What is a black hole?

Not really a hole

A black hole sounds like an empty hole in space, but it is really the opposite. It is a huge amount of matter squeezed into a very tiny space. That makes its gravity, the pull that every object has on other objects, incredibly strong.

Close to a black hole, gravity is so strong that nothing can escape. Not a rocket, not a planet, not even light, the fastest thing in the universe. Because no light can get out, a black hole is completely dark.

That makes black holes very tricky to study. Telescopes cannot see them directly, so scientists have to be clever detectives and watch what happens around them.

The point of no return

The boundary around a black hole is called the event horizon. It is not a solid surface you could stand on, like the ground on Earth. It is more like an invisible line in space. Outside it, something moving fast enough can still get away. Once anything crosses it, there is no way back.

Scientists find black holes by their effects. Gas swirling around a black hole can get so hot that it glows brightly, even giving off X-rays. Stars near a black hole race around it in tight orbits. And a black hole's gravity can bend the light coming from things behind it, like a giant lens.

Anything that strays too close to a black hole gets squeezed and stretched out long and thin, like a noodle. Scientists really do have a name for this: spaghettification!

Where black holes come from

Some black holes are left behind when giant stars die. When a star with more than about eight times the mass of our Sun runs out of fuel, it blows apart in a gigantic explosion called a supernova. If the star had about 20 times the Sun's mass or more, its core collapses into a black hole. These stellar-mass black holes can have anywhere from a few to hundreds of times the mass of the Sun.

Then there are the giants. Almost every large galaxy has a supermassive black hole at its centre, with hundreds of thousands to billions of times the mass of the Sun. Our own galaxy, the Milky Way, has one called Sagittarius A* (say sadge-ay-star). It has about 4 million times the mass of the Sun and is about 27,000 light-years away.

Scientists proved it was there by tracking stars whizzing around the centre of our galaxy, a discovery that won a Nobel Prize in 2020. The nearest black hole we know of, called Gaia BH1, is about 1,500 light-years away, which is very, very far from us.

Do black holes suck things in?

In films, black holes act like giant vacuum cleaners, slurping up everything in space. That is a myth. From far enough away, a black hole's gravity works just like the gravity of any other object with the same mass.

Here is a way to think about it. If the Sun were somehow swapped for a black hole with exactly the same mass, Earth and the other planets would keep going around in their orbits just as before. It would get very dark and cold, but nothing would get pulled in. And this could never happen to our Sun anyway: it is far too small to ever become a black hole.

Taking a picture of the invisible

If black holes are black, how could anyone photograph one? Scientists had to build a telescope as big as the Earth. The Event Horizon Telescope linked eight radio telescopes on mountains and volcanoes in Hawaii, Mexico, Arizona and Spain, in Chile's Atacama Desert and in Antarctica. Working together, they acted like one giant, Earth-sized telescope.

In 2017 they all pointed at the centre of a galaxy called Messier 87. Each telescope recorded so much data that the hard drives were flown to special supercomputers to be combined. More than 200 researchers worked on the project.

On 10 April 2019, they revealed the result: the first image of a black hole, called M87*. It shows a glowing ring of super-hot gas around a dark centre. That dark patch is the black hole's shadow, caused by its gravity bending and capturing light at the event horizon. This black hole is about 55 million light-years away and has about 6.5 billion times the mass of our Sun.

In 2022, the same team revealed the first image of Sagittarius A*, the black hole at the heart of our own galaxy. Seen from Earth, it looks about as big as a doughnut sitting on the Moon would look, which shows just how sharp the Event Horizon Telescope's vision is.

📚 Words to know

Gravity

The pulling force that every object with mass has on other objects. It keeps you on the ground and Earth going around the Sun.

Event horizon

The boundary around a black hole. Once anything crosses it, it can never escape.

Light-year

The distance light travels in one year. Astronomers use it to measure the huge distances in space.

Supernova

The gigantic explosion of a very big star at the end of its life.

Supermassive black hole

A giant black hole, found at the centre of most large galaxies, with up to billions of times the mass of the Sun.

🧪 Try this: bend space with a stretchy sheet

1. You need an old bedsheet or a large piece of stretchy fabric, a few marbles, and something heavy and round, such as a grapefruit. Ask three or four friends to help.
2. Everyone holds an edge and pulls the sheet flat and tight. Roll a marble across it. It travels in a straight line.
3. Now put the heavy object in the middle so it makes a dip. Roll the marble past it again. Does it curve around the dip? If it is slow enough, it spirals in.
4. Try rolling marbles fast and slow. Fast marbles can curve past and escape, but slow ones get caught. A black hole makes the deepest dip of all, so deep that close to it not even light can climb back out.
5. Remember, this is only a model. Real gravity pulls in every direction in space, not just across a flat sheet.