OMI-1 AND OMI-2 CEN (Omicron-1 and Omicron-2 Centauri). Way down in southern Centaurus (the eponymous Centaur), in a tongue of the constellation that dips between Crux (the Southern Cross) and Carina (the Keel of Argo), lies what appears to be a naked-eye double made of two fifth magnitude stars, Omicron-1 (the western one) at magnitude 5.13 and Omicron-2 at 5.15. Most unusual is that they are both bright supergiants, class G (G3) Omi-1 separated from class A (A2) Omi-2 by only 4.6 minutes of arc. Omi-1 (which may be as cool as F7) has even been called a hypergiant. While they do not compose a true double, they are almost certainly related. Both stars are very far away, from their parallaxes 5720 and 5350 light years. While the statistical uncertainties are huge, half the sizes of the measures themselves, the similarity of the distances lend them some credence, so we'll adopt an average of 5530 light years for both. The motions, while not exactly the same, are similar too, both stars moving toward us but to the northwest against the background, Omi-1 at 43 kilometers per second, Omi-2 at 58 km/s. Both stars are considerably dimmed and reddened by interstellar dust. Averaging the observed colors suggests a dimming of 1.22 magnitudes, which again we'll apply to both. The measured (suggested?) temperatures for the stars are out of line with those derived from their classes, which are 5700 Kelvin for Omi-1, 8500 for Omi-2. Corrections for infrared or ultraviolet radiation then range from minimal to zero.

With all this out of the way, Omi-1 shines with the light of 68,000 Suns, Omi-2 with 61,000, which lead to respective radii of 270 and 115 solar radii. With a radius of 0.53 Astronomical Units, Omi-2 is some 35 percent bigger than the orbit of Mercury, while Omi-1 reaches out to 25 percent larger than that of the Earth. While there is no rotational information for Omi-1, Omi-2 (with a projected equatorial rotation velocity of 43 km/s) could take as long as a third of a year to spin just once. Theory gives masses of 17 Suns for Omi-1, 16 for Omi-2, both well above the lower limit of 8-10 solar masses beyond which stars explode as supernovae. Both have ceased fusing hydrogen in their cores and may be preparing to fire up the helium ash (if they have not done so already). They are too far apart to be an orbiting double, the angular separation and distance yielding a physical separation of at least 7.4 light years, one and three quarters the distance between here and Alpha Centauri. At that distance apart, they would take at least 55 million years to orbit, five times longer than the stars' ages of 10-12 million years. While they are likely not a gravitationally-bound binary, they are almost certainly members of some long-lost association of O and B stars (both born as roughly class O9) that is now expanding into the cosmic void (their membership in the Carina OB1 association now rejected). Not surprisingly, the larger of the two, Omicron-1, is slightly variable, wobbling irregularly by a few tenths of a magnitude with maybe an occasional 200-day period. It also has an 11th magnitude "companion" a dozen seconds of arc away that from its motion is surely just in the line of sight. Assuming Omi-1 and Omi-2 are at the same distance from us, from each the other would be about as bright as our quarter moon. If this magnificent pair were in the northern hemisphere, you could be sure they would have been were far more heavily studied.
Written byJim Kaler 4/08/16. Return to STARS.