Anatomy of a Black Hole

Most of what looks bright in a black-hole image is not the black hole itself. The black hole is defined by a boundary in spacetime called the event horizon, from which nothing — not even light — can escape.

Event horizon

The event horizon is not a solid surface. It is the last stable radius from which a light ray can still travel outward. For a Sun-mass black hole it sits at only about three kilometres from the centre. For the supermassive black hole at the centre of our galaxy it is roughly 12 million kilometres.

Singularity

Inside the event horizon, general relativity predicts an infinitely dense point called the singularity. In practice this is a signal that our current theory breaks down — a full theory of quantum gravity is needed. No telescope has ever imaged, or ever will image, the interior.

Accretion disk and photon ring

Matter falling toward a black hole spirals through an accretion disk, heating to millions of kelvin and radiating brightly across the electromagnetic spectrum. Just outside the event horizon, gravitational lensing bends light into a thin photon ring. The Event Horizon Telescope resolved this ring around M87* in 2019 and Sagittarius A* in 2022.

Relativistic jets

Magnetic fields anchored in the accretion disk can launch narrow collimated jets of plasma at close to light speed. These emerge from the region around the black hole, not from inside the event horizon. Dive deeper into what we really know about black holes.