Research

The glowing disc

Most of the light in a picture of a feeding black hole comes from the gas spiralling into it, heated until it glows.

On BlackHoles.fyi, five parts of the picture describe the disc: its shape, how hot it is, why one side is brighter than the other, how thick it is, and the threads of gas in it.

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 shape of the glowing disc

Partly worked out from the physics

Gas falling into a black hole circles it in a flat disc, called an accretion disc, and glows as it heats up.

The inner edge sits at the innermost stable orbit for the hole's spin: the closest that gas can circle without falling in, which moves inward the faster the hole spins. The gas circles at the rate for that orbit. M is a unit of distance set by the hole's mass, so the same drawing fits every black hole at its own scale. The disc is drawn out to 1 500 M, where the disc's own gravity would break it into clumps (Goodman 2003).

How hot the disc is

Follows a published model

The disc's colour and brightness come from its temperature.

The temperature follows the standard model of a thin disc, one much thinner than it is wide, corrected for the strong gravity near the hole (Novikov and Thorne 1973). The black hole on the site's front page is 3C 273, a real quasar: a feeding black hole at the heart of a distant galaxy. Its mass, 260 million suns, and how fast it feeds, 1.2 times the Eddington rate, are its published values. The Eddington rate is the rate at which the light from the falling gas starts to push the rest of it away.

Our research notes on this part

Why one side of the disc is brighter

Worked out from the physics

The disc's light changes in colour and brightness on its way to you.

For each point of the disc we work out how far its light is shifted towards blue or red: by the gas moving towards or away from you, by its light climbing out of the hole's gravity, and by the hole's spin. Gas coming towards you looks brighter and bluer, so one side of the disc outshines the other.

Our research notes on this part

How thick the disc is

Partly worked out from the physics

The disc is much thinner than it is wide, but not flat.

The thickness follows the standard thin-disc model (Shakura and Sunyaev 1973). Above and below its middle the gas thins out in the shape that gas held up by its pressure takes.

The threads of gas in the disc

Partly worked out from the physics

Fine streaks of brighter and fainter gas that curve round the disc.

The threads move at the gas's real orbital speed, so the faster inner gas draws each one into a trailing arc, as it does in a real disc, where the inner gas outruns the outer. We drew their shape to match the gas in published simulations, rather than simulating it ourselves.