Libra the Scales

Libra the Scales
Lying between Virgo and Scorpius in the zodiac, Libra is an ancient constellation. The stars now represent a weighing scale, as they did four thousand years ago in Babylon. But how did its brightest stars come to have names related to a scorpion's claws?

Libra is the only zodiac constellation that represents an inanimate object. In Babylonian astronomy, Libra was MUL Zibanu – the scales or balance. Representing justice and truth, it was sacred to Shamash, the Sun god. But later civilizations had different views. Ancient Egyptians saw the three brightest stars of Libra as a boat. And to the ancient Greeks, the stars of Libra were the claws of Scorpius. In the Almagest, Ptolemy's 2nd century astronomy work, these stars were listed as an asterism called the Claws. (An asterism is a pattern of stars that's part of one or more constellations.)

During Roman times the stars came to be separated from Scorpius, and represented the scales of justice. Here are the main stars of Libra superimposed on an image of scales. The Romans linked the scales with Astraea, the virgin goddess of justice. Disgusted by the wickedness of humanity in the Age of Iron, Astraea ascended to the sky as Virgo.

Libra's main stars have names derived from Arabic that reflect the tradition of the scorpion's claws. The two brightest stars are Zubeneschamali (“Northern Claw”) and Zubenelgenubi (“Southern claw”). Their Bayer designations are Beta Librae and Alpha Librae, respectively.

Even the constellation's brightest star Beta Librae doesn't look exceptionally bright. Yet Beta Librae is some 130 times brighter than the Sun, and with twice the Sun's surface temperature. Although luminous, it's about 185 light years away. Beta Librae has the distinction of being the only known green star. Strictly speaking, green stars don't exist, but in practice Beta Librae is often perceived as green. No one knows why.

Alpha Librae, 75 light years distant, is classed as a binary star. Alpha2 Librae is the primary star, and its official name is Zubenelgenubi. The secondary star Alpha1 Librae is unnamed. Interestingly, both stars are spectroscopic binaries, i.e., each has a close companion detected from the stellar spectrum. So, in fact, Alpha Librae is composed of four stars – it's a quaternary system.

Gamma Librae, traditionally Zubenelakrab (the shears of the scorpion), is an orange giant 70 times more luminous than the Sun and 150 light-years from Earth.

Although not one of the main stars of Libra, HD 140283 deserves a mention.
Popularly known as Methuselah, it's the oldest known star in the Universe, believed to have formed shortly after the Big Bang.

Planetary systems
As of October 2018, there were three stars in Libra with known planetary systems.

HD 141937 is a star of the same type and mass as our Sun, but slightly larger. It doesn't seem to have a planet family, just the gas giant HD 141937b which has some ten times the mass of Jupiter. It may actually be a brown dwarf, a failed star.

A second sunlike star is 23 Librae, 85 light years away. It's slightly more massive than the Sun, and at least 3 billion years older. Two confirmed planets are both gas giants. 23 Librae b orbits at a distance that would put it between Venus and the Earth in the Solar System. We don't know how far 23 Librae c is from its star, but it seems to be somewhat farther out than Jupiter is from the Sun. It takes so long to orbit – probably from 12.5 to 15 years – astronomers couldn't observe an entire orbit.

But the most interesting system is Gliese 581 with its family of planets, three confirmed and two still unconfirmed. The star itself is a red dwarf about twenty light years away from us. It's much smaller than the Sun and its planetary family is much closer to it. Gliese 581's confirmed planets are closer to the star than Mercury is to the Sun.

Gliese 581 is an old star showing little stellar activity. This means the planets have a good chance of keeping an atmosphere, and it reduces the chances of stellar flares that would endanger life. The main focus is on Gliese 581c which seems to be a super-earth, i.e., more massive than Earth, but probably still rocky. It's also in the habitable zone. That's the region around a star in which water might exist as a liquid on the surface – also known as the Goldilocks zone, neither too hot nor too cold.

Since Gliese 581 has a huge comet belt, there is a good chance that the planets have water. If Gliese 581d and Gliese 581g do exist, they're well placed to have liquid water on the surface.

Deep-sky objects
Libra has some deep-sky objects, but nothing dramatic. William Herschel, 18th-century astronomer and discoverer of Uranus, found a few of them. He discovered the globular cluster now catalogued as NGC 5897, which is over 170 light years in diameter and some 50,000 light years from Earth.

Globular clusters are mostly found in the halo of our Galaxy. Not only are their stars some of the Galaxy's oldest, but there are so many of them that their mutual gravity pulls them into a globe shape.

Herschel also discovered two barred spiral galaxies: NGC 5885, around 95 million light years away, and NGC 5792 about 83 million light years distant.

You Should Also Read:
Virgo the Maiden
William Herschel
Searching for Exoplanets

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