OMEGA ORI (Omega Orionis). Draw a line in Orion from Betelgeuse
(the Alpha star) due westward to Bellatrix (Gamma Orionis). Bisect it.
Upward find a fine small company chaired by Meissa (Lambda Ori), while downward you run
into Omega Ori, giving us the Alpha and Omega of the Hunter. And
though fifth magnitude (at 4.57 just barely), a fine star it is, a
hot (19,100 Kelvin) class B (B3) giant (a real one this time,
though there are some that say it's a B2 giant). In an interesting
setting within a local dust cloud, Omega's light reflects
(scatters) off the small grains to create a modest "reflection
nebula" up to more than a light year across, most of it in a
prominent arc to the north of the star. Were Omega a bit hotter,
it would ionize the local gas into a tiny diffuse nebula.
The star's seeming faintness comes from a rather large distance of
1380 light years (with an uncertainty of 190 ly) combined with 0.35
magnitudes of dimming by intervening interstellar dust. These plus
temperature (to account for ultraviolet light) yield a high
luminosity of 15,275 times that of the Sun,
which leads to a radius of 11.4 solar. Theory then gives a mass of
10.5 times that of the Sun and shows that the star, some 19 million
years old (high mass stars not living very long), is just starting
its giant phase, having recently given up core hydrogen fusion.
Having begun life as an even hotter B0.5 star, Omega Ori is close
to and a bit above the limit beyond which stars explode as supernovae, though given all
the uncertainties it might at the end of its life make just a
massive white dwarf.
A rather high projected equatorial velocity of 181 kilometers per
second gives a rotation period of under 3.15 days. Such rapid
rotation is commonly associated with a circumstellar disk that
emits its own radiation, transforming Omega Ori into a well-known
and classical "B-emission" star (the class now B3 IIIe, "III" for
"giant"). Direct determination of angular diameter yields a
stellar radius about double that found above, the measure probably
messed up by the disk. Omega has the distinction of being the
first "Be" star to have had a measured magnetic field (about a
thousand times that of the Earth). The field varies with a period
of 1.29 days, most likely as a result of a tilted magnetic axis
wobbling with the rotation period. If so, the star is spinning at
close to 450 kilometers per second with the rotation axis tilted at
24 degrees from the line of sight. Somewhat variable, as befits a
Be star, Omega changes between magnitudes 4.40 and 4.59 with no
discernable period. Small additional "non-radial" pulsations (some
parts of the star moving inward, others outward) are seen with
periods of 0.97 and 2.19 days. There seem to be no companions, though in the wider
view the star is probably a member of one of the various sub-associations that make up the vast
Orion complex, the constellation holding an immense number of
local wonders.
Written by Jim Kaler 2/17/12. Return to STARS.