system
Given the following post, respond as if you're commenting on Reddit:
human
Why does gravity make planets and stars spherical but galaxies thin disks?

Also, is there a standard shape to super clusters of galaxies?
gpt
Hydrostatic equilibrium. Stars and planets are supported (prevented from collapsing) by the physical pressure of the solids or fluids that make them up. Galaxies, on the other hand, are prevented from collapsing by the velocity of their component stars-- each star is on an orbit around the galaxy. Disk galaxies form out of collapsing clouds of gas (just like solar systems do), which always form disks, because disks are the only stable way to have collisional gas orbiting a central object over many orbits. Otherwise the gas streams will just run into each other and trade angular momentum until they've equilibrated to coplanar orbits.

Superclusters do not have a standard shape. This is essentially because the time it would take for them to undergo many dynamical timescales (the time for galaxies to complete an orbit around the center) and thus become regular in shape is longer than the age of the universe. 

Galaxy clusters often do have a somewhat regular shape, at least in the sense that they have a large cD elliptical galaxy sitting at the middle and a spheroidal distribution of galaxies around it, with a gradually decreasing density of galaxies as you get farther away from the center. But even so, they are much less regular than the shapes of most galaxies, because they take much longer to smooth out the irregularities, and there are more galaxies falling into the cluster all the time.