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.