The shadow and the photon ring
At the centre of a picture of a black hole is a dark patch, which astronomers call its shadow. Round its edge is a thin bright ring, the photon ring, made of light that circled the hole at least once before reaching you.
BlackHoles.fyi draws both by tracing light. For every pixel, the computer follows a ray of light back from your eye, bending it as the hole's gravity would, to find where it came from.
Each part carries a label saying how real it is: worked out from the physics, following a model another team published, chosen for the picture, or partly worked out and partly chosen.
The dark patch at the centre
Worked out from the physics
Astronomers call it the black hole's shadow.
Each pixel is traced along the light ray that reaches it, so the dark patch is traced, not drawn. For a spinning hole, its outline matches the shape the physicist James Bardeen worked out in 1973 to within 1%, at spins up to 0.99, where 1 is the fastest a black hole can spin.
- Bardeen 1973, Timelike and null geodesics in the Kerr metric, in Black Holes (Les Houches lectures)
The thin bright ring round the dark patch
Worked out from the physics
This is the photon ring.
It is light that circled the hole at least once before reaching you. It comes out of the same traced rays as the shadow, and nothing is drawn on top of it.