Eli5: why are galaxies flat and not sphere?
In: Planetary Science
Conservation of angular momentum
Originally there have been clouds/nebulas of particles that have been in more complex shapes and not flat disks. All those particles moved in all kinds of directions so at that time they were orbiting in a sphere and not on a flat disc like now.
If you would count all the individual particles and add where they are moving you would get the total momentum and angular momentum of this nebula which would have a sligth preference in some direction.
Now over a long period of time due to gravity particles would be attracted towards the center of this nebula and eventually form stars and planets. However as this would be happening there would be many collisions between individual particles which would lead to them exchanging momentum (and angular momentum).
Eventually these collisions will lead to varied angular momentum between all ofthese particles averaging out until all of them have one that is closer to each other which means that they will move on a relatively flat plane and not be a sphere.
There might be some innacurracies in this description but it is how it was explained to me why solar system is flat.
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