OMEGA CAP (Omega Capricorni). "Omega stars," those named with the
last letter of the Greek alphabet, get
little respect. They are not helped out much by Omega Cap either,
which is seriously neglected in spite of its size and luster. If
you want Omega-recognition, you instead have to go not to a star,
but to the grandest globular cluster in the Galaxy, Omega Centauri, which, even though 16,000 light years
away, is easily visible to the naked eye. It is now thought to be
the core of a small galaxy that long ago merged with ours. But
that is a digression. Consistent with its name, Omega Cap is a
fourth magnitude (4.11) star that lies at the very bottom of the
classical figure that makes curious figure of the "Water Goat."
Even its class is uncertain, as it is given as both a K (K5) and an
M (M0) giant. There are no
temperature measures, so estimates from class (4100 and 3900 Kelvin
respectively) will have to do. While the 200 degree difference may
seem small, it is critical in assessing the amount of infrared
radiation pouring from this coolish star. From its substantial
distance of 630 light years, the two temperatures give luminosities
of 1650 and 2170 Suns (cooler temperatures
yielding more infrared) and radii of 80 and 102 times solar.
Someone did pay enough attention to the star, however, to measure
an angular diameter of 0.005 seconds of arc, from which we readily
calculate a quite-large true radius of 104 times that of the Sun,
or 0.5 Astronomical Units, half the size of Earth's orbit. Though
we stick here with the traditional K5 class, the cooler M0 version
fits better. No matter the details, the star proves to be hefty,
weighing in at between 5 and 6 solar masses, and is most likely
rather quietly fusing helium into carbon in its deep core, having
started life as a hot class B2 (or so) dwarf. Omega Cap is listed
as a weak barium star, though it probably is not (true barium stars
like Alphard being doubles in which a
now-dead white dwarf at
one time -- when it was a giant -- transferred nuclear-enriched
mass to the currently visible one). Omega is also listed as
variable, which it probably is, though no one seems to have a
handle on the kind or amount of variability. The star, like its
constellation, remains obscure.
Written by Jim Kaler 10/19/07. Return to STARS.