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Black Holes, drawn

generatorThe shadow

The dark patch in the middle of a picture is not the horizon. It is the set of directions from which no light comes, and it is 2.6 times wider than the horizon, because rays from behind get bent around the sides. Bardeen wrote down its exact outline in 1973 for any spin and any viewing angle[36]. This draws it.

One microarcsecond (µas) is the width of a coin seen from the far side of the Earth. The Event Horizon Telescope resolves about 20. M87* at 6.5 billion suns and 16.8 megaparsecs predicts a 40 µas shadow; 42 ± 3 was measured. Sgr A* at 4.3 million suns and 8.3 kiloparsecs predicts 53; 51.8 ± 2.3 was measured.

Four spins at two angles. The flat side faces the direction the hole turns toward you.
Four spins at two angles. The flat side faces the direction the hole turns toward you. · SVG · computed here · Nan · hongdam.net · CC BY 4.0

What a photograph of one is

The 2019 image of M87* is a ring of radio light around a dark centre, at a wavelength of 1.3 mm, made by eight telescopes on four continents recording the same night and combined afterwards as if they were one dish the size of the Earth[44, 70]. The ring is light from hot gas near the photon sphere, bent around; the dark centre is the shadow. Its size measured the mass to within ten percent of what the stars orbiting the galaxy's centre had said. Sagittarius A* followed in 2022, harder because the gas there changes in minutes while the Earth turns for hours[49].

Sgr A*'s horizon and shadow against Mercury's orbit; M87*'s against Neptune's and Voyager 1. To scale.
Sgr A*'s horizon and shadow against Mercury's orbit; M87*'s against Neptune's and Voyager 1. To scale. · SVG · computed here · Nan · hongdam.net · CC BY 4.0
Inside the ring the EHT sees, a thinner one, and inside that a thinner one — the photon ring's subrings. A telescope in orbit could resolve the first of them.
Inside the ring the EHT sees, a thinner one, and inside that a thinner one — the photon ring's subrings. A telescope in orbit could resolve the first of them. · SVG · computed here · Nan · hongdam.net · CC BY 4.0