Black holes

They stretch time, bend space, and push our understanding of the universe to its limit. Black holes have stunned scientists and stargazers for centuries; places where the rules of space stop making sense. But what are they? How do they form? And are they eating the universe?

What is a black hole?

In our Mysteries of Science podcast, astrophysicist Amélie Saintonge told us that black holes form when a star runs out of fuel. For most of its life, a star is held in a tug-of-war between its internal pressure (which pushes outwards) and gravity (pulling inwards). When the star’s fuel starts to run out, gravity wins. The star collapses in on itself, squashing all of its mass into a dense point – and it keeps going.

Nothing can escape the pull of a black hole, including light, which makes them very hard to see and understand. In 1783, the scientist John Michell was the first person to suggest the idea of a star that could trap its own light. The famous scientist Albert Einstein also predicted areas of space with gravity so strong that nothing can escape, but he wasn’t sure his calculations were correct.

Eating the universe

One of the scariest things about black holes is their gravitational pull. At the centre of our Milky Way galaxy exists supermassive black hole known as Sagittarius A*. In the same way Earth and other planets move around the Sun, everything in the Milky Way is moving around Sagittarius A*.

Black holes are surrounded by a zone called an event horizon. Whenever anything crosses the event horizon, it can’t escape the black hole’s gravity – it’s like the point of no return where it will stay forever.

A black hole’s biggest mystery is what happens inside it. The laws of physics (scientific rules about how the universe works) break down near the middle of a black hole (called the “central singularity”) and nobody knows for sure what happens there. Do things vanish? Are they stored? Or are they transformed in some way?

How to see a black hole

As telescopes and technology improve, scientists have started to know more about these strange regions of space. We can detect and watch black holes by seeing how stuff around them behaves. Matter spinning into a black hole heats up and glows, and when two black holes collide, it sends ripples through space known as gravitational waves.

In 2022, NASA’s Event Horizon Telescope snapped a picture of Sagittarius A* for the first time. It shows a central dark region surrounded by a glowing orange ring.

Some people wonder if black holes could connect to other places in the universe, or even to other universes. Einstein’s theory says that black holes might be paired with white holes – areas of the universe where the energy could escape. These connections are called wormholes: tunnels that connect space and time. Scientists have never seen a white hole and most physicists think that wormholes would collapse too quickly for anything to travel through them.

Are we toast?

Luckily, black holes are not cosmic vacuum cleaners. Objects have to pass fairly close to the event horizon to be captured. As Amélie told us, if our Sun was replaced with a black hole of the same mass, Earth’s orbit wouldn’t change (though we would miss sunlight!)

Black holes are not quite eating the universe. But they do remain one of the most interesting parts of space.

Also

Stranger than seeing a black hole would be hearing one. It turns out they don’t just swallow stuff but they can also burp… kind of!

Mysteries of Science guest Dr Kimberly Arcand and her team at NASA transformed data from the Perseus Galaxy Cluster into sound (called sonification). Scientists assign different pitches, volumes, and sometimes even instruments to different values. A bright area might be a high note whilst a dimmer area might be a low note. It’s a mix of art and science. This allows astronomers to hear the activity of stuff in space.

Perseus sounds like a ghost from a cartoon film – woOoO!

It’s not the first time scientists have done this. We’ve also heard the music of Sagittarius A* by turning its images into sound. That sounds like wind or ocean waves.

Sonification doesn’t just sound cool. It’s helping scientists interpret the universe in new ways.

First published in The Week Junior Science+Nature, issue 92, September 2025.