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.
|An Einstein ring. Image credit: Hubble Space Telescope/J.
L. King, University of Manchester.
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.