An Einstein ring is a phenomenon where the light from a distant object, such as a galaxy or quasar, is bent into a ring shape by the gravitational field of a massive foreground object, like another galaxy. This effect is a consequence of Einstein's theory of general relativity.
Source: hyperphysics.phy-astr.gsu.edu
11/04/2021
What happens when two black holes collide? This extreme scenario occurs in the centers of many merging galaxies and multiple star systems. The featured video shows a computer animation of the final stages of such a merger, while highlighting the gravitational lensing effects that would appear on a background starfield. The black regions indicate the event horizons of the dynamic duo, while a surrounding ring of shifting background stars indicates the position of their combined Einstein ring. All background stars not only have images visible outside of this Einstein ring, but also have one or more companion images visible on the inside. Eventually the two black holes coalesce. The end stages of such a merger is now known to produce a strong blast of gravitational radiation, providing a new way to see our universe. This Week is: Black Hole Week at NASA