Why do galaxies appear flat?

Question

Audrey Adams · Accepted Answer

Objects in a galaxy with high tangential (sideways) rotational speed will remain spread out but those with lower sideways speed will come together along an equatorial band, forming an accretion disk.

Every galaxy and solar system begins as an amorphous or almost globular cloud of gas, dust, and rocks; however, as the cloud contracts and shrinks due to mutual gravitational attraction, certain regions are traveling more quickly in a tangential direction than others. If the majority of the side-moving components are along one band of rotation, that band will become the primary "equator" of the rotating system. The components with less side-moving motion will be drawn more toward the center of the rotating cloud than the faster-moving side-moving components, which will maintain their distance from the center due to centrifugal effects. The cloud thus starts to flatten along one major axis and begins forming an accretion disk (shaped like a discus). Those with a high sideways speed will be the furthest from the core, while those with a lesser sideways speed will be closer in and would collide or merge with those objects nearer the equatorial band.

Wesley Alvey · Answer

undefined Galaxies appear flat due to the combined effects of gravity and angular momentum. As gas and dust within a galaxy collapse under gravity, they form a rotating disk shape. This rotation causes the material to flatten out into a disk-like structure, similar to a spinning pizza dough flattening out into a disk. Additionally, the centrifugal force generated by this rotation counteracts the inward pull of gravity, resulting in a stable, flat shape.