Eli5 Why are galaxies flat instead of being spherical like stars and planets?


Eli5 Why are galaxies flat instead of being spherical like stars and planets?

In: 22

Imagine the stereotypical pizza maker. They grab a blob of dough, then fling it into the air while it rapidly spins. The spinning flattens it out into a pizza.

Galaxies started out as blobs spinning rapidly, so they also flattened out into cosmic pizzas.


Stars and planets aren’t perfect spheres, they bulge a bit in the middle due to their spin. The galaxy is the same thing- it’s spinning so quickly (in relation to it’s size) that it’s flattened out.

It’s like when you’re on a merry go round, the faster it goes the more you’re pulled away from the center.

There are other elements to this, like dark matter for example, but that’s the basics

Same reason dollar systems are flat – they are formed as an [accretion disk](https://en.m.wikipedia.org/wiki/Accretion_disk)

Galaxies and solar systems start as clouds of gas and dust. These particles are going in random directions and bounce into each other. When they bounce into each other they transfer momentum, sometimes they stick together. As they stick together they’ll form bigger clumps which will collide and make even bigger rocks, etc.

Throughout this whole process, the total momentum of the system does not change, it just transfers from particle to particle, Rick to rock.

Importantly, while the total momentum of the system is random (as it’s the sum of the momentum of the original randomly moving dust and gas particles) it is not 0, it will have some angular momentum – that is to say the particles at the start are imperceptibly orbiting the center of the overall system.

As the particles and rocks and planets and stars slowly accumulate mass, they will tend to be moving in the same orbital direction as they are made of particles that tended to be orbiting in that general direction.

They won’t all be moving in that direction though, some dust particles will be moving the opposite direction, some rocks will be orbiting perpendicular to the orbital plane, some random asteroid flung from a nearby system may be going the wrong way, but since most of the particles are moving in the same orbital plane, anything going the wrong way will eventually hit something going the predominant direction. If theyre similar sizes, their momentum cancels out and most of the debris from their collision will fall into the center of the system (in galaxies that’s a black hole, in solar systems that’s a star)

Eventually all that’s left in orbit is stuff that happened to have been going the same orbital direction as most of the other stuff in the system

Let’s start by assuming a galaxy starts off sphere-like where stars and planets are orbiting a black hole. Over time, there will be an imbalance as some planets and stars are clumped together in one region of this sphere as orbits go at different velocities. This means that a large of amount of mass is accumulated in that spot. What happens? Gravity happens. So even my miniscule amounts nearby planets and stars would be attracted to this mass. Over a long time this consolidates into a disk-like shape as this mass is concentrated in this equatorial region. Where it may deform is when there is another galaxy nearby, when it is in the process of merging, or post-merger.

The guy who mentioned pizza is fairly good making it easy to understand though.