Here is a video showing two star clusters colliding, lurching round each other, and eventually settling into one. Quite a few stars get catapulted out of the picture in the process.
What I find interesting about this is that it all comes from one simple force, gravity. Gravity pulls things towards one another; or, if they are whirling round each other, it keeps them whirling rather than just zooming off into space. That is all – and yet, looking at the video, one gets a sense of viscosity, as if the clusters were embedded in invisible treacle.