Libra is a constellation of the zodiac. Its name is Latin for weighing scales. It is fairly faint, with no first magnitude stars, and 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. In the equatorial coordinate system, the right ascension coordinates of the constellation 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.
In astrology, Libra is the seventh astrological sign in the Zodiac. It spans the 180-210th degree of the zodiac, between 180 and 207.25 degree of celestial longitude. Under the tropical zodiac, Sun transits this area on average between (northern autumnal equinox) September 23 and October 22, and under the sidereal zodiac, the sun currently transits the constellation of Libra from approximately October 16 to November 17. The symbol of the scales is based on the Scales of Justice held by Themis, the Greek personification of divine law and custom. She became the inspiration for modern depictions of Lady Justice. The ruling planet of Libra is Venus. Libra is the only constellation in the sky represented by an inanimate object. The other eleven signs are represented either as an animal or mythological characters throughout history.
[https://en.wikipedia.org/wiki/Libra_%28astrology%29]
Libra as depicted in Urania’s Mirror, a set of constellation cards published in London c.1825
Libra was known in Babylonian astronomy as MUL Zibanu (the ‘scales’ or ‘balance’), or alternatively as the Claws of the Scorpion. The scales were held sacred to the sun god Shamash, who was also the patron of truth and justice. It was also seen as the Scorpion’s Claws in ancient Greece. Since these times, Libra has been associated with law, fairness and civility. In Arabic ‘zubānā’ means ‘scorpion’s claws,’ and likely similarly in other Semitic languages: this resemblance of words may be why the Scorpion’s claws became the Scales. It has also been suggested that the scales are an allusion to the fact that when the sun entered this part of the ecliptic at the autumnal equinox, the days and nights are equal. Libra’s status as the location of the equinox earned the equinox the name ‘First Point of Libra,’ though this location ceased to coincide with the constellation in 730 because of the precession of the equinoxes.
Libra is a constellation not mentioned by Eudoxus or Aratus. It is mentioned by Manetho (3rd century B.C.) and Geminus (1st century B.C.), and included by Ptolemy in his 48 asterisms. Ptolemy catalogued 17 stars, Tycho Brahe 10, and Johannes Hevelius 20. It only became a constellation in ancient Rome, when it began to represent the scales held by Astraea, the goddess of justice, associated with Virgo.
[https://astronomommy.files.wordpress.com/2012/06/june-2012-looking-south-lines.png]
[http://www.constellation-guide.com/constellation-list/libra-constellation/]
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 for the unaided observer. Traditionally, Alpha and Beta Librae are considered to represent the scales’ balance beam, while Gamma and Sigma are the weighing pans.
Zubeneschamali looks blue in this photo, but stargazers call it green
[http://earthsky.org/brightest-stars/libras-zubeneschamali-the-only-green-star]
Beta Librae, also named Zubeneschamali, is (despite its ‘beta’ designation) the brightest star in the constellation of Libra. The traditional name derived from the Arabic ‘al-zubānā al-šamāliyy,’ meaning ‘the northern claw.’ Its distance is 185 light-years (57 parsecs) from the Sun.
The apparent visual magnitude of this star is 2.6. According to Eratosthenes Beta Librae was observed to be brighter than Antares. Ptolemy, 350 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.
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 12300 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 Earth 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 Beta Librae suggest the presence of a companion star which is not directly observable from Earth. However, it is categorized as a single star.
[https://en.wikipedia.org/wiki/Beta_Librae]
Alpha Librae is a double star and despite its ‘alpha’ designation the second-brightest star in the constellation of Libra. The system bore the traditional name of Zubenelgenubi, derived from the Arabic ‘al-zubānā al-janūbiyy,’ ‘the southern claw,’ which was coined before Libra was recognized as a constellation distinct from Scorpius.
Alpha Librae is about 77 light-years (24 parsecs) from the Sun. The two brightest components of Alpha Librae form a double star moving together through space as common proper motion companions. The brightest member, α² Librae, is itself a spectroscopic binary system. The second member, α¹ Librae, is separated from the primary system by around 5400 AU. It too is a spectroscopic binary with an orbital period of 5,870 days and an angular separation of 0.383 arcseconds; equal to about 10 AU. The system may have a fifth component, the star KU Librae at a separation of 2.6°, thus forming a hierarchical quintuple star system. KU Lib shares a similar motion through space to the Alpha Librae system, but is separated from the other stars by about a parsec. It is sufficiently close to be gravitationally bound to the other members, but has a substantially different metallicity.
The two brightest members of Alpha Librae are separated in the sky by an angular distance of 231" (3'51"). The position angle of the companion is 314 degrees. The brighter of the two is a white star of spectral type A3, with an apparent brightness of 2.8. Its companion is a type F4 star of apparent brightness 5.2. They are probably members of the Castor Moving Group of stars that have a similar motion through space and share a common origin some 200 million years ago.
Alpha² Librae is 0.33 degree from the ecliptic so it can be easily occulted by the Moon and (very rarely) by planets. The next occultation by a planet will be by Mercury on 10 November 2052.
[https://en.wikipedia.org/wiki/Alpha_Librae]
Sigma Librae is a third magnitude star, visible to the naked eye, with an apparent visual magnitude +3.29. It is at a distance of roughly 288 light-years (88 parsecs) from Earth. It has the Latin names Brachium ‘arm’ and Cornu ‘horn.’
Sigma Librae has a spectral class M3/M4 III, which places it in the red giant stage of its evolution. This is a semi-regular variable star with a single pulsation period of 20 days. It shows small amplitude variations in magnitude of 0.10–0.15 on time scales as brief as 15–20 minutes, with cycles of repetition over intervals of 2.5–3.0 hours. This form of variability indicates that the star is on the asymptotic giant branch and is generating energy through the nuclear fusion of hydrogen and helium along concentric shells surrounding an inert core of carbon and oxygen.
[https://en.wikipedia.org/wiki/Sigma_Librae]
Gamma Librae has the traditional name Zuben-el-Akrab, a modification of the Arabic ‘Zuban al Aqrab.’ ‘the claws of the scorpion,’ a name that dates to before Libra was a distinct constellation from Scorpius.
It belongs to the spectral class K0III, and is of the fourth-magnitude, having apparent magnitude +3.91. It is approximately 152 light-years (47 parsecs) from the Earth. This star has 2.15 times the mass of the Sun and is radiating around 71 times the luminosity of the Sun at an effective temperature of 4,822 K.
[https://en.wikipedia.org/wiki/Gamma_Librae]
Size of the Sun (left) and Gliese 581 (right)
Gliese 581 is a star of spectral type M3V (a red dwarf) at the center of the Gliese 581 planetary system, about 20 light years away from Earth in the Libra constellation. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star to the Sun.
An M-class dwarf star such as Gliese 581 has a much lower mass than the Sun, causing the core region of the star to fuse hydrogen at a significantly lower rate. From the apparent magnitude and distance, astronomers have estimated an effective temperature of 3200 K and a visual luminosity of 0.2 percent of that of the Sun. However, a red dwarf such as Gliese 581 radiates primarily in the near infrared, with peak emission at a wavelength of roughly 830 nm, so such an estimate will underestimate the star’s total luminosity. (For comparison, the peak emission of the Sun is roughly 530 nm, in the middle of the visible part of the spectrum). When radiation over the entire spectrum is taken into account (not just the part that humans are able to see), something known as the bolometric correction, this star has a bolometric luminosity 1.3% of the Sun’s total luminosity. A planet would need to be situated much closer to this star in order to receive a comparable amount of energy as the Earth. The region of space around a star where a planet would receive roughly the same energy as the Earth is sometimes termed the "Goldilocks Zone", or, more prosaically, the habitable zone. The extent of such a zone is not fixed and is highly specific for each planetary system.
[https://en.wikipedia.org/wiki/Gliese_581]
The orbits of the Gliese 581 planetary system, as per the 2009 analysis excluding planets g and f.
In the picture, Gliese 581c is the third planet from the star.
The Gliese 581 planetary system is known to consist of at least three planets (possibly up to five) along with a debris disc discovered using the radial velocity method. The confirmed planets are believed to be located close to the star with near-circular orbits. In order of distance from the star, these are Gliese 581e, Gliese 581b, and Gliese 581c. The letters represent the discovery order, with b being the first planet to be discovered around the star.
Gliese 581e is the innermost planet and, with a minimum mass of 1.7 Earth masses, is the least massive of the three. Discovered in 2009, it is also the most recent confirmed planet to have been discovered in this system. It takes 3.15 days to complete an orbit. Initial analyses suggested that the planet’s orbit is quite elliptical but after correcting the radial velocity measurements for stellar activity, the data now indicate a circular orbit.
Gliese 581b is the most massive planet known to be orbiting Gliese 581 and was the first to be discovered.
Gliese 581c is the third planet orbiting Gliese 581. It was discovered in April 2007 by Udry et al., who asserted that if Gliese 581c has an Earth-type composition, it would have a radius of 1.5R⊕, which would have made it at the time the most Earth-like of all known exoplanets.’ The planet initially attracted attention as being potentially habitable, though this has since been discounted.
The minimum mass of the planet is 5.5 times that of Earth. It completes a full orbit in just under 13 days.
The habitable zone of Gliese 581 compared with the Solar System’s habitable zone, showing Gliese 581g near the center.
[https://en.wikipedia.org/wiki/Gliese_581g]
Gliese 581g, unofficially known as Zarmina (or Zarmina’s World), is an unconfirmed (and disputed) exoplanet claimed to orbit within the Gliese 581 planetary system, twenty light-years from Earth. It was discovered by the Lick–Carnegie Exoplanet Survey, and is the sixth planet orbiting the star; however, its existence could not be confirmed by the European Southern Observatory (ESO) / High Accuracy Radial Velocity Planet Searcher (HARPS) survey team, and its existence remains controversial. It is thought to be near the middle of the habitable zone of its star. That means it could sustain liquid water- a necessity for all known life- on its surface, if there are favorable atmospheric conditions on the planet.
The planet is thought to be tidally locked to its star. If the planet has a dense atmosphere, it may be able to circulate heat. The actual habitability of the planet depends on the composition of its surface and the atmosphere. It is thought to have temperatures around −37 to −11 °C (−35 to 10 °F). By comparison, Earth has an average surface temperature of 15 °C (59 °F), while Mars has an average surface temperatures of about −63 °C (−81 °F).
Gliese 581d is an exoplanet that was once considered disputed due to inaccurate analysis caused by noise and stellar activity, but reanalysis suggests that it does in fact exist, despite stellar variability. Its mass is thought to be 6.98 Earths and its radius is thought to be 2.2R⊕. It is considered to be a super-Earth, but remarkable in that its orbit is inside the habitable zone and has a solid surface allowing for any water present on its surface to form liquid oceans and even landmasses characteristic of Earth’s surface, although with a much higher surface gravity. Its orbital period is thought to be 66.87 days long, with a semi-major axis of 0.21847, with an unconfirmed eccentricity. Analysis suggests that it orbits within the star’s habitable zone, where the temperatures are just right to support life.
The Gliese 581 system has been the target of both SETI and Active SETI searches for extraterrestrial life. A Message from Earth (AMFE) is a high-powered digital radio signal that was sent on October 9, 2008, toward Gliese 581c. The signal is a digital time capsule containing 501 messages that were selected through a competition on the social networking site Bebo. The message was sent using the Yevpatoria RT-70 radio telescope radar telescope of the National Space Agency of Ukraine. The signal will reach Gliese 581 in early 2029.
Using optical SETI, Ragbir Bhathal claimed to have detected an unexplained pulse of light from the direction of the Gliese 581 system in 2008.
In 2012, the International Centre for Radio Astronomy Research at Curtin University in Perth, Gliese 581 was precisely targeted by Australian Long Baseline Array using three radio telescope facilities across Australia and the Very Long Baseline Interferometry technique, however no candidate signals were found.
[https://en.wikipedia.org/wiki/Gliese_581_planetary_system]
The ternary star system Gliese 570. The T-type methane brown dwarf Gliese 570D is indicated with an arrow.
Gliese 570, also known as 33 G. Librae, is a ternary star system approximately 19 light-years away. The primary star is an orange dwarf star (much dimmer and smaller than the Sun). The other secondary stars are themselves a binary system, two red dwarfs that orbit the primary star. A brown dwarf has been confirmed to be orbiting in the system. In 1998, an extrasolar planet was thought to orbit the primary star, but it was discounted in 2000.
In the night sky, the Gliese 570 system lies in the southwestern part of Libra. The system is southwest of Alpha Librae and northwest of Sigma Librae. In the early 1990s, the European Hipparcos mission measured the parallax of components B and C, suggesting that the system was at a distance of 24.4 light-years from the Sun. This, however, was a relatively large error as Earth-based parallax and orbit observations suggest that the two stars are actually part of a system with Gliese 570 A, and must actually lie at the same distance.
An artist’s impression of Gliese 570 D showing the primary stars
The primary star of the system (component A) is an orange dwarf star that may just have over three fourths the mass of the Sun, about 77 percent of its radius, and only 15.6 percent of its visual luminosity. It has a separation of 190 astronomical units from the binary components B and C, moving in an eccentric orbit that takes at least 2130 years to complete. Gliese 570 A is spectral type K4V and emits X-rays.
A binary system in their own right, components B and C are both rather dim red dwarf stars that have less mass, radius, and luminosity than the Sun. Component B is spectral type M1V, component C is spectral type M3V, and both emit X-rays.
On January 15, 2000, astronomers announced that they had found one of the coolest brown dwarfs then known. Catalogued as Gliese 570 D, it was observed at a wide separation of more than 1,500 astronomical unit from the triple star system. It has an estimated mass of 50 times that of Jupiter.
The status of Gliese 570 D as a brown dwarf was confirmed by Doppler spectroscopy at the Cerro Tololo Interamerican Observatory in Chile. The surface temperature of this substellar object was found to be a relatively cool 500 degrees Celsius, making it cooler and less luminous than any other known brown dwarf (including the prototype ‘T’ dwarf), and classifying the object as a T7-8V brown dwarf. No X-rays have been reported from this brown dwarf.
In 1998, an extrasolar planet was announced to orbit the primary star within the Gliese 570 system. The planet, identified as ‘Gliese 570 Ab,’ was considered doubtful and the claim was retracted in 2000. No extrasolar planets have been confirmed to exist in this multiple star system thus far.
[https://en.wikipedia.org/wiki/Gliese_570]
HD 141569 and its two companions taken by HST in 2002
HD 141569 is a B-type main-sequence star approximately 320 light-years away in the constellation of Libra. The primary star has two red dwarf companions (orbiting each other) at about nine arcseconds.
In January 1999, NASA announced a protoplanetary disk around HD 141569. The Hubble Space Telescope showed that the disk appears to come in two parts (inner and outer). It superficially resembles the largest gap in Saturn’s rings.
The vast disk is 75 billion miles across (13 times the diameter of Neptune’s orbit). The inner edge of the gap is 21 billion miles from the star. The relatively narrow gap lies approximately halfway between the inner and outer edges of the disk. Though already a fully formed star, HD 141569 is relatively young, probably only 1% through its lifetime as a stable star. The star is nearly three times more massive and 22 times brighter than the Sun.
Dust disks surrounding newly forming stars are common, but only a small number of adult stars are known to have disks; of these, only a handful have been imaged. Astronomers believe these disks must form and/or be replenished when older rocks and debris collide and break up into small particles.
The gap in the disk leads to the conclusion of a protoplanet in formation around the star. The planet does not have to be in the gap, however. It could either be sweeping up the dust and rocks from the disk as it travels in its orbit around the star, or the gravity of the planet could knock the dust out of one part of the disk.
[https://en.wikipedia.org/wiki/HD_141569]
HD 140283, informally nicknamed Methuselah star, is a metal-poor subgiant star about 190 light years away from the Earth in the constellation Libra. Its apparent magnitude is 7.223. The star has been known to astronomers for over a century as a high-velocity star, and an early spectroscopic analysis found it to have a substantially lower metal content than the Sun. Modern spectroscopic analyses find an iron content about a factor of 250 lower than that of the Sun. It is one of the closest Population II stars to us:
[https://en.wikipedia.org/wiki/HD_140283]
Digitized Sky Survey image of HD 140283, the oldest star with a well-determined age in our galaxy
A team of astronomers using NASA’s Hubble Space Telescope has taken an important step closer to finding the birth certificate of a star that’s been around for a very long time. The star could be as old as 14.5 billion years.
This ‘Methuselah star,’ cataloged as HD 140283, has been known about for more than a century because of its fast motion across the sky. The high rate of motion is evidence that the star is simply a visitor to our stellar neighborhood. Its orbit carries it down through the plane of our galaxy from the ancient halo of stars that encircle the Milky Way, and will eventually slingshot back to the galactic halo.
This conclusion was bolstered by the 1950s astronomers who were able to measure a deficiency of heavier elements in the star as compared to other stars in our galactic neighborhood. The halo stars are among the first inhabitants of our galaxy and collectively represent an older population from the stars, like our sun, that formed later in the disk. This means that the star formed at a very early time before the universe was largely ‘polluted’ with heavier elements forged inside stars through nucleosynthesis. (The Methuselah star has an anemic 1/250th as much of the heavy element content of our sun and other stars in our solar neighborhood.)
HD 140283 has seen many changes over its long life. It was likely born in a primeval dwarf galaxy. The dwarf galaxy eventually was gravitationally shredded and sucked in by the emerging Milky Way over 12 billion years ago.
The star retains its elongated orbit from that cannibalism event. Therefore, it’s just passing through the solar neighborhood at a rocket-like speed of 800,000 miles per hour. It takes just 1,500 years to traverse a piece of sky with the angular width of the full Moon. The star’s proper motion angular rate is so fast (0.13 milliarcseconds an hour) that Hubble could actually photograph its movement in literally a few hours.
The star, which is at the very first stages of expanding into a red giant, can be seen with binoculars as a 7th-magnitude object in the constellation Libra. Hubble’s observational prowess was used to refine the distance to the star, which comes out to be 190.1 light-years.
[http://www.nasa.gov/mission_pages/hubble/science/hd140283.html]
Artist’s Impression of the oldest star in the Milky Way galaxy
HE 1523-0901 is the designation given to a red giant star in the Milky Way galaxy approximately 7500 light years from Earth. It is thought to be a second generation, Population II, or metal-poor, star ([Fe/H] = −2.95). The star was found in the sample of bright metal-poor halo stars from the Hamburg/ESO Survey.
The star’s age, as measured by ESO’s Very Large Telescope, is 13.2 billion years. This makes it among the oldest stars and nearly as old as the estimated age of the universe itself (13.8 billion years). The measurement uncertainty in the age estimate is 0.7 to 2.7 billion years, depending upon the assumptions made to estimate the uncertainty, although the uncertainty in the relative age of this and other stars using the same method is smaller.
HE 1523-0901 is the first star whose age was determined using the decay of the radioactive elements uranium and thorium in tandem with measurements of several neutron capture elements. It is believed to have formed directly from the remnants of the first-generation stars that reached the end of their longevity and exploded as supernovae early in the history of known matter.
HE 1523-0901 is approximately 0.8 Solar masses. It can be viewed particularly well from the southern hemisphere with the use of a small telescope. It can also be observed from central European latitudes.
[https://en.wikipedia.org/wiki/HE_1523-0901]
Deep-sky objects in Libra include:
The globular cluster NGC 5897
NGC 5897 is a globular cluster in the constellation Libra. This satellite of the Milky Way, which is quite remote (located about 24,000 light years away), has a diameter of over 170 light years. With its Shapley–Sawyer Concentration Class of XI, it has very low star density even in its center.
The stars of the cluster have only one per cent of the sun’s metallicity, which means that they were formed in a time before the galaxy formed a disk and spiral arms.
NGC 5897 has a diameter of 12.6 arcminutes and an apparent magnitude of 8.5.
[https://en.wikipedia.org/wiki/NGC_5897]
NGC 5792 is a barred spiral galaxy about 83 million light-years away in the constellation Libra. There is a magnitude 9.6 star on the northwestern edge of the galaxy:
NGC 5792: This picture takes its charm from the combination of the bright foreground star (magV 9.5, spectral type M0), the galaxy itself and those hundreds of dim background galaxies, the number of which exceeds the number of foreground stars by far.
[http://www.capella-observatory.com/ImageHTMLs/Galaxies/NGC5792.htm]
Zubenelgenubi and Friends
Moderately bright Zubenelgenubi is the star just off the upper right hand limb of an eclipsed Moon in this telescopic view from Port Elizabeth, South Africa. Actually the second brightest star in the constellation Libra, Zubenelgenubi is fun to pronounce (try zoo-BEN-al-je-NEW-bee...) and rewarding to spot in the night sky as it has a fainter companion star, seen here on the far right. Astronomer Francois du Toit reports that both stars were visible to the unaided eye on the night of May 4th (2004), during the Moon’s total eclipse phase. Orbiting a common center of gravity once every 200,000 years or so, the two stars are both larger and hotter than the Sun. About 77 light years away they are separated from each other by over 730 light hours - about 140 times Pluto’s average distance from the Sun. Zubenelgenubi was once considered the southern claw of the nearby arachnologically correct constellation Scorpius. What star was the northern claw? Zubeneschamali, of course.
[https://apod.nasa.gov/apod/ap040514.html]
[https://en.wikipedia.org/wiki/Libra_%28constellation%29]
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