SADACHBIA (Gamma Aquarii). Aquarius (the
Waterman, or Water Bearer) mostly sprawls gently south of the
celestial equator. It is focused upon the 4-star Water Jar (or
Urn), a somewhat dim but very noticeable triangle that has a star
smack in the middle, the Urn appearing like a "Y" or even as a
small spray of diamonds that lies directly on the equator. Though
at bright fourth magnitude (3.84), not quite the brightest star of
the Urn, and ranking sixth in the constellation (after Sadalmelik, Sadalsuud, Delta, 88, and Zeta Aquarii,
the latter also in the Urn), Sadachbia is one of two of the Urn's
stars to be named (the other "Seat," Pi Aqr).
The name harkens not to the Water Bearer, but
to an older Arabic constellation, and seems to mean "the Lucky Star
of the Tents," the Urn perhaps reminding the Arabs of a desert
tent. The "Sa" prefix on the name relates obscurely to "luck" in
the same way it does in Sadalmelik and Sadalsuud. Sadachbia is for
the most part an ordinary hot class A (A0) star much like Vega and so many others that make our
constellations. Lying 158 light years away, it radiates 62 times
more light than the Sun from a 9500 Kelvin
sphere three times the solar diameter. Theory and long observation
of similar stars in double systems show it to be a hydrogen fusing
main-sequence star with a mass just short of three times solar.
Aside from its prominent location, Sadachbia has two qualities to
recommend it. First, like so many of its stellar kin, it is a
"spectroscopic double," one whose character can be determined only
by examining the spectrum. A star of unknown type orbits Sadachbia
every 58 days from a distance at least 0.40 astronomical units
away, about the distance Mercury is from the Sun. Sadachbia also
has a small 12th magnitude companion 37 seconds of arc away from
it, but it is merely a line of sight coincidence. Of more
significance, Sadachbia, like Kaus
Australis (Epsilon Sagittarii), is a failed "Lambda Bootis"
star. Lambda Bootis stars are otherwise normal stars near class A
that have unusual deficiencies in heavy elements. No one really
knows why. One theory is that during their extreme youth, these
rare stars accreted interstellar matter that was itself highly
deficient in heavy elements as a result of the condensation of such
elements onto dust grains (which is an observational fact).
Sadachbia was once considered such a star, but later investigations
showed instead that the chemical composition was more or less
normal. While thus not an actual example of a Lambda Boo star,
Sadachbia still allows the illustration of the myriad odd stellar
beasts that populate the sky.