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Why nebulae and molecular clouds collapse into accretion discs rather than accretion spheres? I mean why would it collapse into a 2D form in a 3D universe? Same thing with galaxies? Why are they giant spinning discs rather than giant spinning spheres? And why everything has to spin in the universe (moons, planets, stars, galaxies, etc...) ?
The basic answer to your question is Conservation of Angular Momentum. Astronomers have been interested in this question for a long time and are still writing papers on the subject.
Even if you assume that at the time of the big bang there was no net angular momentum in the universe, one expects that there were local fluctuations. When galaxies initially formed by gravitational attraction these net local amounts remained. Now angular momentum is conserved so when the initial collection of stars and gas in one galactic volume start to collapse the net angular momentum has to stay the same. Since angular momentum L =mvr2
, for a given v its easier to conserve L if the r is large. In the orthogonal direction (along the axis of the accidental angular momentum) there is no such hindrance to collapse and so the collection turns out to be disk shaped.
By the way there has been several studies on the net angular momentum of a large collection of galaxies coming from the Sloan Digital Sky Survey and the answer is pretty close to but not quite zero. Stay tuned.
(published on 11/23/11)
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