At magnitude 2.64, Phact (or α Col) is the brightest star in modern constellation Columba, the Dove. It is part of the rather prominent triangle which makes up the principal part of the constellation. Columba was created from the western stars of Argo Navis in the seventeenth century to represent Noah's Dove (with the Argo as Noah's Ark). The star's name comes directly from Arabic, and means "the Ring Dove."
Phact is a fairly hot star of class B7 IVe, with a surface temperature of 12,500 K. From a distance of 270 light years, it radiates just about 1000 times the luminosity of the Sun (including invisible ultraviolet radiation). Seven times the Sun's diameter and 4.5 times its mass, Phact is classed as a subgiant, a star that has just ceased (or is about to cease) fusing hydrogen into helium at its core. Its evolution will now proceed rapidly over only the next few million years; the star will expand and cool to become a bright orange giant.
Emission and Rotation
Phact is more specifically classed as a "Be" star, the "e" standing for "emission," for light radiated by hydrogen at specific wavelengths. Like most class B stars, Phact is spinning rapidly, at least 180 km/sec at its equator. The rapid rotation causes the star to flatten at its poles and to spin off a gaseous envelope about twice its radius, which causes the emission. Similar stars dot the sky, among them Achernar, Alcyone in the Pleiades, and the very odd star Gamma Cassiopeiae.
The rotation speed of hydrogen-fusing stars like the Sun and Phact divides rather neatly a surface temperature around 6500 K. Stars cooler than this limit spin slowly, while those warmer spin very rapidly. The reason is that inside the interiors of the cooler stars, convection causes gases to move turbulently up and down. This generates a magnetic field that interacts with and is "pulled on" by the star's stellar wind. This magnetic drag acts over billions of years to slow the star's rotation down. The Sun must once have been rotating much more rapidly than it does today (and that's a phact!).
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]