At magnitude 4.31, Alpha Sculptoris is the brightest star in the modern constellation Sculptor. It lies just 2.7° southeast of the South Galactic Pole.
Properties and Evolution
α Scl's faintness results from its rather large distance of 670 light years; the star is actually is a warm blue-white class B7 IIIp giant. Its luminosity is around 1700 suns, which with its temperature of 14,000 K yields a radius of 7 times solar, a mass of 5.5 times solar, and an age of 81 million years. The star is right on the edge of the hydrogen-fusing main sequence; fusion has probably stopped in its helium core, as its surface prepares to cool and expand to become a much larger red giant.
Spinning with an equatorial velocity of only 14 km/sec - very slow for a class B star - α Scl takes 21.652 days to make a rotation. The star is also "helium weak", in which the abundance of surface helium is anomalously low - only 45% of normal. On the other hand, other elements like silicon, titanium, and manganese are greatly enhanced. α Scl is, in fact, the prototype of the "Si-Ti helium weak" stars.
This chemical oddness is caused by the slow rotation, which keeps the outer layers undisturbed. This allows some kinds of atoms to drift downward, and others to rise to the surface. The effect is enhanced by a magnetic field that helps concentrate the chemicals into spots. These, in turn, allow the precise measurement of the rotation period, as their movement in and out of sight causes variations in the spectrum. The magnetic field occasionally flips its direction, and controls the behavior of a closely-surrounding cloud of circumstellar gas.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]