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.