OMI-1 CMa (Omicron-1 Canis Majoris). The "Omicrons" of Canis Major are often spoken of in one breath. Situated between Wezen and Sirius, the pair splits the 15th letter of the Greek alphabet into two, Omicron-1 lying two degrees to the west of Omicron-2 (and not part of the Arab's "Virgins," as is Omicron-2). Though at fourth magnitude (3.87) the fainter of the pair, in some ways it is the better known because of its seemingly intimate relation to a star cluster called Collinder 121. The two stars are not a double, and are not gravitationally bound. At an uncertain distance of 2000 light years, Omi-1 is at least 70 light years from Omi-2. Yet the two are oddly related, both brilliant supergiants, Omi-2 blue class B and hot, Omi-1 a much cooler (4000 Kelvin) orange class K (K3) star. Part of the star's relative dimness comes from absorption of starlight by interstellar dust that lies in the Milky Way. Omi-2 is dimmed by only 0.2 magnitudes (about 20 percent), while Omi-1 suffers 1.12 magnitudes of absorption (a factor of 2.8). Were there not dust in the line of sight, Omi-1 would be closer to second magnitude and would just outshine Omi-2. Omi-1, however, still pumps less energy into space, as correction for its infrared radiation is notably less than the ultraviolet correction for Omi- 2. Omi-1's luminosity of 65,000 Suns tells of a magnificent 18 solar mass star with a diameter 530 times that of the Sun, 60 percent larger than the orbit of Mars. Fusing helium in its core, the star is on its way out, and like Omi-2 is destined someday to explode in a grand supernova. While in no way a true double, the two stars do seem to share a heritage in being born from the same complex of interstellar gas and dust, Omi-1 some 18 million years ago. Omi-1's prominent involvement with the cluster, however, seems to be accidental, the cluster set well behind the star. Thanks to Jeff Bryan, who suggested this star.
Written by Jim Kaler. Return to STARS.