An Einstein ring is a rare form of strong gravitational lensing, created when a distant light source (such as a galaxy or quasar), a massive foreground lensing object, and the observer are nearly perfectly aligned, causing the background light to form a circular ring.
The foreground object's gravity symmetrically bends the light from the background source; when alignment is precise, complete or partial rings—known as Einstein rings—appear.
Einstein rings act as powerful cosmic magnifiers—revealing extremely distant objects by boosting their apparent brightness and enabling detailed study of their properties.
Surveys like SLACS have discovered hundreds of Einstein rings. Notable examples include the “Cosmic Horseshoe” and the double-ring system SDSS J0946+1006 imaged by Hubble.
In February 2025, ESA’s Euclid mission captured a striking Einstein ring around galaxy NGC 6505, about 590 million light-years away—a rare and detailed view of space warping and mass distribution.
High-resolution rings—such as Euclid’s recent image and detailed Hubble observations—provide deep insights into the distribution of dark matter and constraints on cosmic expansion.
Beyond natural lenses, theoretical proposals suggest using the Sun as a gravitational lens at around 550 astronomical units, enabling ultra‑high‑resolution imaging of exoplanets.