GIRTAB (Theta Scorpii). Girtab, Bayer's Theta star within Scorpius, the celestial scorpion, stands out in almost any way you can look at it. The name alone is unusual, in that it comes to us from Sumerian rather than Arabic or Greek, and means simply "the scorpion." Moreover it carries an additional name, "Sargas," also Sumerian, whose meaning is unknown. We'll stick here with Girtab, though Sargas is commonly used as well. Though the most southerly extent of the Sun is in neighboring Sagittarius, Scorpius is the most southerly of the zodiacal constellations. And bright second magnitude (1.87) Girtab is the most southerly BRIGHT star in the Scorpion, closely anchoring the southern curve of the scorpion's tail. At almost exactly 40 degrees below the celestial equator (and beat out only by much dimmer third magnitude Eta Scorpii, and then by only 1/6 degree), Girtab is invisible north of 50 degrees north latitude. The star's southerly position has allowed northern observers to use its visibility as a test of the night-sky brightness near the horizon. Scorpius is filled with bright blue-white stars of class B. As a yellow-white class F (F1) bright giant, Girtab is again an exception. From its distance of 272 light years, the star radiates 960 times more energy than the Sun from a surface with a temperature of 7200 Kelvin, its radius 20 times solar, making it a true giant indeed. Though its equatorial rotation speed is high (over 50 times that of the Sun), the large size still gives it a fairly long rotation period of 10 days (or less). Girtab is unusual too in our knowledge of its evolutionary status. There is no question that the star, which weighs in with a mass 3.7 times that of the Sun, is rapidly evolving with a dead helium core toward lower temperatures. One hundred million years ago, it WAS a blue class B star, one that would have fit right in with its current scorpian neighbors. As the star swells and the surface cools, it should in under a million years become a Cepheid variable like Mekbuda, and then become a red giant 5 times brighter than it is now, at which point the helium in its core will begin to fuse to carbon and oxygen, setting the stage for it to become a massive -- and distinctly single -- white dwarf.
Written by Jim Kaler. Return to STARS.