ALPHA PYX (Alpha Pyxidis). If there were no attempt made to
highlight all the sky's constellations with their brightest stars,
or even their Alpha stars (which are not always one and the same),
Alpha Pyx would likely never be paid much attention. This fourth
magnitude (but nearly third, 3.68) star is almost lost above the bright
stars of Vela, the Sails. Not in an
ancient constellation, but in the modern one of Pyxis (the Compass, or Mariner's Compass), Alpha Pyx is
honored by no proper name. It is, however, the luminary of its
otherwise drab figure, though not by terribly much. The star is
dim because it is so far away, some 850 light years, which
diminishes the light of a quite luminous body. At that distance,
and near the Milky Way, Alpha Pyx is also subject to a bit (about
30 percent) of dimming by the pervasive dust of interstellar space.
Were the dust not in the way, the star would indeed appear at third
magnitude (a respectable 3.31). Like many of the stars of
neighboring Argo, Alpha Pyx is a hot, blue-white class B (B1.5)
giant with an impressive surface temperature of 22,900 Kelvin. The
distance and apparent magnitude, coupled with corrections for dust
absorption and for a great deal of invisible ultraviolet light,
yield a luminosity nearly 18,000 times that of the Sun and a diameter of 8 times solar. While
called a giant, the luminosity and temperature conspire to suggest
that the star is really still a hydrogen-fusing 11-solar-mass
dwarf, but one that will soon begin to evolve as the interior
hydrogen is all turned to helium. Born less than 18 million years
ago, it seems to be involved in a circumstellar shell, and may be
a member of the Beta Cephei class of
multi-periodic variables (though no one quite knows for sure).
Like many class B stars near the Sun, Alpha Pyx has a subsolar
metal composition, its iron content about 60 percent solar, the
other elements similar. Most likely, the Sun is a bit richer in
metals than its local surroundings, a gift from the long lost solar
birthplace.