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.
Written by Jim Kaler. Return to STARS.