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Old Nov 22nd 2016, 12:48 AM   #1
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Why gas clouds collapse if the forces are conservative?

Hello all, You all have always kindly helped me as I remember questions I have accumulated since high school days, and have never been answered. Eventually I gave up and forgot them, but as I grow old for some odd reason these things pop back up in my mind, like old ghosts from 35 years ago. So here's a ghost: if you have two small objects in space, they will not collapse. They will just fly around each other (or go in whatever orbit) forever, for the reason that gravity is a conservative force. If you have three small objects, same thing, they will not collapse. Now give me a trillion trillion trillion hydrogen molecules flying in space. Due to the sheer number, some will come close together. But, they will never actually touch each other, as they will repel and just change directions. Electromagnetism is also a conservative force, so if two molecules have some speed when they are a certain distance before that "collision", then after they collide they will have speeds that add up to the same momentum when they reach that distance. So, before collision and after collision the sum of kinetic + gravity potential + electromagnetic potential will be the same. Plus, there are no chemical reactions. If two H2 molecules come very close together, they will not form a H4 molecule, so it's impossible for them to aggregate due to some chemical reaction. Therefore, for the simple fact that energy is conserved, even if a hydrogen cloud starts to collapse, it must bounce back into a cloud, as there's nothing holding it together. So my old 14 years old self came to the conclusion that the movies showing a gas cloud "collapsing under the effect of gravity" must be false (or at least quite incomplete), because gravity and electromagnetism are conservative and there are no chemical reactions in a gas cloud. But I know that gas clouds do collapse, so where did my 14 years old self go wrong with that conclusion?

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Last edited by adamjackson; Nov 22nd 2016 at 08:54 PM.
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Old Nov 22nd 2016, 05:36 AM   #2
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The orbits for three or more bodies are only stable for a small number of quite tightly prescribed arrangements.
Bodies that do not meet in just the precise way required will transfer some kinetic energy between each other and then fly off in different directions, until they encounter another particle.
Note that bodies with a very different velocity with respect to the average for the cloud will tend to be flung out of the cloud.
Eventually you have a cloud of particles that are all moving in pretty much a coherent manner,
because all particles that weren't have gone their own way and been lost in the interstellar vastness.
Now that the spread of velocities between the particles is small, gravity can do its thing.
Note also that there are other forces acting, for example the Van_der_Waals_force
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Old Apr 25th 2017, 04:05 AM   #3
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that's a good question , and a very satisfactory explanation from woody ...

to summarize i guess it's true to say the whole gas cloud does not collapse ,part of it does , and part of it is flung away , and this satisfies the conservation of momentum.
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