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.
Written by Jim Kaler. Return to STARS.