Epoch J2000 Equinox J2000
|Right ascension||15h 17m 00.41382s|
|Declination||−09° 22′ 58.4919″|
|Apparent magnitude (V)||2.61|
|Spectral type||B8 V|
|U−B color index||−0.359|
|B−V color index||−0.106|
|Radial velocity (Rv)||−35.2 km/s|
|Proper motion (μ)||RA: −98.10 mas/yr
Dec.: −19.65 mas/yr
|Parallax (π)||17.62 ± 0.16 mas|
|Distance||185 ± 2 ly
(56.8 ± 0.5 pc)
|Metallicity [Fe/H]||0.33 dex|
|Rotational velocity (v sin i)||250 km/s|
Beta Librae (β Lib, β Librae) is the Bayer designation for a star in the zodiac constellation of Libra. It has the traditional name Zubeneschamali // and the Latin name Lanx Borealis, meaning "the northern scale [of the Balance]". The name Zubeneschamali is derived from the Arabic الزبن الشمالية (al-zuban al-šamāliyya) meaning "The Northern Claw". The apparent visual magnitude of this star is 2.6, making it the brightest member of Libra. From parallax measurements, its distance can be estimated as 185 light-years (57 parsecs) from Earth.
Based upon the features of its spectrum, Beta Librae has a stellar classification of B8 V, making it a B-type main sequence star. It is about 130 times more luminous than the Sun and has a surface temperature of 12,300 K, double that of the Sun. This high temperature produces light with a simple spectrum, making it ideal for examining the interstellar gas and dust between us and the star. Like many stars of its kind, it is spinning rapidly, over 100 times faster than the Sun with a projected rotational velocity of 250 km s–1. The measured angular diameter of the primary star is 0.801 mas. At the estimated distance of this system, this yields a physical size of about 4.9 times the radius of the Sun. This type of massive, hydrogen-fusing star often appears blue-white, and is usually stated to be white or bluish by modern observers, but earlier observers often described Beta Librae as the only greenish star visible to the naked eye. There seems to be no generally accepted explanation for why some observers see it as green.
The small periodic variations in the magnitude of the Beta Librae suggests the presence of a companion star which is not directly observable from earth. However, it is presently categorized as a single star.
According to Eratosthenes Beta Librae was observed to be brighter than Antares. Ptolemy, three hundred and fifty 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. It could simply be caused by Beta Librae being a variable star, showing a present day variability of 0.03 of a magnitude.
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