PHACT (Alpha Columbae). Down below Orion lies Lepus, the Hare, Orion's prey, and below the Hare is a flying bird, Columba, the Dove. A modern constellation carved in the seventeenth century from the western outlying stars of Argo, Jason's ship, Columba was meant to represent Noah's Dove (Argo as Noah's Ark). Yet the little constellation still has some claims to ancient heritage, vague mention of it made almost 2000 years ago. As well there should have been. Set into a fairly blank area of sky, the principal part of Columba is a rather prominent triangle, the brightest star of which, Phact (appropriately the Alpha star), is just over the line into third magnitude (2.64). The star's name comes directly from Arabic, and means "the Ring Dove." Phact is a fairly hot star of class B (B7), with a surface temperature of 12,500 Kelvin. Its distance of 270 light years (allowing for ultraviolet radiation from its hot surface) tells of a star that radiates just about 1000 times the energy of the Sun. Seven times the solar diameter and 4.5 times the solar mass, Phact is classed as a "subgiant," a star that has just ceased (or is about to cease) the fusion of hydrogen into helium in its core. Evolution will now proceed rapidly as over only the next few million years the star will expand and cool to become a bright orange giant. Phact is more specifically classed as a "Be" star, the "e" standing for "emission," for light radiated by hydrogen (and other atoms) at specific wavelengths or colors. Like most class B stars, Phact is spinning rapidly, at a speed of at least 180 kilometers per second at its equator (90 times that of the Sun), and maybe much higher. The rapid rotation causes the star to flatten at its poles and to spin off a low density envelope about twice its radius, from which the emissions come. Similar stars dot the sky, among them Achernar, Alcyone in the Pleiades, and the very odd star Gamma Cassiopeiae. Ordinary hydrogen-fusing stars like the Sun (even such subgiants as Phact) divide rather neatly at a surface temperature of around 6500 Kelvin, those below it spinning slowly, those above very rapidly. The reason is that cooler stars, in which gases move turbulently up and down, generate magnetic fields that are pulled away by winds. The dragging magnetic fields then act over billions of years to slow the cooler stars down. The Sun must once have been rotating much more rapidly than it does today (and that's a phact; sorry, irresistable).
Written by Jim Kaler. Return to STARS.