Libra constellation lies in the southern sky. It is one of the zodiac constellations, first catalogued by Ptolemy in the 2nd century CE. The constellation’s name means “the weighing scales” in Latin. Libra is usually depicted as the scales held by the Greek goddess of justice Dike (or Astraea), represented by the neighbouring Virgo constellation.
Libra is the only zodiac constellation that represents an object, not an animal or a character from mythology. The four brightest stars in the constellation form a quadrangle. Alpha and Beta Librae mark the scales’ balance beam, and Gamma and Sigma Librae represent the weighing pans. The constellation is represented by the symbol ♎.
Libra does not contain any first magnitude stars. It is home to HD 140283, popularly known as Methuselah Star, one of the oldest stars known.
Facts, location and map
Libra is the 29th constellation in size, occupying an area of 538 square degrees. It lies in the third quadrant of the southern hemisphere (SQ3) and can be seen at latitudes between +65° and -90°. The neighboring constellations are Centaurus, Hydra, Lupus, Ophiuchus, Scorpius, Serpens Caput and Virgo.
Libra contains three stars with known planets and does not have any Messier objects. The brightest star in the constellation is Zubeneschamali, Beta Librae, with an apparent magnitude of 2.61. There is one meteor shower associated with the constellation, the May Librids.
Libra contains four formally named stars. The star names approved by the International Astronomical Union (IAU) are Brachium, Zubenelgenubi, Zubenelhakrabi, and Zubeneschamali.
Ancient Greeks knew the part of the sky occupied by the Libra constellation as Chelae, or “claws,” and considered it part of Scorpio constellation. Chelae represented the scorpion’s claws.
The association of this region of the sky with scales was established among the Romans in the first century BC. It is said that Moon was located in Libra when Rome was founded. The Romans considered Libra to be a favoured constellation, one associated with balanced seasons and equal length of night and day. The Sun was at the autumnal equinox in Libra until the year 729, when the precession of the equinoxes shifted the equinox to Virgo. The autumnal equinox will move to constellation Leo in the year 2439.
The Romans were not the first to associate Libra with the idea of balance. The Babylonians called it ZIB.BA.AN.NA, which means “the balance of heaven,” about a thousand years before Christ.
Once Libra became associated with balance, its association with Scorpio’s claws faded and the one with the goddess of justice, the Greek Dike or Astraeia, represented by the constellation Virgo, became stronger.
As a reminder that Libra was once considered a part of Scorpio constellation, the brightest star in Libra, Beta Librae, has the name Zubeneschamali, which means “the northern claw” in Arabic, while Alpha Librae, Zubenelgenubi, is “the southern claw.”
Major stars in Libra
Zubeneschamali – β Librae (Beta Librae)
Beta Librae is the brightest star in the constellation. It has an apparent magnitude of 2.61 and is approximately 185 light years distant from the solar system.
It has the stellar classification of B8 V, which means that it is a blue-white dwarf. Beta Librae is a very fast spinner, with a projected rotational velocity of 250 km/s. It has 4.9 times the solar radius and is approximately 130 times more luminous than the Sun.
The star’s proper name, Zubeneschamali, comes from the Arabic phrase al-zuban al-šamāliyya, which means “the northern claw.” The star’s Latin name is Lanx Borealis, or “the northern scale.”
Beta Librae is classified as a single star, but it shows small periodic variations in luminosity (0.03 of a magnitude), which indicate the presence of a companion star.
Zubenelgenubi – α Librae (Alpha Librae)
Alpha Librae is the second brightest star in Libra. It is a multiple star system whose two brightest components form a binary star and share a common proper motion through space. They are suspected members of the Castor Moving Group of stars, which share a common origin about 200 million years ago.
The Alpha Librae system lies close to the ecliptic and can be occulted by the Moon and, much less frequently, by planets. It will next be occulted by a planet (Mercury) on November 10, 2052.
The name Zubenelgenubi is derived from the Arabic phrase al-zuban al-janūbiyy, which means “the southern claw.” The system is also sometimes known as Kiffa Australis or Elkhiffa Australis. Both names are partial Latin translations and come from the Arabic phrase al-kiffah al-janūbiyy, which means “the southern pan (of the scales).” An older Latin name for the star is Lanx Australis, or “the southern scale.”
Alpha-1 Librae is the dimmer of the two main components in the system. It has an apparent magnitude of 5.153. Alpha-1 Librae is a spectroscopic binary system with an orbital period of 5,870 days, with components separated by 0.383 seconds of arc (10 astronomical units). The system has the stellar classification F4 and an apparent magnitude of 5.153. It is 74.9 light years distant from Earth.
Alpha-2 Librae has an apparent magnitude of 2.741. It is also a spectroscopic binary system, separated from Alpha-1 Librae by about 5,400 astronomical units. It has the stellar classification A3 and an apparent magnitude of 2.741. It is 75.8 light years distant from the Sun.
The star KU Librae might be a fifth component in the Alpha Librae system, lying at a separation of 2.6 degrees. It shares a similar proper motion with the other components, but is a parsec away. This is just close enough for KU Librae to be gravitationally bound to the other stars.
Brachium – σ Librae (Sigma Librae)
Sigma Librae is a red giant star with the stellar classification of M3/M4 III. It has an apparent magnitude of 3.29 and is approximately 288 light years distant from the Sun. The star’s traditional name, Brachium, means “arm” in Latin. It is also sometimes known as Cornu (Latin for “horn”) and Zubenalgubi (“southern claw” in Arabic).
Sigma Librae used to have the Bayer designation Gamma Scorpii even though it lies quite far from the border with Scorpius constellation. It only became Sigma Librae in the 19th century, and the designation was confirmed by the International Astronomical Union on July 31, 1930.
Brachium is a semi-regular variable star with a single pulsation period of 20 days. It exhibits small variations in magnitude of 0.10 to 0.15 over short periods of 15 to 20 minutes every 2.5 to 3 hours or so.
Methuselah – HD 140283
HD 140283 is one of the oldest stars known, believed to have been created shortly after the Big Bang. Nicknamed Methuselah, it is a subgiant star that is very metal poor and consists almost entirely of hydrogen and helium. The star’s iron content is less than 1 percent that of the Sun.
The star’s age is estimated to be 14.46 billion years old, while the Universe is believed to be 13.77 billion years old. The star’s age does not really conflict with the age of the Universe because it is uncertain and recent estimates give a margin of error of 0.8 billion years
HD 140283 has an apparent magnitude of 7.205 and is 202.4 light years distant.
υ Librae (Upsilon Librae)
Upsilon Librae has the stellar classification of K3III, which makes the star an orange giant. It is a multiple star system with an apparent magnitude of 3.60. It is approximately 195 light years distant from the Sun.
τ Librae (Tau Librae)
Tau Librae is a blue-white dwarf with the stellar classification of B2.5V. It has an apparent magnitude of 3.66 and is approximately 445 light years distant. The star has 3.2 times the Sun’s radius.
Zubenelakrab – γ Librae (Gamma Librae)
Gamma Librae is an orange giant belonging to the stellar class K0 III. It has an apparent magnitude of 3.91 and is approximately 152 light years distant from the solar system. It has 2.15 solar masses and is about 71 times more luminous than the Sun.
The star’s traditional name, Zubenelakrab, or Zuben-al-Akrab, is derived from the Arabic phrase al-Zuban al-Aqrab, which means “the shears of the scorpion.”
θ Librae (Theta Librae)
Theta Librae is an orange giant with the stellar classification of K0 III. It has a visual magnitude of 4.136 and is approximately 163 light years distant from Earth. It has a mass about 84 percent greater than the Sun’s and it is approximately 35 times more luminous.
ι Librae (Iota Librae)
Iota Librae has the stellar classification of B9IVpSi and an apparent magnitude of 4.54. It is composed of Iota-1 Librae, a pair consisting of a B9 subgiant and a dwarf star, approximately 377 light years distant, and Iota-2 Librae, a class A3 dwarf star about 240 light years from the Sun.
The components in the Iota-1 Librae system orbit each other with a period of 23.469 years and are separated by only 0.13 seconds of arc. Their combined mass is 6.05 times solar and they are 149 and 94 times more luminous than the Sun respectively.
There is another pair in the Iota-1 system, two 10th and 11th magnitude class G dwarfs.
Zuben Elakribi – δ Librae (Delta Librae)
Delta Librae belongs to the spectral class B9.5V. It is a blue-white main sequence star with an apparent magnitude of 4.43, approximately 300 light years from Earth. It is classified as an eclipsing variable star. It has a period of 2.3272 days and its luminosity varies from 4.43 to 5.81 magnitudes.
The star’s traditional name, Zuben Elakribi, or Zuben-el-Akribi, comes from the Arabic az-zubānā al-ʿaqrab, which means “the claws of the scorpion.”
48 Librae, or FX Librae, is a shell star, a blue supergiant exhibiting irregular variations in luminosity as a result of its abnormally high rotational velocity, which results in gas being ejected from the star’s equator and forming a gaseous equatorial disk around the star.
48 Librae is one of the most rapid rotators known, with a projected rotational velocity of 400 km/s.
48 Librae has the stellar classification B8Ia/Iab. It has an apparent magnitude of 4.94 and is approximately 515 light years distant from Earth. It has 5.8 solar masses, 3.3 times the solar radius, and is about 965 times more luminous than the Sun.
Gliese 581 (HO Librae)
Gliese 581, or HO Librae, is a red dwarf star with the stellar classification of M3V. It has an apparent magnitude that varies between 10.56 and 10.58, and is 20.3 light years distant from the Sun.
It is the 89th closest star to the Sun, and has only a third of the Sun’s mass and 0.2 percent of its visual luminosity. The star lies about two degrees north of the constellation’s brightest star, Beta Librae.
Gliese 581 is classified as a variable star of the BY Draconis type and is sometimes known by its variable designation, HO Librae. BY Draconis variables are typically K or M class main sequence stars that exhibit variations in luminosity as a result of rotation coupled with star spots.
Gliese 581 has a planetary system with at least three and possibly up to six planets. The first extrasolar planet in the system, Gliese 581 c, was discovered in April 2007. It is likely too hot to be a habitable zone, not unlike Venus. An unconfirmed planet in the system, Gliese 581 d, might be within or just outside the system’s habitable zone. Gliese 581 e, discovered in April 2009, was the least massive planet known orbiting a normal star at the time of discovery.
Another unconfirmed planet was claimed to have been discovered in September 2010, and if its presence is confirmed, it will be the planet most suitable for liquid water as it lies in the middle of Gliese 581’s habitable zone. In November 2012, the European Space Agency found a comet belt in the system, one with at least ten times as many comets as the solar system.
23 Librae is a yellow dwarf with the stellar classification of G5 V. It has an apparent magnitude of 6.45 and is approximately 85 light years distant from Earth. It is another star in Libra constellation that has a planetary system with two confirmed planets. The first extrasolar planet, 23 Librae b, was discovered in 1999, and the second one was detected in 2009.
23 Librae is a much older star than the Sun, with an estimated age between 8.4 and 11.1 billion years. It has 107 percent of the Sun’s mass and 125 percent of the solar radius.
HD 141937 is another yellow dwarf (spectral class G2/G3 V) with a confirmed planet in its orbit. The planet is a massive gas giant, discovered in 2001. The star has exactly the same mass as the Sun and a slightly larger radius, 1.06 times that of the Sun’s. It has an apparent magnitude of 7.25 and is approximately 109 light years distant from the solar system.
Gliese 570 (33 G. Librae)
Gliese 570 is a ternary star system, one consisting of at least three stars, located approximately 19 light years from Earth, in the southwestern part of the constellation. It lies southwest of Alpha Librae and northwest of Sigma Librae.
The primary component is an orange dwarf with the stellar classification K4V and an apparent magnitude of 6.79. The star is smaller and less massive than the Sun and has only 15.6 percent of the Sun’s luminosity. It is a known X-ray source.
The system also contains a binary star system consisting of two red dwarfs (spectral classes M1V and M3V) that orbit each other. Both stars also emit X-rays.
A brown dwarf belonging to the spectral class T7Vwas discovered orbiting in the system in January 2001. At the time, it was one of the coolest brown dwarfs known. The dwarf star has a mass 50 times that of Jupiter.
In 1998, an extrasolar planet was believed to orbit the brightest star in the system, but its presence was discounted in 2000.
Deep sky objects in Libra
NGC 5885 is a barred spiral galaxy in Libra. It has an apparent visual magnitude of 11.8. The galaxy was discovered by William Herschel on May 9, 1784.
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 5897 is a relatively large globular cluster in Libra. It has an integrated magnitude of 9 and is approximately 40,000 light years distant from the solar system.