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KAUS AUSTRALIS

(Epsilon Sagittarii). Sagittarius, the Archer, holds a bow and arrow, three stars making the bow, one the arrow's point, the southern one Kaus Australis. The star has one of the few names with mixed parentage, as "Kaus" is from Arabic meaning "bow," while "Australis" from Latin clearly signifies "southern." (It also makes the lower right star of the asterism called "the Teapot"). Deep in the southern hemisphere 34 degrees below the celestial equator, the star is not well known to northerners, though at bright second magnitude it is the 36th brightest star in the sky, just barely outdoing Alkaid at the end of the handle of the Big Dipper. It is also the brightest star in Sagittarius, and as the Epsilon star shows that Johannes Bayer had other things in mind when he assigned Greek letters, at least for some constellations; the second brightest is Nunki, the Sigma star, while Alpha and Beta are dim fourth magnitude stars well to the south of our Epsilon. Kaus Australis also demonstrates that astronomers can have trouble finding a place even for prominent stars. It has traditionally been called a giant and assigned to blue class B (at the cool end, B9.5), with a temperature of 9200 K, while others have more recently assigned it to hot-end class A as a "bright giant" (one notably more luminous than the ordinary giants). Its distance of 145 light years shows it to have a luminosity 375 times that of the Sun, certainly greater than most class A or B giants should have. Whatever the arguments, the star is much brighter than its main sequence (hydrogen-fusing) counterparts and is clearly in a more advanced state, having begun to die. With a mass nearly four times solar, it probably has a core of helium that is shrinking and heating as it prepares itself to fuse to carbon and oxygen. Both the temperature plus luminosity and direct measures of angular diameter agree that Kaus Australis is seven times bigger than the Sun. More confusing is the star's chemical composition. It was long thought to be a rare "Lambda Bootis" star. Such stars seem to be highly depleted in metals. One explanation is that during their development they accreted from interstellar space considerable gas whose metal atoms had already been absorbed by interstellar dust. However, Kaus Australis was later removed from the category. It may instead be a sort of "shell star" in which its high rotation speed (over 70 times that of the Sun) was responsible for creating a shell of gas that hides much of the star within. If nothing else, the star shows how much we have yet to learn of the stellar science.