Editing Ring galaxy (section)

===Bar instability===

A phenomenon where the rotational velocity of the bar in a [[barred spiral galaxy]] increases to the point of spiral spin-out. Under typical conditions, gravitational density waves would favor the creation of spiral arms. When the bar is unstable, these density waves are instead migrated out into a ring-structure by the pressure, force, and gravitational influence of the baryonic and dark matter furiously orbiting about the bar. This migration forces the stars, gas and dust found within the former arms into a torus-like region, forming a ring, and often igniting star formation.

Galaxies with this structure have been found where the bar dominates, and essentially "carves out" the ring of the disc as it rotates. Oppositely, ring galaxies have been found where the bar has collapsed or disintegrated into a highly-flattened bulge. 

Other instabilities have been predicted due to asymmetries caused by spiral arms, which creates a net torque depending on the pattern of the bar. This torque creates a resonance instability that drives matter from the arms both outwards and inwards towards the core, creating the distinct ring and core structure.<ref name=":0">{{Cite web |last=Buta |first=Ronald James |last2=Combes |first2=Françoise |year=1996 |title=Galactic Rings |url=https://ned.ipac.caltech.edu/level5/Rings/frames.html |access-date=2025-02-16 |series=17 |publisher=Fundamentals of Cosmic Physics |pages=95-281 |via=NASA/IPAC Extragalactic Database}}</ref> 

Despite this, observations suggest that bars, rings and spiral arms have the ability to fall apart and reform over the span of hundreds of millions of years, particularly in dense intergalactic environments, such as galaxy groups and clusters, where gravitational influences are more likely to play a role in the morphological and physical evolution of a galaxy without the influence of collisions and mergers.