01 / 05
Approach
A black hole is a region where gravity is so strong that nothing, not even light, can escape. From far away, it pulls like any other object of the same mass.
Its visible signature is bent background light: stars seem to move apart around it. Drag to orbit, or switch to “Launch object” mode in the panel.
rs = 2GM/c²
02 / 05
Accretion disk
Infalling gas does not drop straight in. It swirls into a disk and heats up through friction, to millions of degrees around stellar black holes.
The side moving towards us looks brighter (relativistic Doppler beaming). Without spin, the last stable orbit is at 3 rs. It moves inward as spin increases.
ISCO = 3 rs (a* = 0)
03 / 05
Photon sphere
At 1.5 rs, light can circle the black hole on an unstable orbit. This is the photon sphere.
It traces the thin ring at the edge of the shadow. The apparent shadow has a radius of about 2.6 rs, larger than the horizon itself. Lensing also shows the far side of the disk, above and below.
rph = 1.5 rs · shadow ≈ (√27/2) rs
04 / 05
Event horizon
The event horizon is a point of no return: once crossed, every path leads inward. It has no material surface.
For a non-rotating black hole it lies at rs. At maximum spin it shrinks to half that, GM/c². Rotation also drags spacetime along, which makes the shadow asymmetric (Spin slider).
r+ = (GM/c²)(1 + √(1 − a*²))
05 / 05
Spaghettification
Near a black hole, gravity pulls harder on your feet than on your head. These tidal forces stretch objects. Near a stellar black hole you would be stretched before reaching the horizon. A supermassive one would let you cross the horizon intact. Launch an object too close: it stretches and fades, more so when the mass is low.
Time slows down too: a clock held still at r ticks √(1 − rs/r) times as fast as a distant clock (live value in the panel).
dτ/dt = √(1 − rs/r)