Deep-Sky Objects in Libra

NGC 5897
Image Credit: NGC 5897 in Virgo by San Esteban

Libra is the 29th biggest of the 88 recognized constellations, and the 7th largest of the zodiacal constellations, taking up an area of 538 square degrees of the southern sky. It is also the only constellation of the zodiac that is named after an inanimate object, in this case a pair of scales, and not an animal or a character from mythology. The brightest star in Libra is Zubeneschamali (Beta Librae), a +2.61 magnitude B-type star that is located about 185 light years away.

Libra is visible from between latitudes +65° and -90°, although best viewed in the northern hemisphere at about 9 PM Local Time during the month of June, when it is highest in the sky. However, Libra is not among the most flamboyant or conspicuous constellations, and finding it in light-polluted skies can be a challenge. Nonetheless, the constellation can be found between the bright stars Spica in Virgo and Antares in Scorpius.

Apart from one meteor shower, the May Librids, that has its radiant in Libra, the constellation has no Messier objects and only a few noteworthy deep sky objects, the most notable of which are briefly explored in this list.

Star Cluster

NGC 5897

The 8.5 magnitude globular cluster seen at the top of the page is located about 24,000 light years away, and has in impressive 170 light-year diameter. Unlike most other globular clusters, NGC 5897 has a mass concentration of XI, which means that the stars in the cluster are evenly distributed, and have not undergone a process of core collapse that subsequently collects most of the cluster’s mass around the core. All the stars in the cluster are also very old, with metal contents that are on average less than one percent that of the Sun, meaning that the cluster’s stars all formed before the Milky Way galaxy did. Look for this cluster in Libra at coordinates RA: 15h 17m 24.40s and Dec.: -21° 00′ 36.4″. In good seeing conditions, the cluster will show as a fuzzy ball of light about 6.3′ x 6.3′ in diameter.

Galaxies

NGC 5792

NGC 5792The only other noteworthy deep sky object in Libra is this magnitude 12.1, barred spiral galaxy that is located about 83 million light years away. Note thought that NGC 5792 has a very low surface brightness, and it is therefore not an easy target for small to medium telescopes. With large instruments, however, it might be possible to spot this galaxy as a faint smudge of light around 6.9’ × 1.7’ in size.

Other galaxies in Libra includes NGC 5890, an unbarred lenticular galaxy found 98 million light-years distant with an apparent visual magnitude of 14; and NGC 5885, a barred spiral galaxy situated 92 million light years away that shines with an apparent magnitude of 11.8.

Stars

Methuselah (HD 140283)

Methuselah (HD 140283)While single stars do not strictly qualify as deep sky objects, the image opposite shows the overexposed picture of a magnitude 7.223 star named Methuselah, which is the oldest known star in the Universe, being about 14.5 billion years old. Methuselah is a sub-giant that is believed to have formed soon after the Big Bang, but the fact that its metal content is less than one percent that of the Sun largely rules out the possibility that it is a Population III star, the first stars that formed after the so-called Universal Dark Age.

Investigators used the Hubble telescope to constrain the uncertainties about the star’s distance in an effort to arrive at a more precise age for it, which turned out to be older than the Universe itself which is estimated to be 13.6 billion years old. However, the discrepancy does not mean that some stars are older than the Universe that contains them, since the uncertainty about the ages of both some stars and the Universe is very large. Nonetheless, the uncertainty about Methuselah’s age is only about 800 million years, meaning that if Methuselah is not a Population III star, it must be one of the very first Population II stars to have formed out of the remains of the first stars. By way of explanation, our Sun is a Population I star that formed from the remains of a previous generation of Population II stars.

Methuselah is now passing through the local stellar neighborhood in an orbit that carries it through the plane of the Milky Way galaxy, and particularly through the galaxy’s halo, which is populated mainly by ancient stars.

8 Librae

48 Librae, also known as FX Librae, is one of a class of relatively rare stars called shell stars. These stellar bodies have exceptionally high spin rates, and in the case of 48 Librae, its equatorial rotational velocity of about 400 km/sec is one of the highest known, and fast enough for stellar material to be flung off the star, and collect in a vast ring around the its equator.

48 Librae is classified as a B8Ia/Iab-type star, and has an apparent visual magnitude of 4.94, which varies in an irregular pattern. The star is 5.8 times as massive as the Sun, about 3.3 times as big, and shines with at least 965 solar luminosities.

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