Research · Research notes
Photon ring
BlackHoles.fyi draws real black holes from the physics of light and gas near them. These are our working notes on one part of that picture: what we found while drawing it, and where the research is still unclear.
The photon ring is the thin bright ring just outside a black hole's shadow, the dark centre of the picture where no light reaches the camera. The ring is light that went at least once round the hole before reaching the camera. The site doesn't paint it on. It falls out of the same traced light rays that make the shadow, so its size and shape follow from the hole's mass and spin. Most of the work on it went into drawing a ring thinner than a pixel without dots, shimmer or shake.
The ring is thinner than a pixel at the sizes a phone or a laptop shows it, so a picture with one ray of light per pixel drew it as a row of dots. Each frame now aims its rays at a different point inside each pixel, and near the ring the picture is the average of the last eight frames, which draws a smooth ring.
That first made the whole picture shake up and down. Now only the pixels near the ring are averaged, and the rest of the picture keeps one fixed ray per pixel, so the hot gas circling the hole still streaks as it moves, where averaging would smear it.
When a device draws the picture at less than full size to keep up, the ring gets about half as many rays, and its brightness still wobbles a little along its length. More rays would fix that, at a cost in speed.
Where the research is unclear
- Papers on the photon ring say each further ring, from light that went round the hole once more, is about 4% as bright as the one inside it. That figure is for light that has gone round many times. Our first two rings differ much less (the second is about 17% as bright as the first), so we think the 4% figure describes the faint outer rings better than the first two, which are the ones the picture draws.
- The site has a view that names each part of the hole in turn, and it draws the ring as a full circle. In a real picture, the disc of hot gas round the hole would hide the part of the ring that passes in front of it. We haven't yet decided how to show that.
Updated 8 November 2026