OMI-2 CMA (Omicron-2 Canis Majoris). Down below brilliant Sirius in Canis
Major (as viewed from the northern hemisphere, up above it as
seen from the southern) lies a notable triangle of bright stars
made of Adhara (Epsilon Canis
Majoris), Wezen (Delta), and Aludra (Eta). The old Arabs referred to
the trio as "the Virgins," and included among them a lesser-known
star between Wezen and Sirius with no proper name, and which Bayer
chose to call "Omicron-2." Its Greek letter mate, to which the
star is not gravitationally attached,
Omicron-1, lies two degrees
to the west. Though at mid-third magnitude (3.02) visually fainter
than the Dog's more famed stars, Omi-2 still fits right in as quite
the magnificent star. It is a glorious blue class B (B3)
supergiant 2500 light years away that lights its surroundings with
the radiance of 110,000 Suns, after
correction for a small bit of dimming by interstellar dust and a
rather large amount of ultraviolet radiation from its 14,700 Kelvin
surface. As a supergiant, it is in the process of evolving, the
luminosity and temperature telling of a 20 solar mass star. Some
eight million years old, Omi-2 ceased core hydrogen fusion only
about 15,000 years ago, and has already begun to fuse the resulting
core helium into carbon and oxygen. The star's only fate seems to
be to explode as a supernova. Omi-2's relation with the orange
class K supergiant Omi-1 has long been argued. Though not in any
way a double star, they seem to be part of an extended loose
association of O and B stars (that also includes Wezen) sometimes
called "Collinder 121" after the real cluster that seems to
surround Omi-2 (but that actually lies far in the background).
Omi-1, 2000 light years away, is a bit closer, but the errors on
the measurements are great enough that they could still be at the
same distance. At best, they at least 70 light years apart, at
worst a couple hundred. Yet they were probably born more or less
at the same time from the same complex of interstellar matter, and
are now slowly moving apart. Thanks to Jeff Bryan, who suggested
this star.