What is a stellar-mass black hole?

It is a black hole formed by the gravitational collapse of a massive star, typically between 3 and 150 solar masses. Its event horizon is only tens of kilometres across.

How do black holes form from stars?

When a star above roughly 20 solar masses exhausts its fuel, its core collapses. If the remaining core exceeds about 2.3 solar masses, no known pressure can halt the collapse and a black hole forms, usually accompanied by a supernova.

What is the event horizon?

The event horizon is the boundary where the escape velocity equals the speed of light. Inside it, every possible path leads further in, so nothing — including light — can get back out.

Why does a black hole look like a ring of light?

Gravity bends the light around it. Radiation from the accretion disk behind the hole is lensed up and over the shadow, and light orbiting at the photon sphere forms a thin bright ring at about 2.6 times the horizon radius.

Stellar-Mass Black Hole란 무엇인가?

A stellar-mass black hole is what a massive star becomes when its core collapses and nothing is left to stop it. Between roughly 3 and 150 solar masses of material falls inside its own event horizon — a surface from which not even light can escape. There are probably a hundred million of them in the Milky Way.

The formation is a failure of resistance. Electron degeneracy pressure holds up a white dwarf; neutron degeneracy and the strong force hold up a neutron star. Past about 2.3 solar masses of collapsed core, no known force wins, and the material falls inward indefinitely. What is left behind is not matter at all in any useful sense but geometry: a region of spacetime curved so steeply that every future-pointing path leads inward.

The event horizon is not a wall. It is a boundary defined by where escape velocity reaches the speed of light, and for a 10-solar-mass black hole it lies about 30 kilometres from the centre. Nothing special happens locally when you cross it; you simply lose the option of turning around. Outside it, at 1.5 times the horizon radius, sits the photon sphere, where light can orbit — which is why the rendered model shows a razor-thin bright ring hugging the shadow.

This model is a real ray-trace, not an artistic impression: light paths are integrated through curved spacetime, so the accretion disk behind the hole is bent up and over the shadow, background stars smear into Einstein rings, and the approaching side of the disk is brightened by relativistic Doppler beaming. Cygnus X-1, discovered in 1964 and the subject of a famous bet between Stephen Hawking and Kip Thorne, was the first object widely accepted as one of these.