SHAM (Alpha Sagittae). We are used to thinking of named stars as
those that dominate the sky, classical first magnitude stars like
Vega and Arcturus, second magnitude stars like Mizar and Polaris,
plus several of third. Sham, at faint fourth magnitude, nearly
fifth (4.37), quite violates that general rule, a star with a
proper name that cannot be seen from a modestly lit town. But what
a setting it has within a faint but exquisite constellation, Sagitta, the Arrow, said to be the arrow
of Hercules, perhaps that of Cupid, or
even the strayed arrow of the distant Archer Sagittarius. Whoever it belonged to, it is a jewel of
a figure tucked into the Milky Way to the south of Cygnus, the Swan. Even in its mythological context,
Sham is odd. Of the five stars that make the classical figure, it
is tied for the honor of third brightest. Yet its name, coming directly
from Arabic, meaning "the arrow," stands in for the whole
constellation. Moreover, Bayer gave it the Alpha designation even
though un-named
Gamma and Delta (at the arrow's tip) are notably
brighter. The star itself confuses as well. A quite-luminous
class G giant with a temperature similar to that of the
Sun (5400
Kelvin) and 340 times brighter, it seems faint only because it is
fairly far, 475 light years, away, its temperature and luminosity
leading to a diameter 20 times solar. Most interesting is Sham's
state of evolution. It is in the "Hertzsprung Gap," a region of
stellar temperature and luminosity in which we find few stars.
When stars like the Sun stop fusing hydrogen to helium in their
cores they swell to become giants and cool at their surfaces.
Eventually they brighten, start fusing helium to carbon, and then
dim some. At first Sham looks like a four solar mass star that
within the last million years ago ceased its hydrogen fusion and is
now ready to make its run to higher luminosity. Yet its surface
chemical composition is different than solar, its nitrogen
abundance enhanced, which implies that its surface gases have been
contaminated by nuclear fusion by-products from below, which can
happen only if the star has already gone through the brightening
stage and is now fusing its helium. Moreover, its luminosity and
temperature suggest that it ought to be a Cepheid variable star
like Mekbuda or Polaris, and yet it is not, the dim star
continuing to intrigue us with its mysteries.