Constellations: Corona Australis 'the Southern Crown' Content from the guide to life, the universe and everything

Constellations: Corona Australis 'the Southern Crown'

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The shield of the Science, Mathematics and Engineering faculty of the h2g2 University.Constellations: Overview | Andromeda | Antlia | Apus | Aquarius | Aquila | Ara | Aries | Auriga | Boötes | Caelum
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Not an atom less than the proud laurel shall content my bier
No! By the eternal stars! Or why sit here in the Sun's eye
and 'gainst my temples press
Apollo's very leaves, woven to bless
By thy white fingers and thy spirit clear.

– John Keats To a Young Lady who Sent me a Laurel Crown

The Constellation Corona Australis

Name:Corona Australis (Latin: 'Southern Crown')
Genitive:Coronae Australis
Meaning:The Southern Crown
Short form:CrA
Formerly:Corona Sagittarii
Area:128 sq deg
Co-ordinates1:Right Ascension 19h, Declination −40°
Origin:Ancient

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.

Mythology

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.

History

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.

Stars

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 Variables

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.

Coronet Cluster

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.

Neutron Star

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.

Star Table

StarDesignationName or
catalogue number
Brightness (m)Distance
(light years)
Spectral classification
and/or comments
βbeta CrAHD 178345+4.10500Carbon star
αalpha CrAAlfecca Meridiana+4.11130White dwarf
γ Agamma CrA AHD 177474+4.260Double star system
δdelta CrAHD 177873+4.5175Orange giant
ζzeta CrAHD 176638+4.7187Blue-white dwarf
εepsilon CrAHD 175813+4.8140White dwarf
γ Bgamma CrA BHD 177475+4.9100Double star system
η1eta1 CrAHD 173715+5.5210White dwarf
η2eta2 CrAHD 173861+5.6600Blue-white dwarf
κ2kappa2 CrAHD 170867+5.65440Blue-white dwarf
κ1kappa1 CrAHD 170868+6.3500White sub-giant
RR CrAR Coronae Australis+11 var500Herbig Ae/Be
TYTY CrATY Coronae Australis+12 var500Eclipsing Binary

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).

Johann Schmidt

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

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.

IC 1297

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

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.

NGC/IC Table

CatalogueNameTypeBrightness (m)Distance
(light years)
Remarks
NGC 6541Caldwell 78Globular cluster+6.622,800Dun 473
NGC 6726/7Corona Australis NebulaReflection nebulae+7.9500Discovered in 1861
by Johann FJ Schmidt
NGC 6729Caldwell 68Bright nebula+9.7500Surrounds R CrA
IC 1297PK 358-21Planetary nebula+10.77,800Discovered in 1894
by Williamina Fleming
IC 4812ESO 396-*N12Reflection nebula+9500Surrounds a binary system

Meteor Showers

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

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.

1Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.2Today we know this as 'bay' and use bay leaves in soups and stews. Daphne is still the Greek for bay.3A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion km.4Someone who studies or charts the Moon, derived from selen, the Greek word for moon.

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