Galaxies like ours are mostly a thin, flat disc, wrapped in a loose cloud of older stars called the halo. If the whole thing started out as one big spinning cloud of gas, why didn’t it all end up the same shape?

It comes down to what bumps into what. Gas behaves like a fluid, so when two streams of it run into each other they actually collide. The motion going up and the motion going down cancel out, and the energy gets carried off as heat. The spin can’t go anywhere, though, since angular momentum is conserved. Over a long time the gas keeps losing its vertical motion while holding onto its rotation, and it flattens into a disc.

Stars are a different story. Some of the gas turns into stars along the way, and stars basically never hit each other because the distances between them are enormous. Whatever orbit a star is born on, it keeps.

In the visualizer you can watch both happen. The pink is gas, and you can see it slowly settle into the plane. The gold stars formed early, before the gas had flattened, so they’re still swinging high above and below the disc on tilted orbits. The blue ones formed later, inside the disc, and they stay in it.

So the gas went flat because it could collide, and the early stars stayed puffy because they couldn’t.