Corona Australis (“the southern crown”) is the southern sky’s counterpart to Corona Borealis (“the northern crown”), and despite being rather small and faint, it is nonetheless a rather beautiful constellation on account of its distinctive horseshoe-shaped pattern of relatively equal brightness stars. The most luminous of its stars, Alphekka Meridiana, is a blue subgiant found 130 light years from Earth with a visual magnitude of just +4.10.
Corona Australis is ranked 80th in size out of the 88 recognized constellations, taking up an area of 128 square degrees of the southern celestial hemisphere. It can be seen by observers located between +40° and -90° of latitude, although best seen from May to July. Corona Australis is bordered by Sagittarius to its north and east, Telescopium to its south, Scorpius to its west, and Ara to its southwest.
Hercules Constellation Family
Corona Australis is a member of the Hercules family of constellations, together with Aquila, Ara, Centaurus, Corvus, Crater, Crux, Cygnus, Hercules, Hydra, Lupus, Lyra, Ophiuchus, Sagitta, Scutum, Sextans, Serpens, Triangulum Australe, and Vulpecula
In the 3rd century BC, Greek poet Aratus referred to the pattern of stars that make up the constellation in reference to the neighbouring constellation of Sagittarius, describing it as a circlet of stars under the forefeet of the centaur. By 150 AD, however, Corona Australis was recorded as one of 48 individual constellations by Greek astronomer Ptolemy in his astronomical treatise called Almagest. The Greek name assigned to it at the time, Notios Stephanos, means “southern crown”, and is believed to represent the wreath made of myrtle leaves that the god Dionysus left as a gift for Hades in return for retrieving his mother from the Underworld and taking her to Mount Olympus. Interestingly, this particular legend is also associated with Corona Borealis.
The Corona Australids are a short duration annual meteor shower lasting from March 14-18, with a peak on the 16th when 5 to 7 meteors per hour can be seen. From northern latitudes, the shower is best viewed around 2 hours before dawn, or after the Moon sets from southern locations. The meteoroids associated with the stream are small, though, and also seem to travel in a south to north direction no more than 7 degrees from their radiant in the constellation.
– Alphekka Meridiana (Alpha Coronae Australis), the constellation’s brightest stars, is a blue subgiant situated 130 light years from our solar system that shines with a visual magnitude of just 4.10. This 254 million year old star has around 2.3 times the Sun’s size and mass, and 31 times its luminosity. Alpha Coronae Australis is also a rapid rotator at about 200 km/s at the equator, resulting in a complete revolution once very 14 hours or so.
– Beta Coronae Australis, the second brightest star in Corona Australis, is an orange giant (K0II) found 510 light years away of magnitude 4.117. It is more than 100 million years old, and is around 43 times bigger than the Sun, with 4.5 times its mass, and 730 times its luminosity.
– Gamma Coronae Australis, the constellation’s third brightest star, is a binary system located 56.4 light years away with a visual magnitude of 4.2. Its components, A and B, are both yellow-white dwarf stars (F8V) with 1.2 times the Sun’s mass. They are also separated by 32.8 AU, and have an orbital period of 121.76 years.
– RX J1856.5-3754 is the closest neutron star to our solar system at just 400 light years distant, and is a member of a group of young neutron stars no further than 1630 light years away referred to as the Magnificent Seven. This one million years old neutron star has a diameter of 14 kms, a mass of 0.9 sol, and has an apparent visual magnitude of 25.6.
Other stars of interest in the constellation includes the variable stars R Coronae Australis and Epsilon Coronae Australis; the blue-white subdwarf Zeta Coronae Australis; and the yellow giant Theta Coronae Australis.
Notable Deep-Sky Objects
– Corona Australis Molecular Cloud is a dark molecular cloud located in the constellation’s north that contains several bright reflection nebulae, including the butterfly shaped NGC 6729, and the blue colored NGC 6726–7 and IC 4812. This star forming area of space is 430 light years away, making it one of the closest such regions to our own solar system, and adding to its mass of 7,000 sol are many protostars, as well as very young stars. In the image opposite can also be seen the blue variable giant star R Coronae Australis, and NGC 6723, a globular cluster located in the bordering constellation of Sagittarius.
Other objects on interest in Corona Australis includes the Coronet open cluster, which is found 554 light years light years from Earth; and the globular cluster NGC 6541, which is 100 light years wide, 14 billion years old, and 22,800 light years distant. There is also Bernes 157, a boomerang shaped dark nebula situated 520 light years away; the green-hued planetary nebula IC 1297; and NGC 6768, which consists of a merging elliptical and lenticular galaxy.