Black Holes Explained: What They Are and What We Have Seen
A black hole is a place where gravity is so strong that not even light can escape. Learn how they form, how we know they exist, and what the famous photographs really show.
Black holes sound like science fiction, but they are real. We have measured them, heard them, and even photographed them. Here is what is known.
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 once it gets too close. The point of no return is called the event horizon. Outside it, you could still escape. Inside it, every path leads inward.
A black hole is not a cosmic vacuum cleaner. If our Sun were replaced by a black hole of the same mass, Earth would keep orbiting just as before, only in the dark and the cold.
How do they form?
A massive star lives its life
It fuses atoms in its core, and the energy pushes outward, balancing gravity pulling inward.
The fuel runs out
When the star is much heavier than our Sun and can no longer fuse atoms, the outward push stops.
The core collapses
Gravity crushes the core in a fraction of a second, often with a huge explosion called a supernova.
A black hole is left behind
If enough mass remains, nothing can stop the collapse.
The main kinds
- Stellar-mass black holes: a few to dozens of times the mass of the Sun, formed from collapsed stars.
- Supermassive black holes: millions to billions of times the Sun's mass, sitting in the centres of most large galaxies. Ours, called Sagittarius A\*, is about 4 million times the mass of the Sun.
- Intermediate-mass black holes: a rarer middle group that scientists are still searching for.
How we know they are real
- Orbits of stars. Astronomers tracked stars moving at enormous speeds around the centre of our galaxy, which revealed the hidden mass there.
- Gravitational waves. In 2015, the LIGO detectors recorded ripples in space from two black holes merging, the first direct detection of gravitational waves.
- Photographs. In 2019, the Event Horizon Telescope released the first image of a black hole's shadow, in the galaxy M87. In 2022, it released one of Sagittarius A\* at the centre of the Milky Way.
What the photographs show
The pictures show a glowing ring of hot gas with a dark centre. The dark area is the shadow of the black hole, and the bright ring is light from matter heated as it spins around. The images were created by combining data from radio telescopes around the world, turning the entire Earth into one giant telescope.
What most people miss
- Time slows down near a black hole, as predicted by Einstein's theory of general relativity. A clock close to one ticks more slowly than a clock far away.
- Black holes can shrink very slowly. Stephen Hawking showed they can lose energy, called Hawking radiation, but for large ones this takes unimaginably long.
- There is still mystery. What happens at the very centre is something our current physics cannot fully describe.
