KAUS MEDIA (Delta Sagittarii). Sagittarius is known more for its two "asterisms"
(informal constellations), the "Little Milk Dipper" and the
"Teapot," than it is for the figure it was meant to represent, a
Centaur Archer. And what is an archer without a bow? Sagittarius
has a fine one, not just a bow, but an arrow along with it that
points into the heart of the Milky Way. At the bow's northern end
lies Kaus Borealis, at the southern Kaus Australis, and in the middle and
marking the eastern end of the arrow Kaus Media, the names an odd
mixture of languages, Kaus coming from an Arabic word meaning
"bow," while the other three words respectively mean, from Latin,
"northern," "southern," and "middle." At the bright end of third
magnitude (2.70), Kaus Media ranks second brightest in the bow
(after Kaus Australis) and fourth in the constellation (fitting for
Bayer's Delta star), behind Sigma (Nunki),
Epsilon (Kaus Australis), and Zeta (
Ascella); it's hard to know
what Bayer had in mind! Kaus Media is yet
one more class K (K3) giant star, but one a bit on the bright side.
Its distance of 305 light years and an uncertain temperature of
4300 Kelvin (needed to account for infrared radiation) lead to a
luminosity 1180 times that of the Sun and a
radius 62 times solar, the star three-quarters the size of
Mercury's orbit. Oddly, the temperature has never actually been
measured, and can only be inferred from the K3 spectral class. The
amount of dimming by interstellar dust is also uncertain, and while
it cannot be much, the star may actually be up to 30 percent
brighter and 15 percent larger. With a relatively high mass 5
times that of the Sun, Kaus Media is mostly likely fusing helium
into carbon in its core. The star is an example of how little we
can know about companions, especially within the rich star fields
of the
Milky Way. Catalogues list three faint neighbors, 14th,
15th, and 13th magnitude stars at separations of 26, 40, and 58
seconds of arc. If these are true companions, they are class K and
M dwarfs, are at real distances of at least 2400 to 5400
Astronomical Units (Earth-Sun distances) from Kaus Media proper,
and take at least from 53,000 to 180,000 years to orbit. The
little stars might also be line of sight coincidences. Spectra
display some evidence for a possible close companion as well, which
might be related to the star's uncertain reputation as a weak
"barium star," one that has been contaminated with heavy elements
by an evolving orbiting neighbor. Clearly the star needs more
attention than it gets.