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).