Alpha Sagittae - Sham, Alsham

At magnitude 4.37, Alpha Sagittae is the third brightest star in the constellation Sagitta, the Arrow. The constellation is tucked into the Milky Way south of Cygnus, and may represent the arrow of Hercules, or Cupid, or even Sagittarius. The star α Sge has the traditional name Sham or Alsahm, which means "the arrow" in Arabic.

Properties and Evolution

The star is a class G1 II bright giant with a surface temperature similar to the Sun's (5400 K). It is 340 times more luminous, and seems faint only because it is 475 light years away. Its temperature and luminosity imply a diameter 20 times solar, and a mass of 4 suns.

Evolutionarily, α Sge lies in the "Hertzsprung Gap," a region of stellar temperature and luminosity where few stars are found. Moreover, the star's luminosity and temperature suggest that it ought to be a Cepheid variable, and yet it is not. Sham's surface surface gases have been contaminated by nuclear fusion by-products from below, implying that the star is now fusing helium.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]