system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it an accretion disc and not a spheroid or cloud?

I'd imagine it's the same thing that gives galaxies their shape? I didn't get the opportunity to ask during my intro geology class but it's something I've been wondering for a bit.
gpt
It's all about angular momentum. When you have a cloud of particles, either freestanding or around a stellar body, all the particles are orbiting the center of mass of either the cloud itself or the body. Either way, they're all moving on their own orbits, mostly at random. And when you compound all these movements and orbits, you can calculate the movement of the whole cloud, and it's angular momentum, which is a measure of how much it's spinning.

The cool part about angular momentum is that, unless acted upon by an outside force, it never changes. So what happens is, over time, particles that are moving not on the plane of rotation (the eventual "disc", if you will) will collide with each other and cancel out their perpendicular momentums, but not their rotational ones, and will tend to drift towards the disc plane. Eventually, enough mass accumulates on the disc to start attracting any stray particles and the disc is formed.

I should note that the particles in the disc don't perfectly orbit the center of mass like Earth does around the Sun. Their movement is akin to what the Solar System does around the galactic core, bobbing up and down the disc while rotating around the center.