RUKBAT (Alpha Sagittarii). Alpha stars are, according to logic, tradition, and expectation, supposed to be the brightest in their constellations. Sure there are a number of exceptions -- Alpha Orionis (Betelgeuse), for example, is on the average slightly fainter than Beta (Rigel). Rukbat, of Sagittarius, is among the most dramatic of some two dozen counter-examples, an Alpha star that lies at mid-fourth magnitude (3.97) and is hard to see from any lighted town. Sagittarius's brightest star, second magnitude Kaus Australis, Epsilon Sagittarii, is actually the brightest. The name, which from Arabic refers to "the Archer's Knee," clearly indicates that the star's residence is Sagittarius, and that it is not some interloper. (It even has a second name, "Alrami," that means "the Archer.") Not only did Bayer assign it "Alpha," but in his great star atlas of 1603 (the "Uranometria"), he draws it vastly brighter than it really is (as he does also-dim Arkab, the Beta star). No one is sure why, but he may have mis-interpreted a commentary by Ptolemy. Rukbat is very far south, indeed not even visible north of 50 degrees north latitude, so Bayer or Tycho would not have been able to note the star's brightness directly. An alternative speculation is that Rukbat has simply faded over the past centuries, but rather ordinary class B (B8) hydrogen-fusing dwarfs do not do that, and it seems highly improbable. Rukbat, 170 light years away, radiates 112 solar luminosities from its 12,370 Kelvin blue-white surface, the star 2.3 solar diameters across. Its temperature and luminosity give an ambiguous status. Rukbat may indeed be a dwarf, one of 3.2 solar masses; but it may also be near the end of its hydrogen-fusing lifetime, and at 3 solar masses may be becoming a growing "subgiant." Practically ignored in the scientific literature (mentioned in but one publication per year), the star still has a few things to recommend it. Rukbat's spectrum indicates that it may have a companion -- a careful search for one, however, turned up empty. The star is also a weak source of X-rays. More important, Rukbat is a "Vega- like" star that is surrounded by a cloud (probably in the form of a disk) of dust that is likely the remnant of the star's formation and that for all we know has spawned planets. If so, given Rukbat's proximity to the termination of its hydrogen-fusing life, any that are close-in do not have long to survive.
Written by Jim Kaler 8/31/01. Return to STARS.