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# Why are galaxies flat rather than spherical?

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1 minute ago, Davey-T said:

Learnt all about the conservation of angular momentum at school ( 60 odd years ago ) so a bit hazy, couldn't remember what bump started it all

Dave

If you want a bit of a giggle due to angular momentum go onto the roundabout in a kids play park (helps if you have a grandchild or similar  otherwise you will get strange looks).  Spin the roundabout then stand close to the centre and while holding onto the bars try kicking the centre post - your leg will appear to have a life of it's own.  When you see the groundskeeper coming scarper

Jim

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I was on a roundabout yesterday with my 4 year old grand-daughter.  But the above post wasn't posted then or I would have tried it lol.

I understand the spinning thing flattening out into a pancake, but the rest of the physics once they start getting into formulae gives me a headache.

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20 hours ago, Davey-T said:

So why does everything start spinning why can't it just contract and sit still ? can they not just "fall" into the warped space and sit there unmoving ?

Dave

Because you need some force to stop things moving, which is one of the fundamental laws of physics.  Unless they literally crash into each other which, in space, is extremely rare then the object continues moving past the object until the gravity well pulls it back and hence you have an orbit.

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Ok, I get the spinning Pizza/angular momentum idea, but then you have already flattened the dough beforehand. I would have thought stars that clump together to form galaxies would orbit in arbitrary directions. Obviously I am wrong in thinking that.

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On 27/01/2020 at 01:54, Davey-T said:

Dave

There must have been motion of some kind to trigger the implosion of vast, diffuse gas clouds into stars. A gravitational effect, a supernova shock front, or whatever, gives part of the cloud a nudge, causing denser patches to appear and gravitationally attract more matter. This would never happen in perfect symmetry so there is bound to be a preferential direction of movement which will increase due to the conservation of angular momentum during the collapse.

(This does not come with my normal four-hour guarantee.)

lly

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3 minutes ago, ollypenrice said:

There must have been motion of some kind to trigger the implosion of vast, diffuse gas clouds into stars.

Seems to be a lot of, should've, could've, must've, I want to see a big blackboard covered in equations to prove it

Dave

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Just now, Davey-T said:

Seems to be a lot of, should've, could've, must've, I want to see a big blackboard covered in equations to prove it

Dave

Not from me, you don't, they'd all be wrong! Expecting perfect symmetry in a star-forming collapse would be like expecting to balance a pencil on its point, eventually and with much effort. While I'm not much up on the maths of pencil-on-point balancing I've read that it can be demonstrated mathematically to be impossible. Whether or not this is true I think we all know that it isn't going to happen.

Olly

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Think this applies.

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6 minutes ago, ollypenrice said:

Whether or not this is true I think we all know that it isn't going to happen.

You'll be telling me next there's no Santa Clause

Dave

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7 minutes ago, Davey-T said:

You'll be telling me next there's no Santa Clause

Dave

I'd never tell you that, Dave. All amateur astronomers need to believe in Santa.

Olly

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10 minutes ago, Davey-T said:

You'll be telling me next there's no Santa Clause

Dave

Was “Clause” an intentional misspelling?

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Of course you could have been refering to the movie.

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The simple answer is: Equilibrium and Symmetry.

Spheres are formed when the dominant force resisting gravity is pressure. Pressure has a spherical symmetry and equilibrium is found in a sphere in this cases (planets, stars)

Discs happen when orbital motion is resisting to gravity. This is a circularly symmetric effect and equilibrium is found in a disk shape (solar systems, galaxies, quesars)

Long answer in the newtonian physics...

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21 hours ago, Viktiste said:

Ok, I get the spinning Pizza/angular momentum idea, but then you have already flattened the dough beforehand. I would have thought stars that clump together to form galaxies would orbit in arbitrary directions. Obviously I am wrong in thinking that.

Ah , are we now not talking about two different rotations in different axis of rotation which is closer to reality.   The clump of stars then may well be orbiting around a centre of mass , which will still result in a flattening out of the clump, superimposed on that will be flattening out of stars due to the period rotation of the star itself on its own axis.   I suspect that the greater flattening out effect will arise against the clump of stars itself owing to presumably a higher rotational speed and hence higher centripetal force. Basically if the mass spins in whatever axis or plane it will be subject to the centripetal force which tends to flattening out of the mass.

Jim

Edited by saac

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