Despite its “beta” designation, Beta Librae is the brightest star in the constellation Libra, with an apparent magnitude of +2.6. However, the debate about its true brightness dates back to antiquity; Eratosthenes (276-194 BC) was of the opinion that Beta Libra was brighter than Antares, while 350 years later, Ptolemy thought that Beta Libra was at least as bright as the star now known as Alpha Scorpii. The exact reason for the controversy is not known, but it is thought that Beta Librae might be a variable star with a range in brightness of about 0.03 magnitudes, which could account for the discrepancy.
Quick Facts
• Constellation: Libra
• Coordinates: RA 15h 17m 00.41382s |Dec. -09° 22′ 58.4919″
• Distance: 185 light-years
• Star Type: B8 V (Suspected variable star)
• Mass: 3.5 sol
• Radius: 4.9 sol
• Apparent magnitude: +2.61
• Luminosity: 130 sol
• Surface Temperature: 12,300K
• Rotational Velocity: 250 km/s
• Age: Estimated 80 million years
• Other Designations: Zubeneschamali, Lanx Borealis, 27 Librae, BD-08° 3935, FK5 564, HD 135742, HIP 74785, HR 5685, NSV 7009, SAO 140430.
Visibility
Despite its four brightest stars forming a distinctive quadrangular shape, the constellation Libra is not among the most conspicuous constellations and contains no stars of first magnitude. However, it is one of the zodiacal constellations, which fact makes it easier to spot along the ecliptic between the constellations Virgo to its west, and Scorpius to its east. The entire constellation is visible to observers south of latitude 60°N, and in the northern hemisphere can be seen during spring and summer, although the brightest star in the constellation, Beta Librae, is best seen during the month of June, at about 9 PM, Local Time.
Physical Properties
Beta Librae is a B8 V- type main sequence star, whose high surface temperature of 12,300K produces a clean, simple spectrum which makes it an ideal vehicle with which to examine the interstellar medium between ourselves and the star.
Like most other B-type stars, Beta Librae is a fast rotator, with a projected equatorial speed of 250 km/sec, which represents a significant percentage of the speed at which the star would break apart under the centrifugal force created by its fast rotation. Apart from its uncertain brightness, there is also some debate about its color; many observers throughout history have reported that the star is green, but observations have shown that massive stars of this type are typically blue-white. Modern observers who fail to see green in the star maintain that the peculiar color seen by others may be due to the star’s environment, but there appears to be no scientific basis for either point of view.
Nonetheless, Beta Libra is not the only star thought by some observers to be green, and while some observers have also reported the companion star of Antares as green, Beta Librae is considered the most luminous star that may possibly have a greenish hue.
History
The constellation of Libra has been associated with balancing scales since Babylonian times, but the ancient Greeks would later envisage Libra as the extended claws of the nearby constellation of Scorpius. This tradition was adopted by Arab astronomers, hence the two brightest stars in Libra subsequently being called Zubeneschamali (“the northern claw of the Scorpion”), and Zubenelgenubi (“the southern claw of the Scorpion”).
By Roman times, the super-sized constellation was once more separated into two separate zodiac constellations, but Beta Librae’s Arabic name, Zubeneschamali, stayed in common use. Over the years, its original name has been corrupted to Zuben Eschamali, Zuben el Chamali, Zubenesch, and Zubenelg, with other variations including Kiffa Borealis, which derives from the Arabic term meaning “the northern pan [of the scales]”. In 2016, however, the Working Group on Star Names of the International Astronomical Union formally accepted the name Zubeneschamali, which it is now listed under in the IAU Catalogue of Star Names.