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.