An unusual galaxy with a well-defined luminous ring around a bright core. The ring can appear smooth and regular, or knotted or warped, and may include gas and dust in addition to stars. Such an arrangement may result from a collision of a disk galaxy with another smaller galaxy, which passes through the disk. An example of a collisional ring is the Cartwheel Galaxy. Other rings may be formed by the action of galactic bars or orbital resonances.

http://hubblesite.org/image/2123/news_release/2007-17