BIHAM (Theta Pegasi). Several things draw us to bright-fourth- magnitude (3.53, just fainter than third) Biham, Theta Pegasi (in Pegasus, the Flying Horse), not the least of which is the tortured name. It seems to have been originally applied to both Theta and nearby faint-fourth (4.40) Nu Peg, which lies just 1.5 degrees to the southwest of Theta. Kunitzsch and Smart give that it derives from a longer phrase "s'ad al Biham," meaning the "Lucky Stars of the Young Beasts," Allen's call closely similar. The name is commonly also given as "Baham." Unlike "Sadalbari," which applies to both Lambda and Mu Pegasi, "Biham" has been given over entirely to Theta (by which the stars is clearly best called). Otherwise, Theta Peg is a more or less ordinary class A (A2) dwarf with a temperature estimated at 8570 Kelvin. A distance of 92 (plus or minus 2) light years gives us a luminosity of 25 times that of the Sun and a radius of 2.3 times solar. Application of theory then leads to a mass of 2.1 times that of the Sun and an age of roughly 500 million years, which is about half-way through the star's billion-year hydrogen-fusing lifetime. The chemical composition is more or less solar, with about a factor of two reduction in iron and the like, and about the same enhancement in some lighter elements, not much given the propensity of class A dwarfs to have wildly odd compositions thanks to gravitational settling of some elements and radiative lofting of others. Perhaps the rapid rotational speed of at least 137 kilometers per second keep things stirred up (such speed leading to a rotation period of under 20 hours). Rapid rotation, though leads to other problems, such as uncertainty in temperature (the star having warmer poles and a cooler equator as a result of rotational bulging), which in turn factors into uncertainties in chemistry, even color and class, the star being redder than suggested by its A2 designation. Unlike lots of other class A stars (Vega, Fomalhaut), no circumstellar disk has been detected. Theta Peg had a bit of a reputation as a Delta Scuti (or some sort of) variable, but later work has shown constancy. Biham may be best known as part of a pointer toward one of the sky's grand globular clusters, Messier 15. A line drawn from it through Enif (Epsilon Peg) points right at it. Lying roughly 34,000 light years away, M15 (visible in binoculars) is one of the most centrally-concentrated globulars known. Containing more than half a million solar masses, and perhaps as many as a million stars, M15 has occasionally been suspected as harboring a medium-mass central black hole.
Written by Jim Kaler 10/15/10. Return to STARS.