Gravitational lens

A gravitational lens is formed when the light from a very distant, bright object (such as a quasar) is "bent" around a massive object (such as a massive galaxy) between the bright object and the viewer. The process is known as gravitational lensing, and was one of the predictions made by Einstein's general relativity.

In a gravitational lens, the gravity from the massive object bends light as a lens might. As a result, the path of the light from a bright object behind the massive one is curved around the massive object. The effect of this is that the light appears to come from different directions than the real object, creating multiple "lensed images" of the bright object. If the Earth, a massive object, and a bright object are precisely aligned, one would see a ring of the same bright object, that configuration is known as an Einstein's ring. More commonly, the massive galaxy is off-center, creating different numbers of images according to the relative positions of the object-lens-earth, and the shape of the gravitational well of the lensing system.

References and external links: http://imagine.gsfc.nasa.gov/docs/features/news/grav_lens.html