Black hole facts

How big they get, how dense they are, and what would happen if you fell into one — in real numbers.

How big is a black hole?

A black hole's size is the width of its event horizon: the edge that nothing, not even light, comes back from. It grows in step with the mass — twice the mass, twice the width.

What if the Sun were a black hole?

Squeeze anything hard enough and it becomes a black hole. This is how small things would have to get:

• The Sun: 6 km wide — smaller than a city.
• The Earth: 1.8 cm — the size of a marble.
• The Moon: 0.2 mm — a grain of sand.
• You: billions of times smaller than a proton.

The Earth would keep orbiting exactly as it does now: the Sun's mass, and so its pull at our distance, would not change. The sky would just go dark and cold.

The biggest black holes are thinner than air

Take a black hole's mass and divide it by the space inside its event horizon. For a star-sized black hole that is about 18 billion tonnes in every sugar cube of space.

But double the mass and the space inside grows eight times, so the bigger a black hole is, the thinner it is on average. Sagittarius A* comes out about 1,000 times denser than water, M87* about a third as dense as air, and TON 618 roughly 300 times thinner than air.

Would you be spaghettified?

Near a black hole your feet are pulled harder than your head. At the edge of a star-sized one the difference across a 2-metre body is about 20 million g: you would be stretched thin long before you reached it. Astronomers really do call it spaghettification.

At the edge of a giant like Sagittarius A* the difference is less than a thousandth of a g. You would drift across the event horizon without feeling a thing — and still never get out.

You never see anyone fall in

Watch someone fall into a black hole from far away and you never see them cross the edge. Their light takes longer and longer to climb out, turns redder and dimmer, and seems to freeze just above the horizon until it fades from view.

The sound of a black hole

The black hole at the centre of the Perseus galaxy cluster, about 250 million light-years away, sends pressure waves through the hot gas around it. NASA's Chandra X-ray Observatory measured them: the note is a B flat 57 octaves below middle C, with one wave passing roughly every 10 million years.

Be the black hole

Zoom Void is a free black hole game built on exactly this sense of scale: start as a hole smaller than a bolt, swallow whatever is smaller than you, and grow through satellites, planets and stars to whole galaxies.

Questions

Where these numbers come from

Every width is the event horizon of a non-spinning black hole, 2GM/c², worked out for each mass. The masses are published estimates: Sagittarius A* and M87* from the Event Horizon Telescope collaboration, and TON 618 from quasar measurements, which are less certain.