Open star clusters are usually more or less ball-shaped with a dense nucleus and peripheral stars scattered around it. Obviously their shape derives from the shape of the gaseous cloud from which they emerged and over time the force of gravity of our galaxy slowly tears them apart, sending the member stars on their lonely journey through adulthood. There are of course several exceptions and I refer to my sketch of Berkeley 4 to show you just one of many examples. But even in this case the stars appear merely "smeared out", which can be explained fairly easily with gravity.
NGC6645's annular shape, on the other hand, is much more difficult to explain. It gets even more complicated when analysis of 72 of its member stars revealed that this cluster has an age of some 9.7 billion years! Most star clusters don't survive for more than a few hundred thousand up to a billion years, especially when they reside in the gravitational plane of our galaxy.
Whatever the mechanics behind it, this particular cluster is one of my all-time favourites, not just because of its appearance but also because of its richness and countless starry filaments. I hope that my sketch transmits the emotions it gives me every time I point my telescope at it.