Libra is a constellation of the zodiac visible generally from May through August in the Northern Hemisphere, and April through September in the Southern Hemisphere. The name Libra comes from the Latin word meaning “balance” or “scales” which is what the constellation traditionally represents. In fact, lb. – the abbreviation for pound – is an abbreviation of Libra. Though Libra is quite a faint constellation and has no first magnitude stars, it was one of the 48 constellations recognized by the astronomer Ptolemy. Libra is noticeably the only constellation of the zodiac which depicts an inanimate object.
How to Locate Libra
Libra is not a very prominent looking constellation, but it can still be found easily enough with a little star-hopping. It lies between the zodiac constellations Virgo, to the west, and Scorpius to the east. From the Northern Hemisphere, you can look to the Big Dipper in Ursa Major as a starting point. Then trace the arc of the handle down to the bright, orange-yellow star Arcturus in the constellation Boötes. From there, head down to Virgo where you can spot the brightest star Spica. Libra is right next to Virgo.
From the Southern Hemisphere, you can start at Antares, the orange-red star at the heart of Scorpius and head west to Spica. Libra is wedged right between the two and forms a quadrangle or kite-like shape. It is marked by its two brightest stars, the awesomely named Zubeneschamali and Zubenelgenubi.
Other constellations that Libra borders are Serpens to the north, Hydra to the southwest, a small section of Centaurus to the southwest, Lupus to the south, and Ophiuchus to the northeast.
Libra is visible from most locations on the planet since it is part of the zodiac which lies close to the celestial equator.
Libra Mythology and History
Libra is one of the most ancient constellations. The 1st Century Greek astronomer and mathematician Geminus of Rhodes mentions Libra in his book Introduction to the Phenomena. This beginner’s guide to astronomy described the zodiac, constellations, motion of the Sun and other important foundations of astronomy.
Libra has long been associated with law and fairness. The Babylonians knew the constellation as MUL Zibanu which translates to ”scales” or ”’balance”. They also associated Libra with the sun god Shamash who represented truth and justice.
The ancient Greeks saw Libra as the Scorpius” claws. Strangely enough, the Arabic word zubānā means ”scorpion’s claws”, and closely resembles the Babylonian ”Zibanu”. This could be why Libra became known as the scales over time.
To the Romans, the constellation Virgo embodied the star maiden Astraea; goddess of justice and keeper of the scales of justice. On the other hand, the ancient Egyptians saw the asterism formed by Libra’s three brightest stars as a boat constellation.
One aspect that does make Libra significant is that the constellation was the location of the autumnal equinox in ancient times, so the equinox was also known as the ”First Point of Libra.” This is one of two points where the ecliptic and the celestial equator intersect and marks times of the year where day and night are pretty much equal. It is not a stretch to believe that the depiction of scales may have alluded to the equal lengths of day and night at the equinox. Libra is no longer the location of the autumnal equinox due to the precession of the equinoxes.
Stars in The Constellation Libra
Libra does not have many bright stars, but there are some significant stars within the constellation and a few exoplanets. Libra’s brightest stars form a kite pattern outlining the constellation. The stars Alpha Librae and Beta Librae are generally considered to represent the balance beams of the scale and stars Gamma Librae and Sigma Librae make up the weighing pans.
- Zubeneschamali/ Beta Librae: Zubeneschamali gets its name from the Arabic meaning ”northern claw”, as there was a time when Libra was thought to be part of Scorpius. Another name for Beta Librae is Lanx Borealis, the Latin for ”northern scale”.
In spite of its ”Beta” designation, Zubeneschamali is the brightest star in the constellation Libra. The magnitude 2.61 dwarf star is 100 times more luminous than the Sun. It is also has a radius nearly 5 times bigger than the Sun. Variations in Zubeneschamali’s luminosity suggest that it may have a companion star.
- Zubenelgenubi/ Alpha Librae: Zubenelgenubi gets its name from the Arabic for ”southern claw”. An old Latin name for the star translates to ”southern scale”. Zubenelgenubi is actually the second brightest star in Libra, despite its designation as Alpha Librae. Zubenelgenubi is a multiple star system. The two brightest components of the system form a binary star system.
This multiple star system lies in close proximity to the ecliptic, which is the apparent path the Sun, Moon, and planets follow across the sky. This means that the system and can be occulted by the Moon, or more rarely by the planets. To highlight just how rare such occultations are, the next one will be in the year 2052 when Mercury will obscure the system from view.
- Methuselah Star/ HD 140283: HD 140283 was dubbed the Methuselah Star due to being one of the oldest known stars in the Universe, if not the oldest star. Astronomers believe HD 140283 was formed soon after the Big Bang. Estimates of the star’s age are not perfect. The minimum age calculated for HD 140283 is 13.66 billion years old, while other calculations reach an estimate of 14.46 billion years old. That’s older than the universe! However, the calculation does not necessarily conflict with the age of the universe because many of the variables in both the age of the star and the age of the universe remain uncertain.
HD 140283 is a sub-giant star and it is very poor in metal – it’s iron content is about only 1% that of the Sun. The star is almost entirely made up of hydrogen and helium. The Methuselah Star is certainly one of the closest metal-poor stars to the Sun. It lies only 190 light-years away and has an apparent magnitude just over 7. It is quite an easy stellar target using both binoculars and small telescopes.
- Zubenelakrab/ Gamma Librae: Gamma Librae is an orange giant star lying over 150 light-years from the Solar System. The star has an apparent magnitude of 3.91 and is over 70 times more luminous than the Sun. The name Zubenelakrab derives from the Arabic meaning ”shears of the scorpion”.
- Zuben Elakribi/ Delta Librae: This is a main sequence star lying roughly 300 light-years from Earth. It has a magnitude of 4.43. Delta Librae is an eclipsing variable star. It can dip to magnitude 5.81. Zuben Elakribi comes from the Arabic meaning ”claws of the scorpion”.
- Theta Librae: Theta Librae is another orange giant within Libra. It is more massive than the Sun and over 30 times more luminous too. The star has a magnitude slightly over 4 and lies some 163 light-years from Earth.
- Iota Librae: Iota Librae actually refers to two star systems in the constellation. One of the systems consists of a sub-giant star and a dwarf star about 377 light-years from Earth.
- Brachium/ Sigma Librae: Sigma Librae is a red giant. It has an apparent magnitude of 3.29 and is a semi-regular variable star with a very short period of 20 days. It also varies ever so slightly in magnitude over even shorter periods about every 3 hours. The star lies only 288 light-years from the Solar System. Its name comes from the Latin word for ”arm”. Sigma Librae was designated Gamma Scorpii before receiving its current and official designation in Libra.
- 23 Librae: This yellow dwarf lies about 85 light-years from the Sun and has two confirmed exoplanets in its orbit. The first one was found in 1999, and the other planet was discovered 10 years later. 23 Librae has an apparent magnitude of 6.45. It is older than the sun, with some estimates as high as 11 billion years old.
- 48 Librae/ FX Librae: 48 Librae is an interesting star. It is a blue supergiant with an extremely rapid rotational velocity – possibly as high as 400 km/s. These high rotation speeds lead to ejected gases which form a disk around the star. It is therefore known as a shell star. 48 Librae lies over 500 light-years from Earth and is almost 1,000 times more luminous than the Sun.
- Gliese 570/ 33 G. Librae: Gliese 570 lies only 19 light-years from the Solar System, making it the nearest star to the Solar System with an exoplanet. Gliese 570 is part of a system of at least three stars. The primary star is an orange dwarf and an X-ray source, though not the only one in the system. Gliese 570 also contains a binary star system comprising of red dwarfs emitting X-rays.
- Gliese 581/ HO Librae: Gliese 581, or HO Librae, is a red dwarf star about 20 light-years from the Earth. It is a variable star of magnitude 10.56 to magnitude 10.58. The star is home to its own planetary system containing at least three exoplanets and possibly as many as six.
- HD 141937: HD 141937 is also a yellow dwarf with a known exoplanet; a massive gas giant. HD 141937 has a similar mass to the Sun but a slightly larger radius. The star lies about 109 light-years from Earth.
Planetary systems
The star Gliese 581 has its own planetary system. Three of the planets are confirmed planets while a further two possible planets remain unconfirmed. Exo-planets are always of interest to astronomers and become even more important when it is possible that they could harbor life.
The planets Gliese 581d and Gliese 581g are both thought to be possible candidates for hosting life. Whether Gliese 581g is actually a planet orbiting the star is still up for debate, with not enough evidence to prove or disregard the planet’s existence yet. Gliese 581g is thought to lie near the middle of the habitable zone and is believed to be a tidally locked planet. Together with its possible location in the habitable zone, this could be quite an important discovery since recent studies have shown that tidally locked planets could still support life.
Then there is Gliese 581c. It is considered the first Earth-like exoplanet found within its parent star’s habitable zone. Another one of the star’s planets is Gliese 581e. This planet is one of the exoplanets with the least mass which astronomers have found to date. In spite of the many debates that may revolve around these planets, all of them are very important in the study of possible life outside the Solar System.
Deep Sky Objects
Libra is home to a few galaxies. There are no Messier Objects in the constellation.
- NGC 5793: NGC 5793 is a spiral galaxy lying close to Zubenelgenubi as seen from Earth. The galaxy is roughly 150 Million Light Years away.
- NGC 5885: This barred spiral galaxy was one of the many discoveries made by William Herschel. NGC 5885 has an apparent magnitude of 11.8 making it a good target for small to medium telescopes
- NGC 5890: the American astronomer Ormond Stone discovered NGC in 1785. It is a lenticular galaxy with a magnitude of 14.
- NGC 5897: This is a fairly large and bright globular cluster found about 24,000 light-years from the Sun. The cluster can easily be seen through small telescopes.
Meteor Showers Radiating from Libra
There are no major meteor showers of note radiating from Libra, but there are some minor showers. The May Librids meteor shower has a very short period. It occurs from approximately May 1st to May 9th, generally peaking on the 6th. A maximum of 2 to 6 meteors can be seen per hour.