This is the most detailed and beautiful image of Wd 1 I've seen. Thanks for spotting it and posting it on IIS, Robert. The image clearly reveals the small-scale differential reddening from Wd 1's own dust shell compared with the broader reddening from our Galaxy. Also Wd 1's elongated shape, which similarly shows up in Wd 2 and RCW1 in the heart of the Tarantula Neb in the LMC, and also in the Arches and Quintuplet in the Galactic bulge region.
Elongation is a property of young clusters caused by a non-spherical molecular cloud collapse. Clusters smooth out their wobbly shapes over time through a several-stage process of virializing (bring mass density and energy density into equilibrium). First they get rid of their excess birth energy by shedding unused natal gas, about half their total original mass. They overdo it and suddenly find themselves with about the same energy output as before but only 30% to 50% as much mass. So they try to virialize, or rebalance, through mass segregation. The heaviest stars concentrate into the core and the lightweight stars ease away into the halo or escape altogether.
Then the overdo THAT and find themselves with too much core mass and not enough kinetic energy to hold it all together. A period of binarism ensues, going from roughly 40% binaries to upwards of 70%. Unfortunately, that also ejects more stars to the periphery. When a star is caught up into a binary's rotational path, the lightest of the three stars will be ejected. Finally the core expands slightly in post core-collapse virial equilibrium. What's left of the cluster can finally age gracefully into a billion years of retirement.
Notice that all these interactions are not of mass and energy as such, but rather mass density and energy density. Density level is like a sliding scale of interactive intensity. There's a large number of variables in this, which is why we have such a rich variety of star clusters.
All the above holds only in cases where the cluster doesn't have too many Galactic disc crossings to put up with. Add those into the cluster's travails—to say nothing of chance encounters with massive footloose molecular clouds falling into the Galactic disc—and is it any wonder why we see so few old clusters?
