OMEGA CAR (Omega Carinae). Well, somebody has to be last. And Omega Carinae certainly does that. It was not only given the last letter in the Greek alphabet, but it's the most southerly star in Carina, the Keel of Argo, the mythical Ship of the Argonauts. Though not by much, as at 70 degrees south of the celestial equator, Omega beats out brighter, second magnitude Beta Car (Miaplacidus) for the title by only a third of a degree. (Much too far south for Bayer to see, Carina's Greek letters were applied by Nicolas de Lacaille, who lived between 1713 and 1762). There is not much between Omega Car and the South Celestial Pole except fourth magnitude I ("eye") Car (HR 4102) and Gamma Chamaeleontis (also fourth mag) in eponymous Chamaeleon. None of this discussion, however, tells us anything about the Omega Car, which is a rather bright third magnitude (3.32) class B (B8) "giant" (but see below) that lies 342 light years away (give or take just 3). A temperature of 13,180 Kelvin, from which we can evaluate the amount of ultraviolet radiation, allowance for a six percent dimming by interstellar dust (consistent with the star's lying nearly a dozen degrees from the Milky Way's midline), and distance give a luminosity 1040 times that of the Sun and a radius of 6.2 times solar. Theory then shows the star to be not so much a giant as a subgiant that has given up, or is about to give up, core fusion of hydrogen, and yields a mass of 4.8 to 5 times that of the Sun, depending on the exact state of evolution. Born just under 100 million years ago, Omega Car is destined to slough off its outer envelope and have its core die as a fairly massive white dwarf of 0.85 solar masses. What makes Omega Car more special is a surrounding equatorial disk that makes it into a "B-emission" or "Be" star (the "e" implying emission lines of hydrogen) in the mold of Zeta Tauri, Gamma Cassiopeiae, and many others. The disk is thick enough along the line of sight for Omega Car to be called a "shell star." The "Be" phenomenon is clearly tied (though in an unknown way) to fast stellar rotation, and sure enough, Omega's spectrum reveals it to be spinning with an equatorial speed of at least 235 kilometers per second, which gives it a rotation period of under 1.2 days. Observations of the disk suggest a tilt of 65 degrees to the plane of the sky, which gives an actual rotational speed of 260 kilometers per second, roughly 85 percent of the speed at which the star would begin to come apart. The rotational velocity is high by solar standards (2 kilometers per second), but well below the record. At such a speed, Omega Car must be ellipsoidal, flattened at the poles, which compromises the measured surface temperature (the star being hottest at the pole, coolest at the equator). Such stars are often unstable, as witnessed by Gamma Cas, Dschubba (Delta Scorpii), and a variety of others, though Omega Car shows no sign of variation. On the other hand, is anybody watching? Nor is there any evidence for a companion of any sort, the star notable for its obscurity and its loneliness in addition to its position at the end of the alphabet.

Written by Jim Kaler 3/29/13. Return to STARS.