This is an Einstein ring. It is the deformation of the light from a source (such as a galaxy or star) into a ring through gravitational lensing of the source’s light by an object with an extremely large mass (such as another galaxy, or a black hole). This occurs when the source, lens and observer are all aligned.
The size of an Einstein ring is given by the Einstein radius. It is:
Photo Credit: ESA/Hubble & NASA