Eli5 Why are galaxies flat instead of being spherical like stars and planets?
In: 22
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
Latest Answers