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 (1.85) 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.