What is the photon ring around a black hole?

• A. A bright ring formed by photons orbiting near unstable circular photon orbits near the black hole, contributing to the observed structure around the shadow.
• B. The dark region where light cannot escape.
• C. The region of the accretion disk inside the innermost stable circular orbit.
• D. The event horizon itself.

Answer: (A)

A photon ring forms because gravity is so strong near a black hole that some light rays skim close to the hole and loop around it before escaping to us. Photons that travel near the photon sphere—an unstable set of circular photon orbits around the black hole—can wind around the hole one or more times and then emerge toward us. The result is a bright, narrow ring that sits just outside the dark silhouette of the black hole’s shadow. In a spinning black hole, prograde and retrograde photon paths differ, so the ring can be shaped a bit differently, but the essential idea remains: a bright ring arises from light that has been dramatically bent and partially wound around the hole before reaching our eyes. The dark region seen is the shadow, not the ring, and the event horizon lies inside that shadow, while the inner edge of the accretion disk (the ISCO) is a separate feature.