TAU LIB (Tau Librae). In spite of having two stars with famed names that represent the outstretched claws of Scorpius, Zubenelgenubi (the southern claw) and Zubeneschamali (the Northern Claw), Libra (the Scales) is pretty obscure. It used to hold the autumnal equinox, but even that left for Virgo around the fifth century BC, a victim of axial precession (the 26,000 year wobble of the Earth's axis). If that is not obscure enough by itself, try looking to the bottom of the constellation where we find the most southerly star with a Greek letter, Tau Librae. However, at bright fourth magnitude (3.66, nearly third) it shines with an unexpected luster of its own and with a bit of a mystery too. Just barely north of Libra's border with Lupus and a mere five degrees west of the border with Scorpius, the star belongs more to the Wolf, the Scorpion, and the Milky Way than it does to the Scales. Indeed, at a distance of 367 light years (give or take 8) this hot, blue class B (B2.5) dwarf fits right in, as from its motion it belongs to the Upper Scorpius-Lupus association of hot massive O and B stars (which is centered at 450 light years). After a 0.22 magnitude correction for dimming by intervening interstellar dust (removal of which would bump the star to third magnitude), and the addition of a lot of ultraviolet radiation from a 20,800 Kelvin surface, we find a luminosity of 2600 Suns and a resulting radius of 4.0 times solar. With an equatorial rotation speed of at least 128 kilometers per second, Tau Lib makes a full turn in under 1.6 days. Application of the theory of stellar structure and evolution yields a mass of 7.5 Suns and shows the star to be very young, perhaps just born, with a hydrogen-fusing age in front of it of some 30 million years.

But, as for many of its kind, there is a complication. Doppler shifts in the spectrum reveal the presence of a close companion with an orbital period of just 3.29 days, possibly of class B5, which would give it a luminosity of somewhat over 300 Suns and a mass of perhaps 4.3 times solar. Factoring in the companion would reduce the luminosity of Tau Lib A by just about 15 percent and have only a small effect on its mass. Giving Tau Lib A 7.25 Suns makes for a system mass of 11.8 Suns (don't pay too much attention to the decimal), and with Kepler's third law gives a mean orbital separation of 0.10 Astronomical Units, a quarter the size of Mercury's orbit and just five times the radius of Tau Lib A. A significant orbital eccentricity takes them between 0.13 and 0.07 AU apart. Left alone, when it someday evolves as a giant, Tau Librae A would slough off its outer envelope to become a white dwarf with a mass about that of the Sun, comparable to Sirius B. However, the pair is close enough to interact during evolutionary expansion. It's hard to tell just what is going to happen, but the transfer of mass back and forth as the two evolve may someday lead to wild, if not violent, behavior.

Written by Jim Kaler 5/17/13. Return to STARS.