BETA SGE (Beta Sagittae). Sagitta,
the Arrow, is yet another case of the common violation of the "rule of Alpha," that the brightest
star gets the first letter of the Greek
alphabet. Beta falls by the wayside as well, as the luminary
is oddly third magnitude Gamma Sge,
followed not by Beta but by bright-fourth magnitude Delta. Alpha and
Beta then come in at a faint-fourth magnitude tie for third place
(specifically both 4.37). Even odder, Alpha is the only star in
the constellation with a proper name.
Sometimes in his Uranometria, Bayer used
position rather than brightness, but here even that rule is broken,
as Delta falls between the Alpha-Beta pair and Gamma. But back now
to Beta, which is a rather ordinary class G (G8) helium-burning giant, though one with a higher
mass and bit of a composition anomaly. First though, from the
star's distance of 440 light years (give or take 9), its
temperature of 4860 Kelvin (to account for some infrared
radiation), and a 20 percent adjustment in brightness as a result
of dimming by interstellar dust, we find a luminosity of 429 times
that of the Sun, which leads to a radius of
29 solar and (from a projected equatorial rotation velocity of 9
kilometers per second) a rotation period that could be as long as
160 days. Direct measure of angular diameter, though, gives a
radius of 64 times that of the Sun, more than twice as great, for
reasons unexplained, as most giant stars are far more well behaved
with good agreement. Luminosity and temperature then give us a
mass of around four times that of the Sun and an age of 130-140 or
so million years. Though currently a "yellow giant" that is most
likely quietly fusing its helium core into carbon and oxygen, Beta
Sge started life as a blue-white mid-class-B dwarf, and after
losing 80 percent of its mass though winds mostly after the death
of the helium core, will end life as a 0.8 solar mass white dwarf (of singular
nature, since there seems to be no stellar companion). While the
iron content (relative to hydrogen) is right on solar, Beta Sge is
mildly "cyanogen-rich." The cyanogen (CN) molecule is very common
in the spectra of such coolish giants, and its elevation suggests
that some fresh nitrogen has been moved upward from the stellar
core by convection, that is, the star is slowly changing its
external chemical composition. It would be fun to keep an eye on
it if the processes did not take such a long time. As giants go,
our Beta-of-the-Arrow also seems to have an especially turbulent
outer atmosphere. The star's only other distinction is that it is
useful as a comparison with which to watch the variations of
neighboring Delta Sge, a double that contains a somewhat
unstable class M red giant that shows us what Beta will someday be
before it passes on as a white dwarf.
Written by Jim Kaler 10/01/10. Return to STARS.