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.