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The head of the Corona Australis reflection nebula

Date created: 2001-08-08

Tags: nebula, dust, cluster, globular, reflection

The constellation of  Corona Australis (the southern crown, CrA) is in the far southern sky but visible from the southern states of the USA. The constellation is small but distinctive. The conspicuous globular cluster NGC 6723 is at the western (right) edge of the photograph, but it is in Sagittarius, and is about 30,000 light years distant, while the wispy dust cloud that dominates the picture is about 500 light years away.

The image above is about 3 degrees across and the faint Corona Australis nebula meanders along the Sagittarius–Corona Australis border in the same E-W direction. Almost all the nebulosity here is starlight, reflected from minute grains of dust, some of which gather into darker condensations ('molecular clouds'), blotting out the background stars. Fainter, more extended features can be seen in the deep picture.

By far the largest and densest of the molecular clouds is in the centre of the photo. It is about a degree long, corresponding to eight light years at the distance of the nebula and is extremely opaque — background stars are dimmed by an astonishing 35 magnitudes. However, not all is darkness, and the dusty cloud appears to be tipped by a pair of bright stars, embedded in bright reflection nebulae. The brightest of these is NGC 6726–27 and it contains both a widely spaced visual binary and a variable star. The other bright reflection nebula, IC 4812, is illuminated by rather closer pair of binary stars, while other wispy nebulae in the western part of the dark cloud betray the presence of very young, hidden but still-forming stars.

Photograph made from plates taken in 2001, April, May, June, R, B, G. Top left is NE. Image width is about 3 degrees.

Credit: David Malin

© Australian Astronomical Observatory