Why do discs, like rings of Saturn and the spiral shape of our galaxy form around massive objects, instead of just a (spherical?) cloud of matter?
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1$\begingroup$ Possible duplicates: physics.stackexchange.com/q/26083/2451 , physics.stackexchange.com/q/93830/2451 , physics.stackexchange.com/q/12140/2451 and links therein. $\endgroup$– Qmechanic ♦Commented Oct 24, 2014 at 5:35
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$\begingroup$ @Qmechanic: I didn't find the answer to this particular question in any of the links you gave. $\endgroup$– bright magusCommented Oct 24, 2014 at 6:03
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$\begingroup$ I think this is the best explanation youtube.com/watch?v=tmNXKqeUtJM $\endgroup$– ThainaCommented Oct 24, 2014 at 11:27
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1 Answer
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In the absence of angular momentum, then material would be able to follow radial paths to be accreted (because of their mutual gravitational attraction) and thus have a spherically symmetric distribution.
For many reasons (see linked questions to the right), the "circum-object" material does have angular momentum, which must be conserved. In the co-rotating frame of reference this lends additional centrifugal support to the material against gravity. Thus, whilst gravitational collapse along the rotation axis may be possible, perpendicular to this axis the material can be supported until it loses its angular momentum in some way. The material will find an equilibrium orbit, and only by losing angular momentum ($L \propto r^{1/2}$) can it fall further in.
So material with angular momentum, but comparatively much less internal support in along the rotation axis (e.g not much pressure or velocity dispersion) will tend to form a flattened disk.
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$\begingroup$ But how this angular momentum of the central body is transferred to the gravitating ones? Gravity cannot explain it, since it is a force working in straight line between two bodies, which means it has no tangential component. $\endgroup$ Commented Oct 24, 2014 at 9:10
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$\begingroup$ I'm not sure what you are thinking of. There does not have to be any transfer of angular momentum from the central body. The central body could have zero angular momentum and you would get the same result. It is the particles/gas in the disk that have angular momentum. $\endgroup$– ProfRobCommented Oct 24, 2014 at 11:22
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$\begingroup$ However if there is transfer of angular momentum (e.g. through linkage with magnetic fields or through a magnetised wind) then this simply adds to the angular momentum that the disk material has, reinforcing the mechanism I describe above. $\endgroup$– ProfRobCommented Oct 24, 2014 at 11:24
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$\begingroup$ Apparently I misunderstood you. OK, what is the rotating frame of reference in your description above? What is rotating around what? $\endgroup$ Commented Oct 24, 2014 at 11:45
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$\begingroup$ I mean co-rotating with the material. In this frame of reference objects feel non-inertial forces - the centrifugal force (and the coriolis force, which is not so important here). $\endgroup$– ProfRobCommented Oct 24, 2014 at 12:29