What Is a Black Hole?

Born from a dying star

Most black holes form when a star at least 20–25 times the Sun's mass runs out of nuclear fuel. For millions of years, the outward pressure of fusion had exactly balanced gravity's inward pull; once that balance breaks, the outer layers crash inward in under a second and the core is crushed into a singularity — a point of effectively infinite density, smaller than a single atom. The collapse triggers a supernova visible across galaxies, and what's left behind is the black hole. For scale: our Sun would need to be squeezed into a ball just 6 km across to become one; Earth would need to shrink to 9 millimetres. The supermassive black hole at our galaxy's centre, Sagittarius A*, weighs about 4 million Suns; the nearest known black hole, Gaia BH1, sits roughly 1,600 light-years away.

The event horizon: a point of no return

Surrounding the singularity is the event horizon — not a physical wall, nothing to bump into, just the boundary beyond which every possible direction of travel leads deeper in, never back out. Every object has an escape velocity: about 11.2 km/s for Earth, 617 km/s for the Sun. At a black hole's event horizon, escape velocity reaches the speed of light itself, the universe's hard speed limit — so nothing, not even light, can cross back out. Many black holes are ringed by a swirling accretion disc of infalling gas heated to millions of degrees by friction, radiating the X-rays that let astronomers detect them at all.

Bending time, and slowly fading away

General relativity predicts that gravity slows time — the stronger the field, the slower a clock runs relative to a distant observer. Near a black hole this effect becomes extreme: hover just outside the event horizon and return to Earth, and you would have aged far less than everyone who stayed behind (the premise behind the 7-to-1 time dilation in the film Interstellar). At the horizon itself, an infalling object appears, from the outside, to freeze and fade forever — though it experiences nothing of the sort from its own perspective. And black holes are not eternal: in 1974 Stephen Hawking showed that quantum particle-antiparticle pairs flickering near the horizon let one partner escape while the other falls in, slowly draining the black hole's mass as Hawking radiation. For a stellar-mass black hole this radiation is far too faint to detect, but over roughly 10⁶⁷ years — one of the longest timescales in all of physics — it would evaporate completely.

Frequently asked questions

Try it live

Everything above runs in your browser — open Fun Black Hole, send stars into a cartoon gravity well, and watch them stretch into spaghetti as they cross the horizon. Nothing is installed, nothing is uploaded.