The parts of a black hole
A black hole has parts you cannot see directly. The horizon is the point of no return. The photon sphere is where light can circle the hole. The innermost stable orbit is the closest that gas can circle without falling in. At the singularity, the bending of spacetime grows without limit and the physics we have stops working; it is less a place than a moment.
Round many black holes, gas circles in a flat glowing disc. BlackHoles.fyi can mark these parts with circles round the hole, or cut the hole and its disc in half to show them on the cut face.
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 circles drawn round the hole
Partly worked out from the physics
They mark the horizon (the point of no return), the photon sphere (where light can circle the hole) and the innermost stable orbit (the closest that gas can circle without falling in).
Their sizes are 2 M, 3 M and 6 M, the ones the physics gives. M is a unit of distance set by the hole's mass, so the same drawing fits every black hole at its own scale. They are drawn as if light travelled straight, so they show where each sphere is, not how it would look.
No paper is listed for this part. Their sizes are textbook results for a black hole that does not spin.
The black hole cut in half
Partly worked out from the physics
A view with the near half of the hole and its disc of gas cut away. Lines mark each region on the cut: the ergosphere, where nothing round a spinning hole can stay still; the corona, very hot gas just above the disc; the jet, a beam of gas fired out along the spin axis; the magnetic field threading the gas; and, for a hole that takes in only a little gas, the cloud of hot gas it has in place of a disc. Inside the horizon sit a second, inner horizon and the singularity, a ring where the maths of gravity gives no answer.
The gas on your side of the cut is left out, so the cut face is lit by the disc's gas, not painted on. The lines sit where each region is, as if light travelled straight. The horizon, the ergosphere and the disc's inner edge follow from the maths for the spin the hole is drawn with. The disc's thickness, the cloud's width, the corona's reach and the jet's edge come from the models the rest of the picture is drawn with, and the magnetic field is a pattern we drew, not one worked out. The inner horizon and the ring come from the idealised maths.
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