Alpha Sagittarii is a star in the constellation of Sagittarius, the Archer. It is not particularly bright, with a magnitude of +3.96. Yet not only did Bayer assign it the α designation, but also in his great 1602 star atlas "Uranometria", he drew it vastly brighter than it really is. No one knows why. Rukbat is very far south, not even visible north of 50° north latitude, so Bayer may have had difficulty in knowing its brightness. The star has the traditional names Rukbat and Alrami, derived from the Arabic "Ar-Rukbah Ar-Rami," meaning "the knee of the archer", and clearly indicating that the star's residence is in Sagittarius.
Properties
An alternative speculation might be that Rukbat has simply faded over the past 500 years. But ordinary main-sequence B8 V hydrogen-fusing stars like Rukbat simply do not do that. From 170 light years away, α Sgr radiates 112 solar luminosities from its blue-white 12,370 K surface. The star is 2.3 solar diameters across. Its temperature and luminosity give it an ambiguous evolutionary status: Rukbat may indeed be a main-sequence star of 3.2 solar masses; or it may also be a 3-solar-mass subgiant near the end of its hydrogen-fusing lifetime.
Rukbat is practically ignored in the scientific literature, mentioned in only one publication per year. But it still has a few notable qualities. Rukbat's spectrum indicates that it may have a companion, but a careful search for one turned up empty. The star is also a weak source of X-rays. More important, Rukbat is surrounded by a "Vega-like" disk of dust, that is the remnant of the star's formation and that for all we know has formed planets.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]