Alpha1 Librae, Alpha2 Librae - Zubenelgenubi

At magnitude 2.75, Alpha Librae is the second brightest star in the constellation Libra (despite its Bayer designation as α Lib). It also has the traditional name Zubenelgenubi. The name, from Arabic "al-zuban al-janubiyy", means "southern claw", and was coined before Libra was recognized as a constellation distinct from Scorpius. The alternate names Kiffa Australis and Elkhiffa Australis, partial Latin translations of Arabic "al-kiffah al-janubiyy", mean "southern pan (of the scales)".

Alpha Librae is close to the ecliptic so it can be easily occulted by the Moon and (very rarely) by planets. The next occultation by a planet will take place on 10 November 2052 with Mercury.

Components

Alpha Librae is actually a visual binary consisting of two stars separated in the sky by an angular distance of 231". It is easily seen in binoculars as a bright, wide pair of white and yellow stars. The two are probably a physical pair, as they move through space together, both of them 77 light years away from us. The brighter of the pair, designated α2 Lib, is a white star of spectral type A3 IV, with an apparent magnitude of 2.8. Its companion, designated α1 Lib, is a class F4 V star of apparent magnitude 5.2.

The two are separated by at least 5500 AU, and maybe more since we do not know the exact difference in their distance. At that separation, their orbital period would be over 200,000 years. From α2 Lib, α1 would appear as a brilliant star of magnitude -10, 100 times brighter than Venus does in our sky. From α1 Lib, α2 would appear 10 times brighter yet, and rival our full Moon.

The brighter component, α2 Lib, is itself a double made of two class A stars, one 45% brighter than the other. They are only a few tenths of an AU apart; from Earth they are separated by a mere hundredth of an arcsecond. The brighter component of α2 Lib also has an enhancement of metals in its atmosphere, most likely caused by separation of elements, some rising up, others drifting down.

There is some evidence that this triple star system belongs to a hugely extended group of stars that move together through space (the "Castor Moving Group"); it includes Castor, Vega, and Fomalhaut.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]