An Einstein ring. Image credit: Hubble Space Telescope/J. L. King, University of Manchester.
A gravitational lens effect in which the image of a remote background object, such as a quasar, is distorted into a ring by a foreground galaxy. A perfect ring will only result if the source, the lensing object, and the observer are exactly lined up, and, in addition, the mass of the lensing object is evenly distributed (see Einstein Cross). A few good approximations to Einstein rings have been found, such as MG1131+0546 and B1938+666. In other cases, where the alignment is not perfect, the gravitational lens produces one or more arcs.
