PHI VEL (Phi Velorum). Just-barely fourth magnitude (3.54) Phi
Velorum (of no proper name) has a rather surprising number of
things to set it apart, including some "negatives." The first
positive is in its location as one of the three stars that define
the southern classic outline of Vela, the
Sails of the Ship Argo (visible south of
about 35 degrees north latitude), the others brighter Kappa and Delta
Vel, these two part of the "False
Cross" that resembles Crux, the
proper Southern Cross. Better, it sits practically on the Galaxy's equator, the center line
of the Galactic disk, just 0.1 degree to the north of it. Phi Vel
is also a blue-white and very luminous class B (B5) supergiant (albeit a lesser
one) that appears dim only because of its great distance of 1590
light years (give or take 70). Oddly, even though it sits smack
within the Milky Way, the star appears to be
in a clear path that has little, if any, interstellar dust
absorption to dim it further. Distance and a temperature of 13,600
Kelvin, needed to account for ultraviolet light, reveal a
luminosity 18,200 times that of the Sun and
a radius 24 times solar. The rotation speed is, at 33 kilometers
per second, fairly low, giving it a rotation period under 37 days,
which is not much of a constraint. Then we come to the star's
greatest claim to stellar fame, its mass, which comes in (from
luminosity, temperature, and theory) right on 10 times solar. The
figure represents something of an upper limit to the mass at which
stars turn into white
dwarfs instead of exploding as supernovae, Phi Vel right on
the line and thus in an unusual position. (In truth, the limit is
somewhere between 8 and 12 solar masses, most astronomers opting
for 8, making the star a potential exploder.) Whatever its
forecast, Phi Vel began life some 20 million years ago as a much
hotter B0.5-B1 dwarf, its cooler and larger status now the result
of the depletion of hydrogen in its nuclear burning core. On the
"negative" side, in which the star has been rejected, are a couple
of memberships. Just over half a minute of arc away is a twelfth
"companion," Phi Vel B, whose slow drift relative to Phi Vel itself
seems to rule it out as a real binary member. What really does
it is that the fainter star is touted as a class K giant, whose brightness would put
it 7800 light years away, five times farther than Phi Vel itself
(though closer if dimmed by interstellar dust). So no double.
Early ideas also had the star as a member of the huge, expanding Scorpius-Centaurus
association of hot blue stars. Rejected again, it isn't, its
motion not in line with real members. What will happen to it, only
time will tell, though it might be a million or more years in the
future before some of our distant descendants may know. Before
anything dramatic happens, however, the star, still cooling and
expanding before it fires up helium in its core, will eventually
double its luminosity and grow to some 500 times the solar size, to
a radius of roughly two Astronomical Units.
Written by Jim Kaler 4/09/10. Return to STARS.