by this process. In pass-through galactic collisions, or bullseye collisions, an often smaller galaxy will pass directly through the disc of an often larger spiral, causing an outward push of the arms from the gravity of the smaller galaxy, as if dropping a rock into a pond of still water. These collisions can either launch the bulge and core away from the main disk, creating an almost empty ring appearance as the shockwave pushes the spiral arms out, or shove the core out towards the disk, often creating an oval-shaped ring with the bulge still somewhat intact. In side-swipe and head-on collisions, the appearance of a perfect ring are less likely, with chaotic and warped appearances dominating. Rings formed through collision processes are believed to be transient features of the affected galaxies, lasting only a few ten to hundred million years (a relatively short timeframe considering some mergers can take over a billion years to complete) before disintegrating, reforming into spiral arms, or succumbing to further disturbance from gravitational influence. Intergalactic medium accretion This method has been inferred through the existence of Hoags object, along with UV observations of several other large and ultra-large super spiral galaxies and current formation theories of spiral galaxies. UV-light observations show several cases of faint, ring-like and spiral structures of hot young stars that have formed along the network of cooled inflowing gas, extending far from the visible luminous galactic disc. If conditions are favorable, a ring can form in the place of a spiral structure. Since some spiral galaxies are theorized to have formed from massive clouds of intergalactic gas collapsing and then rotationally forming into a disc structure, one could assume that a ring disc could form in place of a spiral disc if, as mentioned before, conditions are favorable. This holds true for protogalaxies, or galaxies just throughout to be forming, and old galaxies that have migrated into a section of space with a higher gas content than its previous locations. Gallery See also References External links Hoag's Object at Astronomy Picture of the Day. Galaxy morphological types