Corona Australis is said to represent the crown worn by the centaur represented by Sagittarius by some sources, but the constellation is not really tied to any particular myth. Sometimes it is also called Corona Austrina.
Corona Australis was first catalogued by the Greek astronomer Ptolemy in the 2nd century, along with the constellation representing the northern crown, Corona Borealis.
FACTS, LOCATION & MAP
Corona Australis is one of the smallest constellations in the sky. Only the 80th in size, it occupies an area of 128 square degrees.
Corona Australis does not have any stars brighter than magnitude 3.00 or located within 10 parsecs (32.6 light years) of Earth. The brightest star in the constellation is Alpha Coronae Australis, also known as Alphekka Meridiana (Alphekka South). The nearest star is HD 166348 (spectral class K6Vk), located at a distance of 42.26 light years from Earth.
Corona Australis has two stars with known planets. One of these, HD 166724, is a K-class dwarf with a long-period, wide-orbiting planet that is considered one of the three most eccentric planets with a period larger than 5 years. The planet was discovered in 2012.
Corona Australis belongs to the Hercules family of constellations, along with Aquila, Ara, Centaurus, Corvus, Crater, Crux, Cygnus, Hercules, Hydra, Lupus, Lyra, Ophiuchus, Sagitta, Scutum, Sextans, Serpens, Triangulum Australe, and Vulpecula.
Corona Australis does not contain any Messier objects. The best known deep sky objects in the constellation are the Corona Australis Nebula and the Coronet Cluster. There is one meteor shower associated with the constellation, the Corona Austrinids.
Corona Australis, the Southern Crown, was known by the Greeks as a wreath, not a crown. Before Ptolemy introduced its stars as a separate constellation, Corona Australis was known as a circlet of stars near the forefeet of the centaur represented by the neighbouring constellation Sagittarius.
The constellation is sometimes associated with the myth of Dionysus. In the story, Corona Australis represents the crown the god placed in the sky after freeing his mother Semele from the god of the underworld, Hades. This particular myth, however, is also sometimes associated with the Northern Crown, Corona Borealis.
Ptolemy originally assigned 13 stars to the constellation Corona Australis, but one of them was later moved to the constellation Telescopium, where it became Alpha Telescopii.
MAJOR STARS IN CORONA AUSTRALIS
Alphekka Meridiana – α Coronae Australis (Alpha Coronae Australis)
Alpha Coronae Australis is a class A2V star with an apparent visual magnitude of 4.10. It is 130 light years distant.
It is the only named star in the constellation. It got its name, Alphekka Meridiana (Alphekka South) after Alphecca, the brightest star in the northern constellation Corona Borealis.
β Coronae Australis (Beta Coronae Australis)
Beta Coronae Australis is a K-type bright giant (spectral type K0II) with an apparent magnitude of 4.117, approximately 510 light years distant. The star has an absolute magnitude of -1.8.
R Coronae Australis
R Coronae Australis is a well known variable star belonging to the spectral type A5IIev, approximately 26.8 light years distant from Earth.
It is a relatively young star that is still accreting material onto its surface. The star has a mean apparent magnitude of 11.5. It is located in the star forming nebulosity NGC 6726/27/29.
R Coronae Australis is a Herbig Ae/Be star, which is to say a pre-main sequence star, still in the star formation stage, belonging to the spectral type A or B, less than 10 million years in age, surrounded by a circumstellar disk and embedded in an envelope of gas and dust.
Herbig stars have hydrogen and calcium emission lines observed in their spectra, and they are not burning hydrogen in their centre.
RX J1856.5-3754 is a neutron star in the constellation Corona Australis. Located at an estimated distance of 400 light years from Earth, it is the closest neutron star to our solar system. It is moving across the sky at the speed of 108 km/s. The star is believed to have been formed by a supernova explosion of its companion star approximately a million years ago.
RX J1856.5-3754 was first discovered in 1992. Since it was initially believed to be only about 4-8 kilometres in diameter, too small for a neutron star, it was considered to be a candidate for a quark star instead. Further observations, however, confirmed that it was indeed a neutron star and that its diameter was larger, closer to 14 kilometres.
RX J1856.5-3754 is one of the Magnificent Seven, a group of young, cooling, isolated neutron stars emitting soft X-rays, located within 200 to 500 parsecs to Earth. The stars are also known as XDINS (X-ray Dim Isolated Neutron Stars) or XTINS (X-ray Thermal Neutron Stars).
DEEP SKY OBJECTS IN CORONA AUSTRALIS
Corona Australis Nebula
Corona Australis Nebula is a bright reflection nebula, about 420 light years distant, formed by several bright stars caught up in a dark cloud of dust. The cloud is a star-forming region, with clusters of young stars embedded within it.
The Corona Australis Nebula consists of three nebulous regions, NGC 6726, NGC 6727, and NGC 6729, that were originally discovered by the German astronomer Johann Friedrich Julius Schmidt in 1861. The molecular cloud complex in the region is one of the nearest areas with recent or ongoing intermediate and low-mass star formation.
NGC 6541 is a large, bright globular cluster. It was first discovered by the Italian astronomer Niccolò Cacciatore in March 1826 and then later independently found by James Dunlop in July 1826. Cacciatore initially believed the object to be a nebula.
The cluster lies about halfway between the stars Theta Coronae Australis and Theta Scorpii.
NGC 6726/NGC 6727
The reflection nebula NGC 6726/6727 is illuminated by the stars TY Coronae Australis, a variable star with an apparent magnitude varying between 8.8 and 12.6, and HD 176386, which is a Herbig Ae/Be star, like R Coronae Australis.
NGC 6729 (Caldwell 68)
NGC 6729 is a fan-shaped reflection/emission nebula in Corona Australis, found about seven and a half degrees from the star Zeta Sagittarii. The nebula lies between the stars R Coronae Australis and another variable star, T Coronae Australis.
The Coronet cluster is a small open star cluster in Corona Australis. It is approximately 420 light years distant. The cluster lies at the heart of the constellation. It is one of the nearest known regions with ongoing star formation.