Corona Australis or Austrina is a constellation in the Southern Celestial Hemisphere. Its Latin name means ‘southern crown,’ and it is the southern counterpart of Corona Borealis, the Northern Crown. Corona Australis is a small constellation bordered by Sagittarius to the north, Scorpius to the west, Telescopium to the south, and Ara to the southwest. Although fainter than its namesake, the oval- or horseshoe-shaped pattern of its brighter stars renders it distinctive. In the equatorial coordinate system, the right ascension coordinates of the constellation lie between 17h 58.3m and 19h 19.0m, while the declination coordinates are between −36.77° and −45.52°. Covering 128 square degrees, Corona Australis culminates at midnight around the 30th of June and ranks 80th in area. Only visible at latitudes south of 53° north, Corona Australis cannot be seen from the British Isles as it lies too far south, but it can be seen from southern Europe and readily from the southern United States.
Corona Australis, depicted at the forefeet of Sagittarius, on Chart XV of the Uranographia of Johann Bode (1801)
[http://www.ianridpath.com/startales/coronaaustralis.htm]
Corona Australis may have been recorded by ancient Mesopotamians in the MUL.APIN, as a constellation called MA.GUR (‘The Bark’). However, this constellation, adjacent to SUHUR.MASH (‘The Goat-Fish,’ modern Capricornus), may instead have been modern Epsilon Sagittarii. As a part of the southern sky, MA.GUR was one of the fifteen ‘stars of Ea.’
In the 3rd century BCE, the Greek didactic poet Aratus wrote of, but did not name the constellation, instead calling the two crowns ‘Στεφάνοι’ (‘Stephanoi’, Wreaths). The Greek astronomer Ptolemy described the constellation in the 2nd century AD, though with the inclusion of Alpha Telescopii, since transferred to Telescopium. Ascribing 13 stars to the constellation, he named it ‘Στεφάνος νότιος’ (Stephanos notios), ‘Southern Wreath,’ while other authors associated it with either Sagittarius (having fallen off his head) or Centaurus; with the former, it was called Corona Sagittarii. Similarly, the Romans called Corona Australis the ‘Golden Crown of Sagittarius.’ It was known as Parvum Coelum (‘Canopy’, ‘Little Sky’) in the 5th century. The 18th-century French astronomer Jérôme Lalande gave it the names Sertum Australe (‘Southern Garland’) and Orbiculus Capitis, while German poet and author Philippus Caesius called it Corolla (‘Little Crown’) or Spira Australis (‘Southern Coil’), and linked it with the Crown of Eternal Life from the New Testament. Seventeenth-century celestial cartographer Julius Schiller linked it to the Diadem of Solomon. Sometimes, Corona Australis was not the wreath of Sagittarius but arrows held in his hand.
Corona Australis has been associated with the myth of Bacchus and Stimula. Jupiter had impregnated Stimula, causing Juno to become jealous. Juno convinced Stimula to ask Jupiter to appear in his full splendor, which the mortal woman could not handle, causing her to burn. After Bacchus, Stimula’s unborn child, became an adult and the god of wine, he honored his deceased mother by placing a wreath in the sky.
In Chinese astronomy, the stars of Corona Australis are located within the Black Tortoise of the North (Běi Fāng Xuán Wǔ). The constellation itself was known as ti’en pieh (‘Heavenly Turtle’) and during the Western Zhou period, marked the beginning of winter. However, precession over time has meant that the ‘Heavenly River’ (Milky Way) became the more accurate marker to the ancient Chinese and hence supplanted the turtle in this role.
Arabic names for Corona Australis include ‘Al Ķubbah,’ ‘the Tortoise,’ ‘Al Ĥibā,’ ‘the Tent’ or ‘Al Udḥā al Na’ām,’ ‘the Ostrich Nest.’ It was later given the name ‘Al Iklīl al Janūbiyyah,’ which the European authors Chilmead, Riccioli and Caesius transliterated as Alachil Elgenubi, Elkleil Elgenubi and Aladil Algenubi respectively.
The ǀXam speaking San people of South Africa knew the constellation as ≠nabbe ta !nu ‘house of branches’- owned originally by the Dassie (rock hyrax), and the star pattern depicting people sitting in a semicircle around a fire.
The indigenous Boorong people of northwestern Victoria, Australia, saw it as Won, a boomerang thrown by Totyarguil (Altair). The Aranda people of Central Australia saw Corona Australis as a coolamon carrying a baby, which was accidentally dropped to earth by a group of sky-women dancing in the Milky Way. The impact of the coolamon created Gosses Bluff crater, 175 km west of Alice Springs.
The Torres Strait Islanders saw Corona Australis as part of a larger constellation encompassing part of Sagittarius and the tip of Scorpius’s tail; the Pleiades and Orion were also associated. This constellation was Tagai’s canoe, crewed by the Pleiades, called the Usiam, and Orion, called the Seg. The myth of Tagai says that he was in charge of this canoe, but his crewmen consumed all of the supplies onboard without asking permission. Enraged, Tagai bound the Usiam with a rope and tied them to the side of the boat, then threw them overboard. Scorpius's tail represents a suckerfish, while Eta Sagittarii and Theta Coronae Australis mark the bottom of the canoe. On the island of Futuna, the figure of Corona Australis was called Tanuma and in the Tuamotus, it was called Na Kaua-ki-Tonga.
Corona Australis - August 15, 10:00 PM - Latitude 30° North, Longitude 95° West
[http://www.peoplesguidetothecosmos.com/constellations/coronaaustralis.htm]
[http://astropixels.com/constellations/charts/CrA.html]
Though Corona Australis has no stars brighter than 4th magnitude, it still has 21 stars visible to the unaided eye (brighter than magnitude 5.5). Nicolas Louis de Lacaille used the Greek letters Alpha through to Lambda to label the most prominent eleven stars in the constellation, designating two stars as Eta and omitting Iota altogether. Mu Coronae Australis, a yellow star of spectral type G5.5III and apparent magnitude 5.21, was labelled by Johann Elert Bode and retained by Benjamin Gould, who deemed it bright enough to warrant naming.
The only star in the constellation to have received a name is Alfecca Meridiana or Alpha CrA. The name combines the Arabic name of the constellation with the Latin for ‘southern.’ In Arabic, Alfecca means ‘break,’ and refers to the shape of both Corona Australis and Corona Borealis. Also called simply ‘Meridiana,’ it is a white main sequence star located 130 light years away from Earth, with an apparent magnitude of 4.10 and spectral type A2Va. A rapidly rotating star, it spins at almost 200 km per second at its equator, making a complete revolution in around 14 hours. Like the star Vega, it has excess infrared radiation, which indicates it may be ringed by a disk of dust. It is currently a main-sequence star, but will eventually evolve into a white dwarf; currently, it has a luminosity 31 times greater, and a radius and mass of 2.3 times that of the Sun.
Beta Coronae Australis is an orange giant 510 light years from Earth. Its spectral type is K0II, and it is of apparent magnitude 4.11. Since its formation, it has evolved from a B-type star to a K-type star. Its luminosity class places it as a bright giant; its luminosity is 730 times that of the Sun, designating it one of the highest-luminosity K0-type stars visible to the naked eye. 100 million years old, it has a radius of 43 solar radii and a mass of between 4.5 and 5 solar masses. Alpha and Beta are so similar as to be indistinguishable in brightness to the naked eye.
Some of the more prominent double stars include Gamma Coronae Australis- a pair of yellowish white stars 58 light years away from Earth, which orbit each other every 122 years. Widening since 1990, the two stars can be seen as separate with a 100 mm aperture telescope; they are separated by 1.3 arcseconds at an angle of 61 degrees. They have a combined visual magnitude of 4.2; each component is a dwarf star with a magnitude of 5.01.
R Coronae Australis is a variable star in the constellation Corona Australis. It has varied between magnitudes 10 and 14.36. A small reflection/emission nebula NGC 6729 extends from star towards SE. This star is moving toward the Solar System with a radial velocity of 36 km s−1. In roughly 222,000 years, this system may approach within 1.77 light-years (0.54 parsecs) of the Sun. However, this estimate has a considerable margin of error, so the actual distance remains uncertain:
[https://en.wikipedia.org/wiki/R_Coronae_Australis]
Dust and Gas Surrounding Star R Coronae Australis
Young star R Coronae Australis has a dusty home. The dust is so thick on the upper left of the above photograph that little light from background stars comes through. Thinner dust near the stars reflects light from R Coronae Australis (upper right) and neighbor TY Coronae Australis, giving their surroundings a flowing appearance. Were these stars more massive they would emit light energetic enough to ionize much of the nearly invisible surrounding hydrogen gas, causing it to appear bright red. The unusual structure above the center is a Herbig-Haro object, a knot of gas ejected from the star that has impacted surrounding gas. R Coronae Australis is about 500 light-years away, while the region shown is about four light years across.
[http://apod.nasa.gov/apod/ap001016.html]
RX J1856.5-3754 (also called RX J185635-3754, RX J185635-375, and various other designations) is a nearby neutron star in the constellation Corona Australis. It is thought to have formed in a supernova explosion of its companion star about one million years ago and is moving at 108 km/s across the sky. It was discovered in 1992, and observations in 1996 confirmed that it is a neutron star, the closest to Earth discovered. It is one of the Magnificent Seven, a group of young neutron stars at distances between 200 and 500 parsecs (652 and 1630 light years) of Earth:
[https://en.wikipedia.org/wiki/RX_J1856.5-3754]
RX J1856.5-3754 and 3C58: Cosmic X-rays May Reveal New Form of Matter
Chandra observations of RX J1856.5-3754 and the pulsar in 3C58 suggest that the matter in these collapsed stars is even denser than nuclear matter, the most dense matter found on Earth. This raises the possibility that these stars are composed of free quarks or crystals of sub-nuclear particles, rather than neutrons.
By combining Chandra and Hubble Space Telescope data, astronomers found that RX J1856 radiates like a solid body with a temperature of 700,000 degrees Celsius and has a diameter of about 7 miles.
This size is too small to reconcile with the standard models of neutron stars. One exciting possibility, predicted by some theories, is that the neutrons in the star have dissolved at very high density into a soup of ‘up,’ ‘down’ and ‘strange’ quarks to form a ‘strange quark star,’ which would explain the smaller radius.
Observations of 3C58, the remnant of a supernova noted on Earth in AD 1181, reveal that the pulsar in the core has a temperature much lower than expected. This suggests that an exotic, denser state of matter might exist inside this star as well.
These observations demonstrate that the universe can be used as a laboratory to explore physics under conditions that are not accessible on Earth.
[http://chandra.harvard.edu/photo/2002/0211/]
In the north of the constellation is the Corona Australis Molecular Cloud, a dark molecular cloud with many embedded reflection nebulae, including NGC 6729, NGC 6726–7, and IC 4812. A star-forming region of around 7000 M☉, it contains Herbig–Haro objects (protostars) and some very young stars. About 430 light years (130 parsecs) away, it is one of the closest star-forming regions to the Solar System.
R Coronae Australis illuminates, the surrounding nebula, NGC 6729, which brightens and darkens with it. The nebula is often compared to a comet for its appearance in a telescope, as its length is five times its width. Nearby, another young variable star, TY Coronae Australis, illuminates another nebula: reflection nebula NGC 6726–7. TY Coronae Australis ranges irregularly between magnitudes 8.7 and 12.4, and the brightness of the nebula varies with it. The largest young stars in the region, R, S, T, TY and VV Coronae Australis, are all ejecting jets of material which cause surrounding dust and gas to coalesce and form Herbig–Haro objects, many of which have been identified nearby.
Near Epsilon and Gamma Coronae Australis is Bernes 157, a dark nebula and star forming region. It is a large nebula, 55 by 18 arcminutes, that possesses several stars around magnitude 13. These stars have been dimmed by up to 8 magnitudes by its dust clouds:
The star-forming region around NGC 6726 in the southern constellation of Corona Australis (Southern Crown) about 400 - 500 light years distant is a complex association of dust, beautiful blue reflection nebula and unusual objects referred to as Herbig-Haro (HH) objects. The image above is 34 arcminutes square and north us up. Since there are so many different objects present, an annotated image is presented below for reference. NGC 6726/6727 and IC 4812 are blue reflection nebulae, where bright stars are embedded in a large cloud of dust which reflects the blue light of the stars. NGC 6729 is a butterfly-shaped emission/reflection object that is a variable nebula, powered by the irregular variable star, R Coronae Australis (R CrA). R CrA was discovered about 100 years ago. HH objects are shown as delicate, small arcs of glowing gas. They originate as energetic outflows from embryonic stars that are still forming and cloaked inside gas and dust. Bernes 157 is dark nebula that is so dense that stars inside the nebula and behind it cannot shine through. The Corona Australis star forming region has incredible diversity and is a wonderful imaging target.
[http://astrodonimaging.com/gallery/ngc-6726/]
The Coronet cluster is a small open cluster located about 170 parsecs away in the southern constellation Corona Australis, isolated at the edge of the Gould Belt. The Coronet cluster is 3.5 times closer to the Earth than the Orion Nebula Cluster, and its center is composed of mostly young stars:
[https://en.wikipedia.org/wiki/Coronet_cluster]
Coronet Cluster: A Neighbor of Star Formation
While perhaps not quite as well- known as its star formation cousin of Orion, the Corona Australis region (containing, at its heart, the Coronet Cluster) is one of the nearest and most active regions of ongoing star formation. At only about 420 light years away, the Coronet is over three times closer than the Orion Nebula is to Earth. The Coronet contains a loose cluster of a few dozen young stars with a wide range of masses and at various stages of evolution, giving astronomers an opportunity to observe ‘protostars’ simultaneously in several wavelengths.
This composite image shows the Coronet in X-rays from Chandra (purple) and infrared emission from Spitzer (orange, green, and cyan). The Spitzer image shows young stars plus diffuse emission from dust. In the Chandra data only, many of these young stars appear as blue objects, revealing their output of high-energy X-rays and the amount of obscuring dust and gas in the region. The reason for the blue appearance is that lower energy X-rays, which are depicted as red and green, are absorbed by this veil of material and hence are not seen. The Chandra data also support the idea that X-rays from very young stars are generated largely from magnetic activity in the outer atmospheres. Due to the host of young stars in different life stages in the Coronet, astronomers can use these data to pinpoint details of how the youngest stars evolve.
[http://chandra.harvard.edu/photo/2007/coronet/]
NGC 6541
[https://en.wikipedia.org/wiki/NGC_6541]
Halfway between Theta Coronae Australis and Theta Scorpii is the dense globular cluster NGC 6541. Described as between magnitude 6.3 and magnitude 6.6, it is visible in binoculars and small telescopes. Around 22000 light years away, it is around 100 light years in diameter. It is estimated to be around 14 billion years old. NGC 6541 appears 13.1 arcminutes in diameter and is somewhat resolvable in large amateur instruments; a 12-inch telescope reveals approximately 100 stars but the core remains unresolved. The globular cluster was discovered by Nicolò Cacciatore (1780 - 1841) at the Palermo Astronomical Observatory, Sicily, on March 19, 1826. It was independently found by James Dunlop (1793 – 1848) on July 3, 1826.
The Corona Australids are a meteor shower that takes place between 14 and 18 March each year, peaking around 16 March. This meteor shower does not have a high peak hourly rate. In 1953 and 1956, observers noted a maximum of 6 meteors per hour and 4 meteors per hour respectively; in 1955 the shower was ‘barely resolved.’ However, in 1992, astronomers detected a peak rate of 45 meteors per hour. The Corona Australids’ rate varies from year to year. At only six days, the shower’s duration is particularly short, and its meteoroids are small; the stream is devoid of large meteoroids. The Corona Australids were first seen with the unaided eye in 1935 and first observed with radar in 1955. Corona Australid meteors have an entry velocity of 45 kilometers per second.
[https://en.wikipedia.org/wiki/Corona_Australis]
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