Libra Constellation

Libra constellation lies in the southern sky. It is one of the zodiac constellations, first catalogued by the Greek astronomer 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. Its brightest stars, Zubeneschamali and Zubenelgenubi, shine at third magnitude. The constellation is also 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.

The constellation name Libra is pronounced /ˈli:brə /. In English, the constellation is known as the Scales. The genitive form of Librae, used in star names, is Librae (pronunciation: /ˈlaɪbriː/). The three-letter abbreviation, adopted by the International Astronomical Union (IAU) in 1922, is Lib.

Libra belongs to the zodiac family of constellations, along with Aries, Taurus, Gemini, Cancer, Leo, Virgo, Scorpius, Sagittarius, Capricornus, Aquarius and Pisces.

Libra contains four 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 (Sigma Librae A), Zubenelgenubi (Alpha² Librae), Zubenelhakrabi (Gamma Librae A), and Zubeneschamali (Beta Librae).

Myth

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 by the Romans in the 1st 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 the constellation 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

Libra is one of the fainter constellations of the zodiac. It does not contain any first or second magnitude stars.

The three brightest stars in Libra – Zubeneschamali, Zubenelgenubi and Brachium – are easy to find because they shine at third magnitude and appear as the extended claws of the brighter Scorpius.

Together with the fainter Zubenelhakrabi, they form the main constellation figure of the celestial Scales. Zubeneschamali and Zubenelgenubi form the balance beam and Brachium and Zubenelhakrabi mark the weighing pans. Upsilon and Tau Librae are sometimes included as part of one of the weighing pans.

Zubeneschamali – β Librae (Beta Librae)

Zubeneschamali (β Lib) is the brightest star in Libra. It has an apparent magnitude of 2.61 and is approximately 185 light years distant from the solar system.

The star’s proper name, Zubeneschamali, comes from the Arabic phrase al-zuban al-šamāliyya, which means “the northern claw.” The name dates back to the time when the stars of Libra were seen as part of Scorpius and represented the Scorpion’s claws. The star’s Latin name is Lanx Borealis, or “the northern scale.”

Zubeneschamali has the stellar classification B8 V, indicating a hot blue-white main sequence star. It has a mass of 3.5 solar masses and a radius 4.9 times that of the Sun. The star is a very fast spinner, with a projected rotational velocity of 250 km/s. It shines with a luminosity of 130 Suns and has an estimated age of only 80 million years.

Beta Librae is catalogued as a single star, but it shows small periodic variations in luminosity (0.03 of a magnitude), which may indicate the presence of a companion.

Zubenelgenubi – α Librae (Alpha Librae)

Zubenelgenubi (α Lib) is the second brightest point of light in Libra. It lies approximately 75 light-years away.

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 Librae is in fact a multiple system composed of an F-type main sequence star (α¹ Lib) and an A-type star (α² Lib) that is coming to the end of its main sequence lifetime.

The brighter Alpha² Librae has an apparent magnitude of 2.741 and Alpha¹ Librae shines at magnitude 5.153. The two components are separated by 231 arcseconds in the sky, corresponding to a physical distance of about 5,400 astronomical units (AU).

Both visual components – Alpha¹ and Alpha² Librae – are themselves spectroscopic binary stars. The components of Alpha¹ orbit each other with a period of 5,870 days at an angular separation of 0.383 arcseconds (10 AU). The components of Alpha² Librae have masses of 1.95 and 1.79 solar masses.

The Alpha Librae system may have another component. The star KU Librae lies at a separation of 2.6 degrees, corresponding to a physical distance of about 1 parsec. Even though it has a similar proper motion and may be just close enough to be gravitationally bound to the other stars, KU Librae has a different abundance of metals and may not be related to the system.

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.

Brachium – σ Librae (Sigma Librae)

Brachium (σ Lib) is a red giant of the spectral type M2.5III. It has an apparent magnitude of 3.29 and lies approximately 288 light-years away. It has 2.2 times the Sun’s mass and a radius 108 times that of the Sun. The evolved giant star has an effective temperature of 3,596 K and shines with 1,820 solar luminosities.

The star’s traditional name, Brachium, is Latin for “arm.” Sigma Librae was historically also known as Cornu (Latin for “horn”). It once shared the names Zubenalgenubi or Zuben el Genubi (Arabic for “the southern claw”) with Alpha Librae and Zuban Alakrab (“the claws of the Scorpion”) with Gamma Librae.

Brachium only got the Bayer designation Sigma Librae in the 19th century. It was previously known as Gamma Scorpii, even though it appears quite far from the border with the Scorpius constellation. The designation was confirmed by the International Astronomical Union (IAU) on July 31, 1930.

Brachium is classified as 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. The star’s brightness varies between magnitude 3.20 and 3.46.

Brachium has a 16th magnitude companion at a separation of over an arcminute.

Methuselah – HD 140283

HD 140283 is one of the oldest stars known, believed to have been created shortly after the Big Bang. Nicknamed the Methuselah Star, 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.

Based on the data obtained with the Hubble Space Telescope, the star’s age was estimated to be 14.46 billion years in 2013. This would make it older than the universe itself. However, the value is uncertain, with a margin of error of 0.8 billion years.

More recent studies have derived an age of 13.7 billion years or 12 billion years.

HD 140283 has an apparent magnitude of 7.205 and lies 202.4 light years away. It has a mass of 0.81 solar masses and a radius 2.04 times that of the Sun. With a surface temperature of 5,787 K, it is 4.82 times more luminous than the Sun.

HE 1523-0901

HE 1523-0901 is a very old red giant located approximately 9,900 light-years away. It has an apparent magnitude of 11.129.

Like its Libra neighbour Methuselah, HE 1523-0901 is among the oldest stars discovered to date. It has an estimated age of 13.2 billion years. The star is believed to have formed from the remnants of the earliest stars that went out as supernovae when the universe was still very young.

HE 1523-0901 has a mass of 0.80 solar masses and a radius of 27 solar radii. With an effective temperature of 4,742 K, it shines with 730 solar luminosities. In 2015, it was discovered to have a spectroscopic companion with a mass of at least 11 Jupiter masses.

υ Librae (Upsilon Librae)

Upsilon Librae is a binary star located around 224 light years away. It has an apparent magnitude of 3.628.

The primary component is an orange giant of the spectral type K3III. It has a mass of 1.67 solar masses and a radius 31.5 times that of the Sun. With a surface temperature of 4,135 K, it is 309 times more luminous than the Sun. The star has an estimated age of 3.14 billion years.

The companion is much fainter, shining at magnitude 10.8. It is separated by 2 arcseconds from the primary star.

In about 3.2 million years, Upsilon Librae will reach a peak brightness of -0.46 and be the brightest star in the Earth’s sky.

τ Librae (Tau Librae)

Tau Librae is a double star with an apparent magnitude of 3.68. It lies 367 light-years away. The star system is a spectroscopic binary star with an orbital period of only 3.3 days.

The primary component is a hot blue main sequence star of the spectral type B2.5 V. It has 7.25 times the Sun’s mass and a radius 3.2 times that of the Sun. It has a surface temperature of 17,990 K and a bolometric luminosity of 2,705 Suns. The star’s estimated age is 31.5 million years.

Tau Librae is a heartbeat star system, a pulsating variable binary star with a highly eccentric orbit. Heartbeat stars can come as close to each other as a few stellar radii and be as far apart as 100 times that distance as they orbit. At their closest approach, the tidal forces cause vibrations and rapid fluctuations in the stars’ shapes, which makes the brightness of these systems variable.

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

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

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

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

The constellation Libra does not have many bright deep sky objects that can be easily observed in binoculars and small telescopes. It does not host any star clusters, galaxies or nebulae listed in the Messier and Caldwell catalogues. However, it is home to an object listed in the Herschel 400 catalogue, the globular cluster NGC 5897.

NGC 5897

NGC 5897 is a relatively large globular cluster located 40,800 light-years away from the solar system and 24,100 light-years from the Galactic Centre. With an apparent magnitude of 8.52 and an apparent size of 12.6 arcminutes, it can be observed in amateur telescopes.

The cluster was discovered by the German-born British astronomer William Herschel on March 10, 1785.

NGC 5885

NGC 5885 is a barred spiral galaxy that appears near Zubeneschamali in the sky. It has an apparent magnitude of 11.8 and an apparent size of 3.5 by 3.1 arcminutes. The galaxy lies 105 million light-years away. It was discovered by William Herschel on May 9, 1784.

NGC 5792

NGC 5792 is a barred spiral galaxy located approximately 70.27 million light-years away. It has an apparent magnitude of 12.1 and an apparent size of 6.9 by 1.7 arcminutes. It was discovered by William Herschel on April 11, 1787.

The galaxy is a member of the Virgo III Cloud (the Virgo III Groups), a series of galaxy clusters and galaxies stretching east of the Virgo Supercluster.

NGC 5793

NGC 5793 is a type II Seyfert active spiral galaxy located 150 million light-years away. It appears almost edge-on. It has an apparent magnitude of 13.32 and an apparent size of 1.160 by 0.394 arcminutes.

The galaxy was discovered by the American astronomer Francis Leavenworth in 1886.

NGC 5728

NGC 5728 is a barred spiral galaxy with an apparent magnitude of 13.40, located 145.8 million light-years away. The galaxy is about 97,400 light-years across and has an apparent size of 203.3 arcseconds. It was discovered by William Herschel on May 7, 1787.

NGC 5728 has an active galactic nucleus and a supermassive black hole at its centre. It is classified as a type 2 Seyfert galaxy.

A supernova was discovered in the galaxy in February 2009. Designated SN 2009Y, it shone at magnitude 16 at discovery.

NGC 5917

NGC 5917 is a spiral galaxy located approximately 90.4 million light-years away. It has an apparent magnitude of 14.5 and an apparent size of 1.6 by 0.9 arcminutes.

The galaxy is interacting with the neighbouring galaxy PGC 54817, which appears 4.2 arcminutes away. The galaxies are connected by tidal tails.

A type II supernova was discovered in NGC 5917 in June 1990. Designated SN 1990Q, it peaked at magnitude 18.

NGC 5917 was discovered by the English astronomer John Herschel on July 16, 1835.

NGC 5890

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.

IRAS 14348-1447

IRAS 14348-1447 is a pair of merging galaxies located approximately 1 billion light-years away. The colliding pair is also catalogued as PGC 52270. It has an apparent magnitude of 16.58. The interaction between the galaxies has caused the formation of tidal tails of stars pulled from the galaxies.