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Comment by wkat4242 | Hacker News Reader

I wonder why galaxies tend to be shaped like a disc rather than a ball. Doesn't really make sense to me.

If you started with a galaxy that is a nearly uniform ball, the gas and bodies near the equator would attract the gas and bodies that are further out.

Those further out would tend to collapse towards the equator and eventually that initial ball would become more disk shaped.

Then there is this explanation refering to the conservation of angular momentum: https://van.physics.illinois.edu/ask/listing/18318

It sort of does to me - disks made up of concentric orbits are have no intersections, and so collisions are less likely. In a ball, debris would have different directional vectors that would eventually collide with one another. The disk is the stable evolution of that state.

I believe it's not that collisions (and gravitational interactions) don't happen when it's in a disk, but the galaxy as a whole has an overall rotational momentum about some axis, and collisions within the galaxy can't change that. This means that it can flatten along the axis, but it can't shrink in the perpendicular plane.

If everything were twice as close to the axis (as gravity would like it to be), it would need to be moving twice as fast to conserve rotational momentum, but that's faster than the velocity of a circular orbit at the new radius (assuming everything was in a circular orbits to begin with).

The same principle is why almost everything in the solar system approximately lies within a plane.

Because galaxies spin. A spinning ball will compress into a disc after a while. The parts which are close to the axis just fall into the middle and some amount of friction slows them down until they stay close to the plane.

Elliptical galaxies are ball-shaped.

https://en.wikipedia.org/wiki/Elliptical_galaxy

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