Sigma Librae is one of a small number of "linking stars" that traditionally belong to two constellations. The others are Alpheratz (α And and δ Peg) and Elnath (β Tau and γ Aur); their latter names are no longer used since they lie within the modern boundaries of Andromeda and Taurus. Zubenhakrabi is a more extreme case. The star, whose name means "the scorpion's claw," was originally part of Scorpius. As such, Bayer called it Gamma Scorpii. But it was so far west of Scorpius that in the nineteenth century B. A. Gould named it Sigma Librae, which is how it is referred to today. To add to the confusion, early in that century Elijah Burritt applied the name "Zubenhakrabi" to Eta Librae as well!
Properties
Appearing at magnitude 3.29, Zubenhakrabi is a rather-luminous class M3 II red giant with a cool 3600 K surface. From a distance of 290 light years, it radiates 1900 solar luminosities, swollen to a radius 110 times that the Sun's (0.52 AU). It is a subtle SRb-type semi-regular variable that changes its brightness by 0.16 magnitudes over a 20-day period.
This two-solar mass star, with a dead carbon-oxygen core, is expanding and brightening as a giant for the second time, fueled by internal nuclear-burning shells of helium and hydrogen. (The first brightening was with a dead helium core.) It is on its way to becoming a much larger, brighter, Mira-type long-period variable; it will eventually slough off its outer envelope, and its now-quiet carbon-oxygen core will become a white dwarfs. The star has been well-examined for various quirks, but has no indication of a surrounding dusty shell or any anomalies in surface chemical composition.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]