How can gravity make one distant galaxy look like a ring?

A distant galaxy can appear as an arc or ring around a foreground object, and one background source can appear at several sky positions. This is gravitational lensing. Mass changes the geometry of spacetime, and light follows paths through that geometry. The result can redirect light toward the observer along more than one route without creating additional galaxies.

The apparent direction of arriving light need not be the direction the source would have without the deflector. A lens mapping connects those two directions. Its form depends on the foreground mass distribution and the relative distances of the observer, lens, and source. Unlike an ordinary glass lens, no transparent refracting material is required.

The illustration uses an ideal isolated point-mass lens, with small deflections approximated at one thin lens plane. Angles are measured in units of its Einstein angle, a scale set by mass and distances. For a source offset β and signed image direction θ along their shared axis, the model gives β = θ − 1/θ. Away from exact alignment, two solutions give two ideal point-image positions.

With exact alignment, rotational symmetry makes every direction at radius one Einstein angle a solution, producing the illustrated ring. The ring is a set of apparent images, not a physical ring of newly made stars around the lens. Real extended sources and extended or asymmetric lenses can produce arcs, incomplete rings, or more complicated image arrangements. Their brightnesses, time delays, and detailed shapes require information beyond this simple positional model.