system
Given the following post, respond as if you're commenting on Reddit:
human
Why do galaxies collapse into disks, but stars and planets collapse into spheres?
gpt
One way to answer your question is that stars *don't* collapse into spheres. Stars collapse from a cloud of gas and dust, and while most of that goes into the central spheroid (the star), there's a lot left over which collapses into a disk, and it is from this disk which planets are made.

The difference between the sphere and the disk is that the sphere is made of stuff that's "self-gravitating"; it's massive enough to attract nearby stuff.

A famous example of a planet which didn't collapse into (just) a sphere is Saturn. The main body formed the spherical mass, but smaller masses of ice which didn't fall into the central body collapsed into a disk forming the rings. But the rings don't contain anything massive enough and near enough other parts of the ring to cause it to clump up.

So, disks form around massive bodies when the material in the disk is spread out enough for it to miss future collapse, and spheres form when the material is dense enough to fall into itself.