A collapsing star?
Fusing nucleons?
Neither, it's 1024 circles attracting each other in Algodoo. The circles are perfectly elastic so they can't get rid of the KE that they gain as they fall towards each other, this makes the formation a bit unstable even though they do manage to kick out some KE with the ejected particles. Add air resistance and the particles settle down into a circle. Lots of possibilities here.