star. It was given its variable star designation, W Canis Majoris, in 1912. W CMa is classified as a slow irregular variable star. Detailed analyses have found only very weak and probably spurious periods of approximately a month. It is a carbon star, an asymptotic giant branch star where carbon and s-process elements have been dredged up to the surface during thermal pulses of the helium-burning shell.