Simulated light curve from an eclipsing binary. Image credit: ESA.
An Einstein ring is 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). An Einstein ring image can be up to 30 times brighter than the image of the distant galaxy would be in the absence of the lensing effect.
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.