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The Corona Australis reflection nebula

Date created: 1992-08-28

Tags: nebula, dust, emission, reflection

This spectacular reflection nebula in Corona Australis is the result of a few bright stars caught up in a large, dusty cloud. If that is all there was here, this region would be considered to be like the Pleiades, an accidental association of dust and stars. However, there are features here that show the dark cloud to be an active star forming nebula, though most of the action is hidden from view.

The peculiar yellowish curved streak just below the brightest reflection nebulae surrounds an intriguing object best seen in infrared light. It is R CrA, a young star still accreting interstellar material on to its surface. It seems to be the source of two compact but distinctly red patches, roughly equidistant from the yellow curlicue. These are are Herbig-Haro objects, often the first visible signs of star formation occurring deep inside dark clouds. These compact nebulae are ejected from proto-stars during the later stages of star formation and sometimes appear in pairs, moving in opposite directions from the hidden star-forming region. The R CrA complex is about 500 light years distant, one of the nearest star-forming regions.

The two bright blue nebulae are reflections of the light from embedded stars, not hot enough to excite an emission nebula, which would be red, but bright enough so that a small amount of light is scattered by surrounding dust grains. The grains are minute, more like smoke than dust. The star in the southernmost nebula (IC 4812) shows two distinct diffraction spikes, artefacts of the optical system used to photograph it. This implies that there are two stars here, and this is a the well known double star BrsO 14, discovered and catalogued at Governor Brisbane's observatory in Parramatta, near Sydney, in the 1820s. The brighter reflection nebula also contains a widely spaced pair of double stars.

Photograph made from plates taken in June 1991.
Top left is NE. Image width is about 23 arc min.

Credit: David Malin

© Australian Astronomical Observatory