Corona Australis

CORONA AUSTRALIS the ‘Southern Crown’.

It has been known as austrina, meridiana, meridionalis and notia, also the ‘Centaur's Crown’ and ‘Corona Sagittarii’. It was Ptolemy's ‘Southern Wreath’, La Landes ‘Sertum australe’, the ‘Southern Garland’ and Caesius' ‘Spira australis’, the ‘Southern Coil’. The Germans called it ‘Süliche Krone’, the French ‘Couronne Australe’ and to the Italians it was their ‘Corona Australe’. The Arab desert dwellers saw it as a ‘Womens Tent’ and also Al Kubbah, the ‘Tortoise’, paralleling the Chinese with their celestial ‘Tortoise’. Corona Australis is a poorer version of its northern cousin Corona Borealis, the ‘Northern Crown’. However, the treasures held within its borders outweigh the other, in diversity as well as in beauty.

Alpha and beta are the two brightest stars, both magnitude 4.1 and found in the north eastern quadrant of Corona Australis. Alpha is known as Alfecca Meridiana and lies 129.7 (± 5.4) light years away. This A-type main sequence white star shines with the luminosity of 29.8 (± 2.5) Suns. Beta lies 1.4° to the south, an older K-type bright giant star of a yellow hue, lying 510 (± 110) light years away and shining with the light of 460 (± 200) Suns.

Gamma Coronae Australis lies on the other side of alpha, 1° north and slightly west. Shining at an apparent magnitude of 4.2, only slightly inferior to alpha and beta, a well collimated mirror with high power will bring forth two yellow suns of magnitude 4.8 and 5.1. Both components are main sequence stars of spectral type F8 and orbit a common centre of mass every 120.42 years. The combined luminosity for this system is 5.41 (± 0.47) Suns and the computed distance is 58.4 (± 2.5) light years.

Only 1.5° to the west of gamma brings us to magnitude 4.8 epsilon, a rapidly spinning F-type main sequence or possibly subgiant star. Epsilon is also a spectroscopic binary, both atmospheres in contact as the companion rips around the primary every 0.591 days. As the stars eclipse each other, they cause the system's light to vary from magnitude 4.7 at brightest to magnitude 5.0 at minimum. The combined measured light is 8.70 (± 0.71) times that of the Sun and the distance is a relatively close 97.6 (± 4.0) light years.

Moving 1° east and slightly north of epsilon brings us to one of the most fascinating fields in the heavens. Here we find a tantalising field of heavy obscuring dust and gas, lit up here and there by the stellar furnaces ignited within. This is the playground of the hyperactive and macho T-Tauri stars, still maturing and yet to learn the rules of civilised behaviour, their unpredictable outbursts demanding attention as they develop towards the main sequence. A tour through this region is a lesson in stellar evolution, where the young progeny of interstellar clouds mingle with the older generation stars of their neighbourhood. Those of you who have access to a dark sky have an advantage here. Much of the emission and reflection nebulosity and even the dark nebulae, so beautifully delineated in a black velvety sky, are lost forever to city observers with medium apertures.

NGC 6726 and NGC 6727 are a pair of faint, round nebulae apparently in contact, forming a haze filled ‘figure 8’. The south-western portion is lit by a magnitude 7.2 A-type star that lies 440 (± 110) light years away and shines with the light of 19.9 (± 9.8) Suns. The north eastern glow contains the erratic variable star TY CorA. The spectral type is B or perhaps a later A and the light fluctuates from magnitude 8.8 at maximum to a faint magnitude 12.5 at minimum.

Only 4.7 arc minutes to the south-east lies NGC 6729, a delicate, comet-like wisp, complete with ‘tail’ which encloses the faint variable star T CorA, varying between magnitude 11.6 and 14.3. The position of nucleus is taken by another erratic variable, R CorA, the star ranging in brightness from magnitude 10.0 to 14.3. The nebula NGC 6729 itself varies in brightness as the stars within change their energy output and changes in the shape of the nebula have also been observed. Some of these changes may be due to a band of obscuring gas and dust eclipsing the nebula itself or perhaps shadow effects. These variations were first seen by J. Schmidt in 1866.

Around 9.2 arc minutes to the west of R CorA lies the variable star S CorA, ranging in brightness from magnitude 10.8 to 12.5.

Only 6.4 arc minutes to the south of S CorA lies a delightful surprise, a pair of white ‘cat’s eyes’ shining at magnitudes 6.6 and 6.8. These B8-type stars, catalogued as BrsO 14, are separated by 12.7”, far enough apart to resolve easily but close enough to enhance their jewel-like appearance.

Having a look back at my observing notes, observations with a 32.5cm telescope from Kumeu in 1991, using a power of 50x, showed all these objects within the same wide telescopic field. The nebulae were hard to see but the globular cluster NGC 6723, just over the border in Sagittarius, was an included bonus. This bright and beautiful stellar city appeared brighter towards the middle with the outer edge resolved and chains of stars radiating outwards.

If we move on back to beta then swing 1.4° east and slightly south, we will come upon a small blue fuzzy star-like glow. Changing to a higher power will magically bring forth a lovely pale blue disk, IC 1297, a planetary nebula lying around 13,300 light years away. There is no central star visible but the nebula should be seen in as little as a 15cm aperture.

There are two globular clusters in the ‘Southern Crown’ to focus upon. Both are found in the south western portion, near the border with the magnificent Scorpius. NGC 6541 lies within a beautiful starry field, its ball of family suns mostly resolved into glittering star points with larger apertures. This globular is one of the more compressed clusters with a class of 3 and the probable distance is 22,800 light years. Remember, class 1 globulars are extremely compressed, the stars packed together so tightly that resolution is difficult. On the other hand, as we range toward a class 12 globular, the stars are easier to distinguish as the cluster is progressively more scattered.

NGC 6496 is one of these scattered class 12 globulars. It lies right on the border with Scorpius and is usually catalogued as being in that constellation. I have taken the liberty in borrowing it for Corona Australis, after all, the Scorpion has more than enough. However, we come across a difficulty when viewing this one. Absorption of light by gas and dust in this area means that the stars are difficult to resolve, appearing rather faint, if at all. Those deep sky obsessives with larger telescopes will have better luck, but for mere mortals with smaller apertures, be happy with just seeing it. NGC 6496 lies around 20,500 light years distant.

Corona Australis is not over-endowed with celestial treasures but what it does have provides a cosmic laboratory of the first order.

Clear skies and good hunting ~ CosMos