|2. The Universe / Space, Stars and Galaxies / Constellations|
Constellations: Corona Australis 'the Southern Crown'
Camelopardalis | Cancer | Canes Venatici | Canis Major | Canis Minor | Capricornus | Carina | Cassiopeia | Centaurus
Cepheus | Cetus | Chamæ leon | Circinus | Columba | Coma Berenices | Corona Australis | Corona Borealis | Corvus
Crater | Crux | Cygnus | Delphinus | Dorado | Draco | Equuleus | Eridanus | Fornax | Gemini | Grus | Hercules | Horologium
Hydra | Hydrus | Indus | Lacerta | Leo | Leo Minor | Lepus | Libra | Lupus | Lynx | Lyra | Mensa | Microscopium | Monoceros
Musca | Norma | Octans | Ophiuchus | Orion | Pavo | Pegasus | Perseus | Phoenix | Pictor | Pisces | Piscis Austrinus
Puppis | Pyxis | Reticulum | Sagitta | Sagittarius | Scorpius | Sculptor | Scutum | Serpens | Sextans | Taurus
Telescopium | Triangulum | Triangulum Australe | Tucana | Ursa Major | Ursa Minor | Vela | Virgo | Volans | Vulpecula
Not an atom less than the proud laurel shall content my bier
The Constellation Corona Australis
Corona Australis (sometimes referred to as Corona Austrina) is a tiny southern constellation which has been in existence since time immemorial. In ancient times, laurel crowns were a sign of victory, and this curved line of stars forms the same shape when joined up in the imagination. There is no doubt this star grouping was revered as auspicious.
It is bordered on two sides by mighty Sagittarius, too close for comfort to the stinging barb of Scorpius, has a tiny connection with Ara in one corner, and the rest with Telescopium. When you look towards this portion of the sky, approximately half of the constellation has the Sagittarius arm of the Milky Way as a backdrop.
The Greek god of music, Apollo, was the conductor of the heavenly choir of muses, from which all music on Earth derived. He loved the nymph Daphne unrequitedly, thanks to one of the arrows of the mischievous god of love, Cupid. Rather than succumb to Apollo's advances, Daphne fled and requested the assistance of another god, who acquiesced. When Apollo finally caught up with Daphne he tried to take her in his arms, but as soon as he touched her she morphed into a laurel tree. Unable to reverse the other god's command, devastated Apollo gathered some of the leaves and declared the tree sacred. This is why Apollo is depicted wearing a crown of laurel leaves2 upon his head, in honour of the love he lost.
No-one knows who originally delineated this star formation as a crown. Although it featured in the Greek astronomer Ptolemy's 48 constellations of Almagest, Ptolemy had studied the work of Hipparchus, who figured out how to predict solar and lunar eclipses over two centuries prior to Ptolemy's time. Hipparchus' original work does not survive today, but he is referenced by many astronomers in different cultures.
Despite its southerly position, the ancient Greeks knew this circlet of stars as Corona Sagittarii. The crown of the centaur archer, however, lies at the feet of Sagittarius, so this small constellation could commemorate a deposed king.
The sanctuary of Delphi in central Greece held the Pythian Games, honouring the god Apollo. Crowns of laurel leaves, from the sacred tree of Apollo, were awarded to the winners. Therefore, the crown of stars which make up Corona Australis could be honouring a winning athlete awaiting his laurels.
Corona Australis has featured in other cultures as representing a turtle or tortoise, a golden crown, an ostrich nest, a bowl, a little crown, and even a harem enclosure, according to the Arabs. Chinese astronomers recognised it as a tortoise, and named it Pee.
The scientific star names are Greek letters combined with the genitive of the constellation name, known as the 'Bayer designation', after the man who devised the system. Some stars have proper names as well - for example, alpha Coronae Australis is Alfecca Meridiana. Other stars are known by their catalogue number. More recently discovered variable stars, like R Coronae Australis, are given upper case English letters.
Alfecca Meridiana is the only named star in this constellation. Although it was designated alpha by Bayer, beta, a carbon star, is fractionally brighter.
Gamma Coronae Australis is a double-star system comprising two white dwarfs of almost the same magnitude; gamma A is +4.2 and gamma B is +4.9.
Kappa Coronae Australis is a binary system consisting of a blue-white dwarf and a white sub-giant. Due to the dwarf kappa2 being closer to us, it appears brighter, even though its partner, kappa1, is many times more massive.
Irregular variable star TY Coronae Australis is surrounded by and illuminates gaseous material, creating the gorgeous blue nebulae NGC 6726/27. R Coronae Australis is a protostar (still being formed) and lights up NGC 6729. Astronomers suspect R Coronae Australis, the heart-shaped star on the right of the image, is a binary system.
One of the closest star-forming regions to Earth is 424 light years3 distant. Through the heart of Corona Australis is a star-forming region which has been imaged by the Chandra X-ray observatory. The enigmatically named Coronet Cluster has a nursery of up to 30 young stars varying in both mass and age.
RX J185635-3754 is a neutron star which was detected by the Hubble Space Telescope in 1997. Neutron stars, although small, are extremely heavy because of their density, which is the greatest known to science. This rapidly rotating remnant of a supernova is the closest-known neutron star to us, about 420 light years distant, and is heading in our direction.
Around a million years ago, RX J185635-3754 was one component of a binary system. Its partner, the star now known as zeta Ophiuchi, was expelled from its original system by the force of the catastrophic death of its companion. Zeta Ophiuchi is still travelling, rather like a cosmic cricket ball having been hit for a galactic six. The trajectory of RX J185635-3754 has been plotted and it is not classed as a threat to Earth, because the closest it will pass is at a distance of 170 light years, around 300,000 years in the future.
New General Catalogue (NGC) and Index Catalogue (IC)
The New General Catalogue (NGC) was compiled by John Louis Emil Dreyer (director of the Armagh Observatory from 1882 to 1916). Since the NGC was created, improved detection methods have uncovered other wondrous sights which are registered in the Index Catalogue (IC).
German astronomer and selenographer4 Johann Friedrich Julius Schmidt (1825 - 1884) spent the latter part of his career as director of the observatory in Athens, Greece. Among his discoveries were NGC 6726/27 and NGC 6729, now collectively known as the Corona Australis Nebula. This region is home to the irregular variable star R Corona Australis whose magnitude fluctuates between +9.7 and +12. Another variable star, TY Corona Australis, illuminates NGC 6726/27, causing the surrounding nebula to glow and dim, respectively.
NGC 6541 is a tight ball of stars known as a globular cluster. These are among the oldest known 'objects' in the Universe. NGC 6541 was discovered by Nicolò Cacciatore (1780 - 1841) of the Palermo Observatory, Italy, on 19 March, 1826. He catalogued it as Cacciatore 1826, but mistakenly listed it as a 'new nebula'. Less than four months later, on 3 July, Scottish astronomer James Dunlop correctly identified it as a globular cluster and registered it as Dun 473. It was listed in the NGC by Dreyer, and Sir Patrick Moore featured it in his catalogue for backyard astronomers as Caldwell 78.
Scottish astronomer Williamina Paton Stevens Fleming (1857 - 1911) made many astronomical discoveries, including the famous Horsehead Dark Nebula in Orion, while working for Professor Edward Charles Pickering of the Harvard College Observatory. IC 1297 is a planetary nebula which Fleming found on a photographic plate in 1894. Dreyer attributed the discovery to her boss, registering it as PK (Pickering) 358-21.
Pickering's list of discoveries is impressive, but he claimed nothing in the constellation Corona Australis.
IC 4812 was originally classed a Reflection nebula, but since the data was revised it has been reclassified as a Diffuse nebula or Supernova remnant. The original data appears in the table below.
The meteor shower connected with this constellation is called the Beta Coronae Australids. Due to the low southern declination, this shower is limited to observers in the southern hemisphere. Although the zenithal hourly rate (ZHR) is three during the peak in mid-May, the display produces a few noted bright meteors.
Dark nebulae are 'clouds' of interstellar gas and dust with no set form, and are constantly changing - a kind of creeping tentacle covering what lies beyond. Some, like the Doodad Dark Nebula in Musca, are not even catalogued.
Bernes 157 is a boomerang-shaped dark nebula that is 520 light years distant in Corona Australis. There is so much content, and it is so tightly packed, that it totally obliterates the background stars. It stretches around the Corona Australis Nebula like a huge, draping black scarf.
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