This curious collection of un-named bright and dark nebulae is similar to the cometary globule seen in AAT 71, but is much more massive. Its swept-back shape is moulded by radiation from very luminous stars (notably Scorpii zeta-1) in the open cluster NGC 6231, the Scorpius OB association of very hot, young stars. The cluster is over a degree to the SE and is not seen here. If the dark nebula pictured is at the same distance from the sun as NGC 6321 (about 6000 light years), then the open cluster and the dark cloud it is eroding are at least 100 light years apart.

The direction of the radiation source can be seen from the flow pattern in the dark cloud and the extensive bright red rims (emission nebulae) where the gassy dust is exposed to radiation from the hot stars in NGC 6231. This part of the nebula is reminiscent of the famous Horsehead nebula in Orion, which is also the consequence of starlight destroying a dark cloud. It even contains small blue reflection nebula, like NGC 2024 near the Horsehead.

Photograph made from plates taken in March 1991 (B, G) and July 1992 (R). Top left is NE. Image width is about 32 arc min.