What you are looking at
A
black hole sits in front of a field of distant stars. You cannot see the object itself —
it emits no light — only its effects: a circular
shadow, a brilliant
photon
ring, a swirling
accretion disk of superheated gas, and the starlight behind it
smeared into arcs by the bending of space. (The image is a schematic, not a full ray-traced solution, but it
captures the real geometry.)
The event horizon
Pack enough mass into a small enough region and the escape velocity at its surface reaches the speed of
light. The boundary from which nothing — not even light — can return is the
event horizon, a
sphere of radius given by the Schwarzschild formula:
r_s = 2 G M / c² ≈ 2.95 km × (M / M☉)
A black hole the mass of our Sun would be just ~3 km across; the supermassive one at the centre of our galaxy
is millions of times heavier. Outside the horizon, light that strays close enough is forced into orbit at the
photon sphere (1.5 r_s), and gas can only orbit stably down to the
innermost stable
circular orbit (3 r_s) before spiralling in.
Gravitational lensing
Mass curves spacetime, and light follows that curvature. Rays from stars directly behind the black hole are
bent around it and reach your eye from the side, so a single star can appear smeared into a ring — an
Einstein ring. This simulation warps the background using the lens equation
β = θ − θ_E²/θ, which is why the starfield streaks and doubles near the edge. The same lensing lifts the far
side of the accretion disk up and over the top, the now-famous halo shape.
Time slows near the edge
Gravity also dilates time. A clock hovering at radius r runs slow compared with a distant observer by
dτ/dt = √(1 − r_s/r) 1 + z = 1/√(1 − r_s/r)
Move the hovering clock inward and watch its rate fall toward zero at the horizon — to a faraway observer an
infalling object appears to freeze and fade to red as its light is stretched (redshifted) without limit.
Things to try
Increase the
mass and watch the shadow and lensing ring grow. Slide the
hovering clock from far away down toward 1 r_s and watch its tick rate collapse and its
redshift soar. Toggle the
grid to see spacetime distortion directly, and the orbit markers to
locate the photon sphere and ISCO. Real images of this geometry were captured by the Event Horizon Telescope
(M87* in 2019, Sagittarius A* in 2022).