The bright rings you see in images of black holes come from two distinct sources: a swirling disk of superheated matter falling toward the black hole, and a trick of extreme gravity that bends light into circular paths around it. These aren’t solid rings like Saturn’s. They’re the visible signatures of some of the most intense physics in the universe, where gas reaches millions of degrees and space itself curves so sharply that light can orbit in circles.
The Accretion Disk: A Whirlpool of Superheated Gas
The most prominent ring around a black hole is its accretion disk, a vast, flattened structure of gas and plasma spiraling inward. When nearby gas, dust, or even material stripped from a companion star gets pulled toward a black hole, it doesn’t fall straight in. Instead, it carries sideways motion (angular momentum) that forces it into orbit. The same basic principle explains why water spiraling down a drain forms a vortex rather than dropping straight through.
Over time, all this orbiting material settles into a flat disk for the same reason the solar system is roughly flat: collisions between particles cancel out random vertical motion, while the shared direction of rotation keeps the disk spinning. Three forces shape the disk’s structure: the black hole’s gravity pulling material inward, the outward push of pressure from the intensely hot gas, and the rotational forces of the orbiting matter itself.
Here’s the key question: if the gas is orbiting, how does it ever actually fall into the black hole? It needs to lose angular momentum, and ordinary friction between gas particles is far too weak to do the job. Instead, magnetic fields threaded through the disk interact with the spinning plasma to generate turbulence. This process, called the magneto-rotational instability, uses magnetic tension forces combined with the fact that inner parts of the disk orbit faster than outer parts. The resulting turbulence acts like an effective friction, shuffling angular momentum outward so that matter can creep inward.
As gas spirals closer to the black hole, it converts gravitational energy into heat. The numbers are staggering: a particle falling through the disk from a distant orbit to the inner edge releases roughly 10% of its rest-mass energy. For comparison, nuclear fusion in stars converts less than 1% of mass into energy. This makes accretion disks extraordinarily efficient powerhouses, and that released energy is what makes the disk glow across the spectrum, from radio waves to visible light to X-rays.
Why the Ring Glows So Brightly
The material in an accretion disk isn’t ordinary gas. By the time it reaches the inner regions near the black hole, it has been heated to millions of degrees and exists as plasma, a state where atoms are stripped of their electrons. This plasma radiates intensely, producing the bright glow captured in images like the Event Horizon Telescope’s famous photograph of the black hole in the galaxy M87.
The brightness isn’t uniform. The inner regions of the disk, where matter orbits fastest and experiences the strongest gravitational compression, are far hotter and brighter than the outer edges. Material also moves at a significant fraction of the speed of light in these inner orbits, which creates a visible asymmetry. The side of the disk where material moves toward you appears brighter (its light gets boosted in energy), while the receding side appears dimmer. This is why one side of the ring in the M87 image looks noticeably brighter than the other.
The Photon Ring: Light Trapped by Gravity
Beyond the accretion disk, there’s a second, more subtle ring structure that exists purely because of how black holes warp space: the photon ring. At a specific distance from a black hole, gravity is strong enough to force light itself into a circular orbit. This critical boundary is called the photon sphere, and for a non-spinning black hole, it sits at about 1.5 times the radius of the event horizon.
At this exact distance, a photon can theoretically circle the black hole forever. In practice, though, this orbit is wildly unstable. Any tiny deviation, even an infinitesimally small one, causes the photon to either spiral inward and get swallowed or escape outward. Mathematically, this orbit is what physicists call a saddle point: balanced on a knife’s edge where the slightest nudge sends things careening in one direction or the other.
The photon ring you actually see in images is formed by light (from the accretion disk, background stars, or any other source) that passes close to this critical orbit and gets bent sharply before escaping toward your eyes. Some of that light loops around the black hole once, some twice, and in principle, some light can orbit an arbitrary number of times before escaping. Each successive loop produces a thinner, fainter sub-ring nested inside the previous one. For a distant observer, the apparent radius of this ring is about 2.6 times the Schwarzschild radius (the event horizon radius), regardless of the viewing angle.
Gravitational Lensing Shapes What You See
A black hole doesn’t just pull matter in. It bends the fabric of space around it, and light follows those curves. This effect, called gravitational lensing, fundamentally changes how a black hole looks to a distant observer. Light from stars or gas behind the black hole gets deflected around it, creating warped, brightened images. A star lined up almost directly behind a black hole would appear as a stretched arc or even a complete ring of light (an Einstein ring) to someone looking from the other side.
This lensing is what makes the black hole’s “shadow” visible in the first place. The dark center of the ring isn’t literally the event horizon. It’s the shadow, a region where light aimed at the black hole gets captured rather than deflected toward you. The boundary of this shadow is defined by the photon sphere: light that passes just outside the critical capture radius gets bent around the black hole and reaches your telescope, while light that passes just inside is lost forever. The sharp contrast between the bright lensed ring and the dark interior is what produces the iconic donut-shaped image.
Gravitational lensing also lets you see parts of the accretion disk that should be hidden. Light from the far side of the disk, and even from the underside, gets bent up and over the black hole so that it reaches you. This means you’re effectively seeing the disk from multiple angles simultaneously, which adds to the apparent brightness and thickness of the ring in images.
Why a Disk and Not a Sphere
A common question is why the material forms a flat ring rather than a spherical shell around the black hole. The answer comes down to angular momentum. Gas falling toward a black hole almost always carries some net rotation, even if it’s slight. As the gas collapses inward, conservation of angular momentum speeds up that rotation (like an ice skater pulling their arms in). Collisions between particles dissipate energy in the vertical direction, flattening the cloud, but angular momentum in the horizontal plane is much harder to get rid of. The result is a thin, flat disk aligned with the overall rotation axis.
In rare cases where infalling gas has very little angular momentum, the flow can be more spherical. But for most real black holes, especially those actively feeding on gas from their surroundings, the disk structure dominates.
Two Rings in One Image
When you look at an image of a black hole, you’re seeing the combined effect of all these phenomena at once. The broad, bright glow comes from the accretion disk’s superheated plasma. The sharp bright edge at the boundary of the shadow is dominated by the photon ring, where gravitational lensing concentrates light. And the asymmetric brightness is caused by the relativistic motion of plasma in the inner disk. Separating these components is one of the active challenges for astronomers working with the Event Horizon Telescope, because the photon ring carries direct information about the black hole’s mass and spin encoded in its precise size and shape.