Why Do Star Systems and Galaxies Flatten in the 3rd Dimension?

[Dropout]

Like entropy tends towards chaos, why do star systems or galaxies tend to reduce their movement through the 3rd "Z" dimension even though all particles can freely move in all 3 dimensions? Flattening out.

Is there some sort of word or phrase which describes this behavior? Is it possible, interacting matter naturally tends to reduce the number of dimensions they are operating in?

 

[NeoDevin]

It's due to conservation of angular momentum.

Galaxies and star systems form out of a cloud with some fixed angular momentum. They will tend to flatten out in the plane normal to their angular momentum.

It has nothing to do with entropy.

 

[Dropout]

I know it has nothing to do with entropy, but is there some sort of sub-topic word describing this behavior other then simple angular momentum?