A ring galaxy is a galaxy with a circle-like appearance. Hoag's Object, discovered by Art Hoag in 1950, is an example of a ring galaxy. The ring contains many massive, relatively young blue stars, which are extremely bright. The central region contains relatively little luminous matter. Some astronomers believe that ring galaxies are formed when a smaller galaxy passes through the center of a larger galaxy. Because most of a galaxy consists of empty space, this "collision" rarely results in any actual collisions between stars. However, the gravitational disruptions caused by such an event could cause a wave of star formation to move through the larger galaxy. Other astronomers think that rings are formed around some galaxies when external accretion takes place. Star formation would then take place in the accreted material because of the shocks and compressions of the accreted material. Formation theories Ring galaxies are theorized to be formed through various methods including, but not limited to, the following scenarios: 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. 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. Galactic collisions Another observed way that ring galaxies can form is through the process of two or more galaxies colliding. The Cartwheel Galaxy, galaxy pair AM 2026-424, and Arp 147 are all examples of ring galaxies thought to be formed