RUKBAT (Alpha Sagittarii). Alpha stars are, according to logic,
tradition, and expectation, supposed to be the brightest in their
constellations. Sure there are exceptions -- Alpha Orionis (Betelgeuse) is slightly fainter than
Beta (Rigel) -- but they are minor and
understandable. Rukbat, of Sagittarius,
is among the most dramatic of 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 1602 (the
"Uranometria"), he draws it vastly brighter than it really is (as
he does also-dim Arkab, the Beta star). No one knows why. Rukbat
is very far south, indeed not even visible north of 50 degrees
north latitude, so Bayer may have had difficulty in knowing its
brightness. An alternative speculation might be that Rukbat has
simply faded over the past 500 years, but rather ordinary class B
(B8) hydrogen-fusing dwarfs do not do that. 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.