# Eli5: why are galaxies flat and not sphere?

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Eli5: why are galaxies flat and not sphere?

In: Planetary Science

Some galaxies are spherical!

The reason why they are rare and most are flat is because stuff orbiting in different planes is bound to collide (well, the chance of directly colliding is quite small, but it’s enough if they get close enough to affect each other via gravity). Such arrangement is not stable, so stuff tends to end up orbiting along one plane.

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.

Spinning blobs of stuff tend to flatten into discs over time. Similar to how a person making pizza will throw a blob of dough into the air with a bit of spin to get it to flatten into a nice pizza crust.

Step 1 : make a ball of pizza dough.

Step 2 : spin it.

Step 3 : it will become a flat disk !!

Step 5 : put it in the oven.

Step 6 : have delicious pizza.

you start out with a bunch of stuff orbiting in random directions in a sphere. Over time, random stuff going “up” crashes into random stuff going “sideways” or even “down”. Over time this deorbits everything moving the wrong direction into the galactic core, or it changes its orbit to a mostly flat plane with everything orbiting in the same direction.

Same principal as why the solar system is a flat disk

There is an excellent visualization of this in https://www.youtube.com/watch?v=MTY1Kje0yLg&t=190s

Galaxies can be all sorts of shapes, but there is something called the conservation of angular momentum, which relates to how the gravity pulls the stars towards each other and how new stars join the group.

Galaxies are actually spherical!

Galaxies are mostly composed of dark matter which is invisible to us, but if we could see it galaxies would look like a sphere.

The reason the luminous matter or stuff we see is flat and disk shaped is due to the spinning motion and interactions between all the gas/dust/stars (which dark matter doesn’t feel the same way)