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BATEN KAITOS

(Zeta Ceti). Four named stars define the main figure of Cetus, the Sea Monster, or Whale: Menkar in his head, the great variable Mira in his neck, Deneb Kaitos, which marks his tail, and Baten Kaitos, which links the long neck to the body. Of these Baten Kaitos is the faintest, excepting of course when Mira is on the down-side of its huge variability. At the bright end of fourth magnitude (3.73) and seventh brightest in this large but relatively dim constellation, the star consistently received the Zeta designation from Bayer. The name, appropriate to the star's position, comes directly from a two-word Arabic phrase meaning the "sea monster's belly." Like so many stars that make our constellations, Baten Kaitos is an orange "class K" giant with (being on the warm side of its class) a color and temperature (4600 Kelvin) similar to much brighter Pollux. At a distance of 260 light years, this star shines 260 times more brightly (including its infrared radiation) than the Sun, from which we calculate a radius 25 times solar as befitting a true giant. Its spectrum shows it to be accompanied by a smaller star of unknown nature that orbits it with a period of 4.5 years and that is separated from it by about Jupiter's distance from the Sun. A dying star of around 2.5 solar masses, Baten Kaitos now appears to be fusing helium into carbon in its deep core. It has been listed as a mild "barium star," one (like Alphard) with an overabundance of the element barium (as well as overabundances of some others). Such stars do not come by their odd abundances on their own, but have been contaminated by matter that flowed onto them from now-dead companions whose own nuclear processes had grossly changed THEIR own abundances when THEY were giants. Baten Kaitos's observed companion seems too far away for that, however. In Baten Kaitos's case the barium effect is so minimal that the star was quite likely misclassified. Otherwise, its chemical composition is fairly normal, the iron abundance about three-fourths that of the Sun. Since the Sun appears rather high in metals for its position within the Galaxy, Baten Kaitos fits right in with its stellar surroundings. Baten Kaitos serves to highlight the immense differences among stars even of the same class. Oddly, its constellation-mate Deneb Kaitos, with which it also shares part of a name, is also a class K giant with almost the same temperature yet a luminosity only about half as great. Deneb Kaitos, however, is a strong X-ray source, whereas Baten Kaitos is not. Shedar (Alpha Cassiopeiae), on the other hand, is over three times as bright as Baten Kaitos. The differences are caused by differences in mass and in the state of evolution, which over millions of years can cause changes of a factor of 1000 in the luminosities of dying stars.