ALPHA SCL (Alpha Sculptoris). East of bright Fomalhaut, the luminary of the Southern Fish (Piscis Austrinus), lies the modern constellation Sculptor, the Sculptor's Studio, one of the few constellations in which the first four Greek letters actually fall in order of brightness, surely by coincidence because the differences among them are so small. Alpha Sculptoris, toward the faint end of fourth magnitude (4.31), tops Beta by just 0.07 magnitude. It lies just 2.7 degrees to the southeast of the South Galactic Pole (the perpendicular to the center line of the Galactic disk) could thus carry the name "Polaris Galacticus Australis," in line with 31 Comae Berenices, " Polaris Galacticus Borealis." Alpha Scl's faintness results from its rather large distance of 670 light years, and belies a curious character. At first it seems like just one more warm (14,000 Kelvin) blue-white class B (B7) star, of the kind that abound in the naked-eye sky. Classed as a giant, it radiates 1700 times more brightly than the Sun, which with temperature yields a radius of 7 times solar, a large mass of 5.5 times solar, and an age of 81 million years. The star is thus right on the edge of the hydrogen- fusing main (dwarf) sequence and has probably quenched the nuclear engine in its now-helium core as it prepares to make a run to cooler surface temperatures and to become a much larger red giant. Spinning with an equatorial velocity of only 14 kilometers per second, very slow for a class B star, it takes a precisely-known 21.652 days to make a rotation. Alpha Scl is part of a rare breed called "helium weak" stars, in which the abundance of surface helium is anomalously low, here only 45 percent of normal (which is usually 10 percent of hydrogen). On the other hand, other elements like silicon, titanium, and manganese are greatly enhanced. This oddness is caused by the slow rotation that keeps the outer layers undisturbed, allowing some kinds of atoms to drift downward, others to rise to the surface. The effect is enhanced by a magnetic field (which renders Alpha Scl a "Bp" star) that helps concentrate the chemicals into spots that in turn allow the precise measurement of the rotation period, as their movement in and out of sight causes variations in the spectrum (which were at one time interpreted as the result of a non-existent orbiting companion that could have been a black hole!). Alpha Sculptoris is thus the prototype of the few-known "Si-Ti helium weak" stars"(rendering them "Alpha Scl stars"). The magnetic field occasionally flips its direction, and controls the behavior of a close-in cloud of circumstellar gas.
Written by Jim Kaler. Return to STARS.