ALPHA PIC (Alpha Pictoris). The third-magnitude (3.27) luminary of
the rather nondescript modern constellation of Pictor, the Easel (sometimes the "Painter's Easel"),
Alpha Pictoris is far overshadowed by the number two star, Beta Pictoris, whose fame rests on a thick
surrounding dusty disk that may well harbor a family of planets.
Yet Alpha has its own mystery about it, albeit a small one. This
class A (A7) subgiant (which implies that the star is giving up
core hydrogen fusion if it has not done so already) shines to us
from a distance of almost exactly a century of light years (the
actual measure giving us 99). The temperature is not well
constrained, various estimates falling between 7370 and 8610
Kelvin, the average of 7950 however falling right on the mark for
an A7 star. No matter, since in this range the amount of
ultraviolet or infrared radiation to be added to the visual is not
very dependent on the exact value. The total luminosity then comes
out to be 35 times that of the Sun (high for
an A7 star and consistent with subgiant status), from which we
derive a radius of 3.1 solar. The exact mass depends on whether
the star has truly stopped hydrogen fusion or is just about to, 2.1
solar if the former, 2.2 if the latter, the age of the star close
to a billion years (far under the Sun's 10 billion year total
hydrogen-fusing span, the higher mass shortening the Alpha Pic's
life). Like many of its breed, Alpha Pictoris is a notably fast
rotator, the equatorial rotation velocity measured to be at least
205 kilometers per second, 102 times faster than the Sun's, which
yields a rotation period of less than three-fourths of a day.
Lower mass stars rotate more slowly because their outer convection
layers combined with rotation generate magnetic fields. Their
winds drag the fields outward, and the fields in turn act as drags
on the rotations. As we go upward in temperature, the convection
diminishes, and hotter than class F5 or so (about 6600 Kelvin), the
average spin rates increase. By A7, convection quits. The fields
also generate outer coronae that radiate X-rays. With no
convection, Alpha Pic should produce no X-rays, yet it does, which
implies a hidden, lower mass companion. Variations in positional
measure by the Hipparcos satellite suggest the same thing, yet the
little one, if it exists at all, cannot be seen or confirmed. Not
much of a mystery compared with Beta's, but better than none at
all.