Accretion is a process in which a massive object grows by gravitationally attracting more matter, typically gaseous matter in an accretion disc. My question is: How do accretion disks gain angular momentum in the first place? Also when the accretion disk starts to accrete more matter, why does the accretion disk then not lose its angular momentum?
Accretion disks not gaining any angular momentum, only the angular velocity is going crazy because the matter is getting sucked into a small region of space. For the second question, I think it will lose its angular momentum if the matter being sucked was moving in "wrong" direction in the first place.
But why do accretion disks have angular velocity in the first place? Why do they not just get sucked into a small region of space in a linear or straight line?
Good question. Ben, you should move this to Astronomy/Cosmology. I have theorized about this in prior threads in Astronomcy/Cosmology along these lines: When a supernova goes off somewhere near a large, cold cloud of H/He gas (which there are lots of in galaxies), it will do so somewhat off-set from the center of the cloud, and will impart a momentum of hot gas into the cold cloud of gas from the supernova residue, at very high temperature. That initial injection of hot gas imparts the angular momentum of the cloud of cold gas, which as it contracts and compresses will evenly mix the hot gas, warming the cloud, yet retaining that angular momentum, causing the cloud to begin rotating about a center. Voila.
It helps your intuition if you first consider a small object whose initial velocty is directed toward the center of mass of the attractor. It will fall directly to the attractor. Then consider a small object whose initial velocity is not directed toward the center of mass of the attractor. This object might orbit the attractor. It might approach the attractor along a spiraled path. A lot of gas or small objects like the latter will form an accretion disk.
A small object approaching a larger mass can do so from any direction and off set. Why then does the resultant encounters result in a disc and not a spherical shell of orbiting objects? I understand that there will be collisions and mutual gravitational attractions that could continually alter the nature and composition of this sphere, but cannot see how it always degrades to a disc. My biggest question related to this is why doesn't matter fall into a black hole from ALL directions?
Well usually a black hole sucks in matter from something like a nearby star and they are usually orbiting each other in some way, so you have your accretion plane right there. They aren't usually just smack in the middle of a big gas cloud or something. But, even if they were, the total system would have some average angular momentum, about some axis which defines a plane. I can imagine that as the system condenses the collisions between particles become more frequent and the transverse components would eventually cancel each other out.
