Why Do Galaxies Have a Spiral Shape? Understanding Gravity's Role

[Sniperman724]

If gravity acts on all angles of a mass, then why do galaxies appear to all have the same basic spiral shape with two relatively flat areas on opposite sides of it? Shouldn't all galaxies technically be big sphere looking shapes?

 

[russ_watters]

Spheres are tough because they would require orbits of random, different inclinations, which is unlikely for a galaxy that started as a single, rotating cloud of gas...

...just like our solar system.

 

[zhermes]

If gravity acts on all angles of a mass, then why do galaxies appear to all have the same basic spiral shape with two relatively flat areas on opposite sides of it? Shouldn't all galaxies technically be big sphere looking shapes?

Like russ said, the main problem is angular momentum. With a bunch of angular momentum, the galaxy can't turn into a spheroid; instead it will end up more like a disk, and then instabilities lead to spokes, bars, etc.

But its worth pointing out that large, old galaxies that have undergone mergers due become much more spheroidal---these are called 'elliptical galaxies' (because of their shape).

 

[Chronos]

A spherical mass that is rotating will naturally tend towards a disc shape due to centrifugal forces - especially with a dark matter halo.