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OMEGA CAP (Omega Capricorni). "Omega stars," those named with thelast letter of theGreek alphabet, getlittle respect. They are not helped out much by Omega Cap either,which is seriously neglected in spite of its size and luster. Ifyou 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 yearsaway, is easily visible to the naked eye. It is now thought to bethe core of a small galaxy that long ago merged with ours. Butthat is a digression. Consistent with its name, Omega Cap is afourth magnitude (4.11) star that lies at the very bottom of theclassical figure that makes curious figure of the "Water Goat." Even its class is uncertain, as it is given as both a K (K5) and anM (M0)giant. There are notemperature measures, so estimates from class (4100 and 3900 Kelvinrespectively) will have to do. While the 200 degree difference mayseem small, it is critical in assessing the amount of infraredradiation pouring from this coolish star. From its substantialdistance of 630 light years, the two temperatures give luminositiesof 1650 and 2170Suns (cooler temperaturesyielding more infrared) and radii of 80 and 102 times solar. Someone did pay enough attention to the star, however, to measurean angular diameter of 0.005 seconds of arc, from which we readilycalculate a quite-large true radius of 104 times that of the Sun,or 0.5 Astronomical Units, half the size of Earth's orbit. Thoughwe stick here with the traditional K5 class, the cooler M0 versionfits better. No matter the details, the star proves to be hefty,weighing in at between 5 and 6 solar masses, and is most likelyrather quietly fusing helium into carbon in its deep core, havingstarted life as a hot class B2 (or so) dwarf. Omega Cap is listedas a weak barium star, though it probably is not (true barium starslikeAlphard being doubles in which anow-deadwhite dwarf atone time -- when it was a giant -- transferred nuclear-enrichedmass to the currently visible one). Omega is also listed asvariable, which it probably is, though no one seems to have ahandle on the kind or amount of variability. The star, like itsconstellation, remains obscure. Written byJim Kaler 10/19/07. Return toSTARS.
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