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.
Written by Jim Kaler. Return to STARS.