What makes an accretion disk flatten and spin?

Question

Also - why don't globular clusters flatten and spin?

Answer 1

For any grouping of gas, it's extremely unlikely that the rotation about its center of mass is zero, so it will have a tendency to rotate along a certain axis when gravity begins to pull the particles together.  As for forming a disk, the gas perpendicular to the axis of rotation will have a tendency to maintain its path, whereas other gas will have a tendency to oscillate towards that gas.  When the gas collides, its energy is absorbed into the disk, preventing most of it from coming out the other side.

As for globular clusters, they do spin, but they are much larger than the accretion disk on a single body, so the effect is not as great.  Additionally, local gravitational effects play a much larger role because globular clusters are so tightly bound, shooting stars here and there within the cluster itself.

Answer 2

They are gravitationally attracted (spacetime is warped in the direction of) the tremendous mass at its centre. When a non rigid mass rotates it assumes an oblate shape, such as may be easily seen in photographs of Jupiter.

This is because of centrifugal force, which is at a maximum at the "equator", and a minimum, at the poles. With both liquids and gases, those forces are distributed in a similar manner, due to random collisions of molecules and atoms, or with stars, plasma, and all known stars rotate.

The law of conservation of angular momentum causes them to accelerate their rate of rotation, as they shrink, like an ice skater, drawing in her arms and rotating faster. This causes more centrifugal force to be distributed by those collisions, resulting in a disk shape.

Globular clusters are bound gravitationally to a lesser extent, without such a relatively huge mass in their centre, and their is a great deal of "vacuum" between them, unlike gases, plasma, or liquids, where electromagnetic fields are brought into such close proximity as to interact strongly.

When a coconut is dropped from atop a skyscraper, it is the electromagnetic fields interacting, which bring it to a sudden halt on the concrete below, showing that electromagnetism is a much stronger force than gravity.

Answer 3

I am not sure why accretion disks flatten exactly, but...

From http://en.wikipedia.org/wiki/Accretion_d...

"Instabilities within the disc redistribute angular momentum, causing material in the disc to spiral inward towards the central body."

From http://www.scholarpedia.org/article/Accr...

"According to a semi-analytic understanding developed over the past thirty years, the high angular momentum of matter is gradually removed by viscous stresses and transported outwards. This allows matter in the accretion disc to gradually spiral down towards the gravity center, with its gravitational energy degraded to heat."

As for the globular clusters thing...  I'm pretty sure that globular clusters DO spin.

Answer 4

Globular clusters do not spin they stay still and collect more stars. The accretion disk flatten and spin do to the unbalanced force of gravity.