48 LIB (48 Librae). Little Libra, one
of the fainter constellations of the Zodiac, is hardly famed for its
hot blue stars, which are more the province of Centaurus, Scorpius, Cygnus, and other denizens of the Milky Way, the disk of our Galaxy where stars are born, and
where massive stars, which do not live long, generally reside. We
tend to forget, however, that Libra does border Scorpius; after
all, its two brightest stars, Zubenelgenubi and Zubeneschamali, represent the Scorpion's
outstretched claws. Not that far off the Milky Way, eastern Libra
holds a fine member of the Upper Scorpius association of hot,
massive stars (which includes Antares,
Rho Ophiuchi, and Rho Scorpii), 48 Librae, a fifth magnitude
(4.88) class B (B5) emission-line (rendering it a "Be star") giant that lies a hefty 515 light
years away. Things are, however, not always what they at first
seem. The standard class is problematic, as more recent views have
it as a B8 supergiant, while
luminosity and temperature (see below) clearly reveal it to be a
core-hydrogen-fusing dwarf. Even the temperature itself is a
problem. 48 Lib is one of the most rapid rotators in the sky, with
a projected equatorial rotation speed of 400 kilometers per second.
To be that high, the star's rotation axis must be nearly
perpendicular to the line of sight, which is consistent with 48
Lib's special Be-star status as a "shell star." "Be" stars are
surrounded by gaseous equatorial disk that somehow are linked to
their rapid spins (no theory yet satisfactory), the classic example
being Gamma Cassiopeiae. Shell stars
have their disks lined up along the line of sight, making them look
especially thick, 48 Lib's observed to be expanding. The rapid
rotation also forces such a star to bulge at its equator, making it
subject to "gravity darkening," in which the equator, farther from
the star's center, has a distinctly lower temperature than the
poles, which are hotter and brighter. There is thus no unique
temperature from which to calculate other parameters: the "apparent
temperature is 17,645 Kelvin, while a "non-rotating" temperature
would be 20,100 K (both of which are high for the class). Using
the first to estimate the amount of invisible ultraviolet, 48
Librae would shine with the light of 965 Suns, which in turn gives a sort of average
"radius" (whose meaning is compromised because of the star's shape)
of 3.3 times solar, a rotation period of under 10 hours, and a mass
of 5.8 solar. The higher temperature yields 1260 solar
luminosities, 2.9 solar radii, and 6.5 solar masses. In either
case, the star is seen to be very much a hydrogen-fusing dwarf, and
a fairly young one at that (relative to its 50-55 million year
dwarf lifetime). Well under the limit for supernova explosion, will die
as a white dwarf that
will hold just under a solar mass, having first sloughed off its
outer envelope when it evolves as a giant star.
Written by Jim Kaler 6/27/08. Return to STARS.