GAMMA SGE (Gamma Sagittae). South of Cygnus is the faint but exquisite constellation of Sagitta, the Arrow. The figure blatantly
violates the oft-stated rule that the brightest star of a
constellation carries the Greek Alpha
designation. Bayer more went in order of position, giving Alpha
and Beta to the notably fainter "feathers," while the
constellation's luminary, at the arrow's point, got Gamma (Delta
falling in the arrow's middle). Only much dimmer Alpha has a
proper name (Sham). Shining redly at the
edge of third magnitude (3.47), Gamma Sagittae is one of the few
naked eye class M stars, though just over the line at M0 (even
having been classified as K5). At a distance of 275 light years,
it radiates at a power 640 times that of the Sun. The diameter of this classic red giant
has been intensively studied through the use of interferometry, in
which the way the light from different parts of the star interferes
with itself to produce a fringe pattern allows the measure of
angular size. The angular diameter of 0.00618 seconds of arc leads
to a physical radius 55 times that of the Sun, or 0.26 Astronomical
Units, about half the size of Mercury's orbit. The angular size
and total luminosity observed at the Earth (accounting for a good
deal of infrared radiation) give an accurate temperature of almost
exactly 4000 Kelvin. With a mass of 2.5 times that of the Sun,
Gamma Sagittae began life 750 million years ago as a white class B9 star.
It has gone through its long solar-type life of fusing hydrogen to
helium in its core and has also progressed through its helium-
fusing state. Slightly variable, it now has a dead carbon-oxygen
core (that is surrounded by shells of fusing helium and hydrogen)
and is in the preparatory stage of becoming a full-blown Mira-type
variable sometime in the not-too-distant future. All by itself,
with no known companions, it will lose its outer envelope, and its
core will expire as a cooling white dwarf about the size of the
Earth, the fate expected of the Sun.