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.