A gravitational lens happens when astronomers on Earth look toward a huge galaxy or galaxy cluster, so massive that its gravity distorts any light passing near. The massive object acts like a lens in space, spreading the light out, often to produce multiple images of a more distant object that happens to be shining behind it. Or, if the distant background object and the intervening massive galaxy are perfectly aligned, the gravitational lens may spread the light to produce an image of a ring in space.

A ring-shaped image produced in this way is known as an Einstein Ring. The ring itself not a real physical structure in space, but just a play of light and gravity, a result of the gravitational lensing effect. And yet these Einstein rings have revealed some of the mysteries of the cosmos to the astronomers who study them.
Astronomers in Asia recently analyzed the clearest-ever images of a gravitational lens called SDP.81. They carefully studied the Einstein Ring produced by this system, in order to calculate that a supermassive black hole located near the center of SDP.81 – the lensing galaxy – may contain over 300 million times the mass of our sun.