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.