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.