The anatomy of a black hole

Collapse a star, a gas cloud or a filing cabinet, and what settles down is described by two numbers: how heavy it is and how fast it turns. Charge is a third, and astrophysically it is zero: anything charged pulls in the opposite sign and neutralises. Mass sets the size, spin sets the shape, and there is no third thing to know. Distances below are in gravitational radii, r = GM/c², the only ruler a black hole comes with.

Working inward

The horizon is where leaving stops. Nothing is there to touch. It is the surface at which every path that led outward has tipped over to point in, and a still hole puts it at 2 r. For Cygnus X-1 that is a sphere 125 km across. For M87* it is 257 astronomical units, over four times the width of Neptune’s orbit.

Spin it, and a second surface lifts away from the first. Inside the ergosphere space is dragged round faster than anything can swim against it, so standing still stops being possible. You can still leave, but not without going round on the way. The ergosphere meets the horizon at the poles and sits at exactly 2 r on the equator, at every spin.

Further out is the photon shell, where gravity bends light into a closed circle. A still hole has one photon sphere, at 3 r. Spin splits it in two: at the fastest an accreting hole turns, light going with the rotation circles at 1.07 r, and light going against it at 4.00 r.

Then the last stable orbit. Inside it, circling does not work — nudge inward and you spiral rather than settle. It falls from 6 r to 1.24 r as the spin rises, and everything between it and the horizon is falling rather than orbiting.

At the centre, if the hole turns at all, the singularity is a ring rather than a point.

Where the light comes from

Gas cannot fall straight in. It arrives with sideways motion, settles into a disc and grinds its way inward, shedding energy as light, and then it stops dead at the last stable orbit, because there is no orbit below one. Whatever binding energy the gas has not released by then goes down the hole unseen.

So the last stable orbit sets the entire budget, and spin sets the last stable orbit. A still black hole radiates 5.7% of everything it swallows. One turning at 0.998, the fastest a disc can drive it, radiates 32.1%. Hydrogen fusion gets 0.71% out of the same kilogram.

The jet runs on spin by a second route. Magnetic field threaded through the turning horizon is wound up and thrown out along the axis, at a power that goes as the square of how fast the horizon turns, so a hole that does not turn has no jet at all. M87*’s reaches 5,000 light-years in optical light.