Gravity taken to its limit. Around the black disk of the event horizon — the point of no return, at the Schwarzschild radius Rs = 2GM/c² — a glowing accretion disk of gas spirals in at nearly light speed, one side beamed bright as it sweeps toward you and the other dimmed as it turns away. Encircling it all is the thin photon ring, where gravity bends light into orbit. Change the mass and read off the real sizes of the horizon, the photon sphere and the innermost stable orbit. It all runs on your device.
The bright side of the disk is beamed toward you — relativistic Doppler beaming
Mass10 M☉
Schwarzschild radius Rs—
Photon sphere (1.5 Rs)—
Innermost stable orbit (3 Rs)—
One radius sets the scale of everything. A non-rotating black hole is defined by a single number, its mass, and from it comes the Schwarzschild radius Rs = 2GM/c² — the size of the event horizon, only about 3 km for the Sun's mass. The photon sphere where light can orbit sits at 1.5 Rs, and the innermost stable circular orbit that the accretion disk cannot cross lies at 3 Rs. This view is illustrative — sizes and colours are not to scale — but those three radii are the exact formulas.
How It Works
A horizon set by the mass, a disk that beams itself, and a ring of trapped light.
1
The event horizon
Squeeze enough mass into a small enough space and escape velocity reaches the speed of light. The surface where that happens is the event horizon, a sphere of radius Rs = 2GM/c². Cross it and there is no path back out — not for matter, not for light. Its size is set purely by the mass, so a heavier black hole is simply a bigger one.
2
The accretion disk beams itself
Gas falling in cannot dive straight down; it settles into a disk and spirals inward, heating to millions of degrees and glowing. Orbiting at a large fraction of light speed, the side coming toward you is Doppler-beamed brighter and bluer while the receding side dims — which is why the disk looks lopsided. The disk's inner edge is the innermost stable circular orbit at 3 Rs; inside that, matter simply plunges in.
3
The photon ring
At 1.5 Rs — the photon sphere — gravity bends light so hard that photons can circle the hole. Light grazing this radius loops around and comes back out, forming the sharp, bright photon ring around the dark shadow. It is exactly the feature the Event Horizon Telescope captured, and it is a direct measure of how severely a black hole warps spacetime.
What is a black hole?
A black hole is a region of space where gravity is so strong that nothing, not even light, can escape once it crosses the boundary called the event horizon. It forms when enough mass is squeezed into a small enough region — for example the collapsed core of a massive star. From outside you cannot see in; you can only see matter and light behaving in extreme ways around it, like the glowing disk in this visualisation.
What is the Schwarzschild radius and the event horizon?
The Schwarzschild radius Rs = 2GM/c² is the size of the event horizon of a non-rotating black hole of mass M — the distance from the centre at which escape would require the speed of light. Anything that crosses it is trapped. For the Sun's mass it is only about 3 km; it grows in proportion to the mass, so a ten-solar-mass black hole has a horizon about 30 km across. Change the mass here and the Schwarzschild radius updates from that exact formula.
What is the photon ring or photon sphere?
The photon sphere sits at 1.5 times the Schwarzschild radius, and it is the radius at which gravity bends light so sharply that photons can orbit the black hole. Light skimming past at that distance loops around, producing the bright, thin photon ring you see encircling the dark shadow of the hole in real images like the Event Horizon Telescope's. It is a direct fingerprint of how strongly the black hole curves spacetime.
Why is one side of an accretion disk brighter?
Matter spiralling into a black hole forms a hot, glowing accretion disk that orbits at a large fraction of the speed of light. The side of the disk sweeping toward us is brightened and blue-shifted, while the side moving away is dimmed and red-shifted — an effect called relativistic Doppler beaming. That is why real and simulated black-hole disks look lopsided, with one side far brighter than the other, as shown here.