ARKAB POSTERIOR (Beta-2 Sagittarii). One of the most glaring
exceptions to the general rule that the brightest star in a constellation is called
"Alpha" involves Sagittarius, in which fourth magnitude Rukbat is far fainter than the luminary,
bright-second-magnitude Kaus Australis
(Epsilon) as well as several others of the Archer's stars. Even
though there are two Betas, Arkab
Prior (Beta-1) to the west of Arkab Posterior (Beta-2), each is
also still just fourth magnitude. Far to the south, as far down as
the lower curve of Scorpius's Tail, for
northerners Alpha and the Betas are pretty much out of easy sight
and thus out of mind. Even southerners don't pay much attention to
them. While linked by apparent closeness (just a third of a degree
apart) and a common Arabic name that
refers to the Archer's achilles tendon, they have nothing to do
with each other, Beta-2 139 light years away, Beta-1 378. While
this class F (F2) star is also classed as a "giant," it still has a long way
to go. An ill-determined temperature of 6900 Kelvin implies little
correction for infrared or ultraviolet radiation, the star's
luminosity of 27 times that of the Sun
coming mostly from ordinary visual radiation. Luminosity and
temperature then give a radius of 1.4 times solar and a mass of
twice solar. Two things then capture interest. First, from
stellar structure theory, Arkab Posterior is clearly not a giant,
but is more an old dwarf that is in the end stages of core hydrogen
fusion. At best it's a subgiant, the age about 1.2 billion years.
(The Sun's five billion year age is the result of lower mass and
slower fusion rates.) Second is the rapid projected rotation
velocity of 126 kilometers per second, 63 times that of the Sun,
giving a rotation period of under 1.4 days. As an F2 dwarf, Arkab
Posterior is just a bit warmer than "rotation break" around class
F5, at which as you go up in mass and in class from M to O,
rotations quickly increase. The reason is not that warmer stars
are particularly spun up, but that cooler stars like the Sun are
spun down thanks to their magnetic fields, which are caused by
outer convection currents and rotation and are dragged out by the
solar wind. Still anchored to the Sun, the magnetic fields then
slow cooler stars like ours and indeed are continuing to do so.
Written by Jim Kaler 8/01/08. Return to STARS.