Libra (bottom right edge of the painting of various constellations above) is a constellation of the zodiac. Its name is Latin for weighing scales. It is fairly faint, with no first magnitude stars, and lies between Virgo to the west and Scorpius to the east. Libra is bordered by the head of Serpens to the north, Virgo to the northwest, Hydra to the southwest, the corner of Centaurus to the southwest, Lupus to the south, Scorpius to the east and Ophiuchus to the northeast. Covering 538.1 square degrees and 1.304% of the night sky, it ranks 29th of the 88 constellations in size. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 12 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 14h 22m 08.08s and 16h 02m 17.23s, while the declination coordinates are between −0.47° and −30.00°. The whole constellation is visible to observers south of latitude 60°N.
Overall, there are 83 stars within the constellation’s borders brighter than or equal to apparent magnitude 6.5. The brightest stars in Libra form a quadrangle that distinguishes it from the unaided observer. Traditionally, Alpha and Beta Librae are considered to represent the scales’ balance beam, while Gamma and Sigma are the weighing pans. Alpha Librae, called Zubenelgenubi, is a multiple star system. The primary (Alpha2 Librae) is a blue-white star of magnitude 2.7, and the secondary (Alpha1 Librae) is a white star of magnitude 5.2 and spectral type F3V that is 74.9 ± 0.7 light-years from Earth. Its traditional name means “the southern claw.” Zubeneschamali (Beta Librae) is the corresponding “northern claw” to Zubenelgenubi. The brightest star in Libra, it is a green-tinged star of magnitude 2.6, 160 light-years from Earth. Gamma Librae is called Zubenelakrab, which means “the scorpion’s claw,” completing the suite of names referring to Libra’s archaic status. It is an orange giant of magnitude 3.9, 152 light-years from Earth. Iota Librae are complex multiple stars, 377 light-years from Earth, with both optical and true binary components. The primary appears as a blue-white star of magnitude 4.5; it is a binary star with a period of 23 years. The secondary is a star of magnitude 9.4: a binary with two components, magnitudes 10 and 11. There is an optical companion to Iota Librae; 25 Librae is a star of magnitude 6.1, 219 light-years from Earth. Mu Librae is a binary star, 235 light-years from Earth. The primary is of magnitude 5.7, and the secondary is of magnitude 6.8. Delta Librae is an Algol-type eclipsing variable star, 304 lightyears from Earth. It has a period of 2 days, 8 hours; its minimum magnitude of 5.9 and its maximum magnitude is 4.9. FX Librae, designated 48 Librae, is a shell star of magnitude 4.9. Shell stars, like Pleione and Gamma Cassiopeiae, are blue supergiants with irregular variations caused by their abnormally high speed of rotation. This ejects gas from the star’s equator. Sigma Librae (the proper name is Brachium) was formerly known as Gamma Scorpii despite being well inside the boundaries of Libra. It was not re-designated as Sigma Librae until 1851 by Benjamin A. Gould.
Libra is home to the Gliese 581 planetary system, which consists of the star Gliese 581, three confirmed planets, and two unconfirmed planets. Both Gliese 581d, and Gliese 581g are debatably the most promising candidates for life, although Gliese 581g’s existences have been disputed and have not been entirely confirmed or agreed on in the scientific community. Gliese 581c is considered to be the first Earth-like extrasolar planet to be found within its parent star’s habitable zone. Gliese 581e is possibly the smallest mass exoplanet orbiting a normal star found to date. All of these exoplanets are of significance for establishing the likelihood of life outside of the Solar System. The family of candidate habitable planets was extended in late September 2010 to include exoplanets around red dwarf stars because of Gliese 581g, which is a tidally locked planet in the middle of the habitable zone. Weather studies show that tidally locked planets may still have the ability to support life.
Libra is home to one bright globular cluster, NGC 5897. It is a loose cluster, 50,000 light-years from Earth; it is fairly large and has an integrated magnitude of 9. NGC 5792 is another barred spiral galaxy in Libra. It has an apparent magnitude of 12.1 and is approximately 83 million light years distant from the Sun. NGC 5890 is an unbarred lenticular galaxy in Libra. It was discovered by the American astronomer Ormond Stone in April 1785. It has an apparent magnitude of 14. NGC 5885 is a barred spiral galaxy in Libra. It has an apparent magnitude of 11.8. The galaxy was discovered by William Herschel on May 9, 1784. Credit: Wikipedia.