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. They form when massive stars die, and they bend space, slow time, and will eventually evaporate over trillions of years.

How Black Holes Form

Most black holes form when a massive star (at least 20–25 times the mass of our Sun) reaches the end of its life. For millions of years, the star burns nuclear fuel at its centre — the outward pressure from the explosion exactly balances the inward pull of gravity.

When the fuel runs out, this balance collapses. In less than a second, the outer layers crash inward with tremendous force. The core is compressed so tightly that it becomes a singularity — a region of effectively infinite density, smaller than a single atom.

This collapse triggers a supernova explosion visible across galaxies. What remains: a black hole.

9 mm Size the entire Earth would need to be compressed to to become a black hole

6 km Schwarzschild radius of our Sun if it became a black hole

4 million ☉ Mass of Sagittarius A*, the supermassive black hole at the Milky Way's centre

1,600 ly Distance to Gaia BH1, the nearest known black hole to Earth

Anatomy of a Black Hole

The Singularity

At the very centre is the singularity. Our current physics breaks down here — the density predicted by classical general relativity is infinite, which signals that we need a better theory of gravity (quantum gravity) to describe what actually happens inside.

The Event Horizon

Surrounding the singularity is a spherical boundary called the event horizon. This is not a physical surface — it has no wall, no material, nothing to bump into. It is simply the point of no return: once you cross it, every possible direction you could travel leads deeper into the black hole, not out.

The Accretion Disc

Black holes are often surrounded by a swirling disc of hot gas called an accretion disc. In-falling gas is heated to millions of degrees by friction and compression, radiating enormous amounts of X-rays. This is what makes black holes actually visible — we can detect their accretion discs.

Why Nothing Escapes

Imagine throwing a ball upward. The faster you throw it, the higher it goes. For any given planet, there is a speed — called escape velocity — above which the ball escapes to space and never returns.

For Earth, escape velocity is about 11.2 km/s. For the Sun, it's about 617 km/s. For a black hole, the escape velocity at the event horizon equals the speed of light, which is the cosmic speed limit. Since nothing can travel faster than light, nothing can escape.

Black holes don't suck: A common misconception is that black holes behave like cosmic vacuum cleaners. In reality, from a safe distance they follow completely normal Newtonian gravity. If our Sun magically became a black hole of the same mass (without exploding), the Earth's orbit would be completely unchanged.

Black Holes Slow Time

Einstein's general relativity predicts that gravity slows time. The stronger the gravitational field, the slower time passes relative to somewhere far away.

Near a black hole, time slows dramatically. If you hovered just outside the event horizon (with a very powerful rocket) and then returned to Earth, you would have aged far less than people on Earth. This is the premise of the film Interstellar — the planet near the black hole has a 7-to-1 time dilation relative to the distant crew.

The mathematics predicts that at the event horizon itself, time would appear to stop completely from an outside observer's perspective. An object falling in appears to freeze and fade from the outside view — but experiences no such freezing from the inside.

Hawking Radiation

In 1974, Stephen Hawking showed that black holes are not completely black. Due to quantum effects at the event horizon, black holes slowly emit thermal radiation now called Hawking radiation.

The mechanism involves quantum fluctuations: pairs of virtual particles and antiparticles pop into existence everywhere in space. Normally they immediately annihilate and disappear. Near the event horizon, one particle falls in and the other escapes — the black hole loses mass to produce this radiation.

For stellar-mass black holes, this radiation is extraordinarily faint — far below any detector on Earth. But it means that over timescales of 10⁶⁷ years or more, black holes will eventually evaporate completely. This is one of the longest timescales in all of physics.

Are We Safe?

Yes. The nearest known black hole (Gaia BH1, discovered in 2022) is about 1,600 light-years away — roughly 15,000 trillion kilometres. It poses absolutely no threat to Earth.

There is a supermassive black hole (Sagittarius A*) at the centre of our own Milky Way galaxy, about 26,000 light-years away with a mass 4 million times that of the Sun. It also poses no threat — though streams of stars orbit it at thousands of kilometres per second.

First image of a black hole: In 2019, the Event Horizon Telescope (a network of radio telescopes spanning the Earth) captured the first direct image of a black hole — M87*, 6.5 billion times the mass of the Sun, 55 million light-years away. In 2022, they imaged Sagittarius A* at the centre of our own galaxy.

Try It Yourself

N-Body Gravity Simulation — Add a heavy central object and watch surrounding masses spiral in under gravity, similar to stars orbiting a galactic black hole.
Orbital Mechanics — Explore how objects orbit massive bodies and what happens when orbits decay.

The rubber-sheet analogy: Imagine a heavy ball on a stretched rubber sheet — it creates a deep well that makes nearby balls roll toward it. This is how GR describes gravity: massive objects curve spacetime, and other objects follow the curves. Note: the real curvature is in 4D spacetime, not just in space — which is why even light travelling at c is deflected.

🪐 Open N-Body Gravity →