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

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Discussion Overview

The discussion revolves around the reasons behind the spiral shape of galaxies, particularly focusing on the role of gravity and angular momentum. Participants explore theoretical aspects of galaxy formation and structure, including comparisons to the solar system and the influence of dark matter.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question why galaxies predominantly exhibit spiral shapes instead of spherical forms, suggesting that gravity acts uniformly on mass.
  • Others argue that the initial conditions of galaxies, such as starting as a rotating cloud of gas, lead to a preference for disk shapes rather than spheres.
  • It is noted that angular momentum plays a crucial role in preventing galaxies from becoming spheroidal, resulting in disk-like structures with potential instabilities that create features like spokes and bars.
  • One participant points out that large, old galaxies that have experienced mergers can become more spheroidal, leading to the classification of these as elliptical galaxies.
  • A rotating spherical mass is said to naturally evolve into a disc shape due to centrifugal forces, particularly when influenced by a dark matter halo.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the formation and shape of galaxies, with no consensus reached on the primary factors influencing their spiral structure.

Contextual Notes

The discussion highlights the dependence on initial conditions and angular momentum, as well as the influence of dark matter, but does not resolve the complexities involved in galaxy formation.

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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?
 
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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.
 
Sniperman724 said:
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).
 
A spherical mass that is rotating will naturally tend towards a disc shape due to centrifugal forces - especially with a dark matter halo.
 

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