<p>If we assume that mass was initially uniformly distributed in an area, then why does it flatten out after long time?</p>

<p>For example, consider our solar system or most galaxies out there.</p>

CC BY-SA 3.0
2017-03-16T17:33:06.827Z
Some confusion here. Your question seems to be about galaxies, but you mention our solar system as an example. The solar system is not a galaxy. Can you edit your question to clarify.
James K
CC BY-SA 3.0
2017-03-16T18:45:57.780Z
Minutephysics has a very nice quick video that explains this for solar-systems.    https://www.youtube.com/watch?v=tmNXKqeUtJM
userLTK
CC BY-SA 3.0
2017-03-16T20:05:13.627Z
Also [Why are galaxies disk shaped?](http://astronomy.stackexchange.com/questions/1145/why-are-galaxies-disk-shaped) deserves a better answer, because it's not enough to say "gas will radiate away some components of angular momentum".
kubanczyk
CC BY-SA 3.0
2017-03-16T20:51:01.983Z
I agree with @kubanczyk. Physics.SE has a much better answer: http://physics.stackexchange.com/a/93844/70207
pela
<solar-system><galaxy>
Why do most galaxies or planetary systems have disc shape?
Udayan Joshi
# Why do most galaxies or planetary systems have disc shape?

(Asked by: username_0 on 2017-03-16)

If we assume that mass was initially uniformly distributed in an area, then why does it flatten out after long time?

For example, consider our solar system or most galaxies out there.

**Tags:** solar-system, galaxy
> **Comments:**
> **username_1**: Some confusion here. Your question seems to be about galaxies, but you mention our solar system as an example. The solar system is not a galaxy. Can you edit your question to clarify.  
> **username_2**: Minutephysics has a very nice quick video that explains this for solar-systems.    https://www.youtube.com/watch?v=tmNXKqeUtJM  
> **username_3**: Also [Why are galaxies disk shaped?](http://astronomy.stackexchange.com/questions/1145/why-are-galaxies-disk-shaped) deserves a better answer, because it's not enough to say "gas will radiate away some components of angular momentum".  
> **username_4**: I agree with @kubanczyk. Physics.SE has a much better answer: http://physics.stackexchange.com/a/93844/70207  

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