SIGMA BOO (Sigma Bootis). Two stars in Bootes (the Herdsman) provide guideposts to fainter, but significant, stars. Just south of Izar (Epsilon Boo) lies the class M3 red giant variable W Boo, while to the southeast of Rho Boo lies fainter Sigma. While Rho and Sigma may at first appear as a naked-eye double, the angular proximity is just line-of-sight. Rho is a just-barely-fourth magnitude (3.58) class K (K3) giant 150 light years away, whereas Sigma is a near- fifth magnitude (4.46) more solar type class F (F2) 6900 Kelvin hydrogen fusing dwarf only 50 light years away. (That Rho is visually brighter attests to the power of giant stars.) Like many of its spectral class, Sigma Boo is apparently a variable of the Delta Scuti variety, which vary subtly with multiple periods of hours. There is, however, no confirmation of the variation, let alone data on it. A relatively modest star, Sigma Boo is only 3.1 times more luminous than our Sun, has a radius 1.2 times solar, and carries just 1.3 times the solar mass. There may or may not be companions. A tenth magnitude star lies 233 seconds of arc away, while one of 11th magnitude lies in a different direction at an angular distance of 237 seconds. If they are true companions, and not mere line-of- sight coincidences, the brighter would have to be (from it's true visual luminosity) a class K8 dwarf, while the fainter would be M3. At minimum distances of 3600 Astronomical Units, they would each take at least 150,000 years to orbit the primary F2 star. So far, nothing too special. Sigma, however, is both low metal (20 percent solar) and relatively young, and possibly near the so-called "ZAMS," the "zero-age main sequence" of hydrogen fusing stars. Better, it is a "Vega" type star that, even though much cooler than that star, is surrounded by a disk of warm infrared-radiating dust that indirectly implies a possible planetary system, the dust the leftover debris (though not actual planets have been found). At a temperature of 50 Kelvin, the dust extends some 60 Astronomical Units away from the star, comparable to the "Kuiper Belt" of our own Solar System, which consists of primitive cometary debris that never evolved to form its own planet.
Written by Jim Kaler. Return to STARS.