Star Constellation Facts: Libra, the Scales

Star Constellation Facts: Libra, the Scales
Alexander Jamieson’s Celestial Atlas (1822)

Libra is a faint constellation that is the 29th biggest in the night sky, and the only zodiac sign not to represent a living creature. Instead, Libra is depicted in the heavens as “weighing scales” placed next to the hand of the neighbouring constellation Virgo, a goddess of justice who had scales as an emblem of her office. The measuring symbol for pounds (“lb”) is an abbreviation of the word Libra.

Mythology: Represents Weighing Scales

The constellation’s association with scales is thought to have originated in ancient Babylonia in about the year 2000 BC, when the Sun’s position in front of the constellation’s stars marked the start of the autumnal equinox, during which time the day and night, being weighed, would be found equal. However, with precession over time the Sun has now moved south of the ecliptic into Virgo, which currently marks the autumn equinox.

Location: A Southern Constellation

Libra is a southern sky constellation that can be observed from latitudes +65° and -90°. The constellation’s most luminous stars form a quadrangular pattern that can be found between Virgo to the north-east, and Scorpius to the south-west, by using their brightest stars, Spica and Antares, as guides. Other neighbouring constellations include Centaurus, Hydra, Ophiuchus, Serpens Caput, and Lupus.

Best Seen: Spring and Summer

In the northern hemisphere, Libra is best seen in spring and summer, and in the Southern hemisphere from autumn to winter.

Notable Stars: 2nd Magnitude or Fainter

Libra takes up a 538 sq/deg area of the sky, with the constellation’s “scale” shape formed by the stars Alpha and Beta Librae that form the “cross-beam”, and the stars Gamma and Sigma Librae that represent the “weighing pans.”

Libra Virgo Scorpius

Zubeneschamali (Beta Librae), the most luminous star in Libra, is located about 185 light years away, and shines with an apparent magnitude of 2.61. The star is a blue-white dwarf around 5 times as big as the Sun, and at least 130 times as luminous. It is also a fast rotator, with a rotational velocity of about 250 km/sec at its equator. Beta Librae is thought to be binary star, since small, periodic variations in brightness have been observed, but to date, no companion has been identified.

– Zubenelgenubi (Alpha Librae), the second most luminous star in Libra, is a binary system found 75 light years, and shining with a magnitude of 5.153. Alpha Librae forms the primary pair of the system, and both stars are thought to be members of the Castor Moving Group of stars, since they share a common proper motion, which implies a common origin about 200 million years ago.

Contains Oldest Known Star

Methuselah (HD 140283) is located 190 light years distant, and is a seventh magnitude star that is currently in its pre-red giant phase. It is also the oldest known star, with an estimated age of about 14.46 billion years, give or take 800 million years or so. At first glance, the star’s age seems to contradict the age of the Universe, which is estimated to be about 13.77 billion years old, but the huge uncertainties in the ages of both the star and the Universe means that there is no real conflict. Methuselah consists almost entirely of hydrogen and helium, with only trace amounts of heavy atoms, which means that the star is among the oldest surviving second-generation stars.

Notable Objects: Few Celestial Objects

Libra contains no Messier objects, or any other notable deep sky objects for that matter, apart from the 12th magnitude barred spiral galaxy, NGC 5792 loctaed about 83 million years away; a lenticular galaxy (NGC 5890); and the 9th magnitude NGC 5897, a relatively large globular cluster about 40,000 from Earth.

Planets: 3 Stars with Planets

Of the three stars in Libra with confirmed planets, one of the most interesting is Gliese 581, a red dwarf star some 20 light years away. It has more than four confirmed planets, including Gliese 581, which is located in the star’s habitable zone, and is now considered one of the most promising planet yet discovered for extra-solar life or human habitability.

Meteor Showers: May Librids

The only meteor shower associated with Libra, the May Librids, has a very short duration period that runs from May 1st to 9th, with its peak on the 6th, during which time a maximum of 2 to 6 meteors per hour can be seen.

Astrology: Sept 23 to Oct 23

Date of Birth: Sept 23 to Oct 23
Sign Ruler: Venus
Element: Air
Birth Stone: Coral, Amber
Metal: Quicksilver
Color: Brown
Characteristics: Idealistic, fair-minded, strong social skills, charming, artistic
Compatibility: Aquarius, Gemini and Libra

Once Part of Scorpius Constellation

In ancient Greece, Libra was considered to be part of the constellation Scorpius, and represented the creature’s claws. During the reign of Julius Caesar, however, the constellation Libra was created but its two brightest stars still retain their original names; Zubeneschamali (The Northern Claw), and Zubenelgenubi (The Southern Claw).

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