Beta Librae - Zubeneschamali

Beta Librae is the brightest star in Libra, the Scales, at magnitude 2.61. It also has the traditional name Zubeneschamali, and the Latin name Lanx Australis ("The southern scale of the balance"). The name Zubeneschamali is derived from the Arabic "al-zuban al-samaliyyah", meaning "The Northern Claw". Libra's two brightest stars once represented the outstretched claws of Scorpius, the Scorpion.

Properties

Zubeneschamali is a hot main-sequence (hydrogen fusing) class B8 V star with a surface temperature of 12,000 K - double the Sun's. While such stars are normally considered blue-white in color, Zubeneschamali has long had a reputation of being the only naked eye star that oddly appears green to the human eye. From its distance of 160 light years, the star has a calculated luminosity about 130 times the Sun's. Its high temperature gives it a simple spectrum, and it is therefore ideal for examining the interstellar medium of gas and dust between it and the Sun. Like many stars of its kind, β Lib is spinning rapidly, over 100 times faster than the Sun.

Variability

Small periodic variations in the magnitude of the Beta Librae may imply the presence of a companion star which is not directly observable from earth. Though not considered a variable star, ancient astronomers claimed β Lib to be as bright or brighter than first magnitude Antares right next door in Scorpius. According to Eratosthenes, β Lib was brighter than Antares; Ptolemy, 350 years later, said it was as bright as Antares. The discrepancy may be due to Antares becoming brighter, but this is not known for certain. We will probably never know if that is true, or if it is, how Zubeneschamali could have faded so fast.

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