What Is a Black Hole?
A black hole is a place in space where gravity is so strong that nothing can escape from it, not even light. Anything that crosses the edge is gone forever. You cannot see a black hole because no light comes out, but you can spot one by what it does to the things around it.
Black holes are the most extreme objects in the universe. The biggest one we know weighs as much as 66 billion Suns squashed into a single point.
- What is it?A region of spacewith gravity so strong nothing escapes
- Visible?No, completely invisibleNo light can leave it
- Smallest knownapprox. 3 SunsA tiny stellar one called XTE J1650
- Biggest known66 billion SunsA galaxy-centre giant called TON 618
- Closest to Earth1,560 light-yearsLight takes 1,560 years to reach us from it
- First one photographed2019A black hole in galaxy M87, by the Event Horizon Telescope
How big are black holes?
By mass. The biggest one weighs 66 billion times more than the smallest.
A "stellar" black hole is the smallest type, made from one dying star. The giants in the middle of galaxies are millions or billions of times heavier. Nobody is sure how the very biggest ones got so heavy in the time the universe has existed.
What is a black hole made of?
A black hole is not really an object the way a planet or a star is. It is a region of space with extreme gravity. At the very centre is a tiny point called a singularity, where all the mass is squashed into something smaller than an atom. Around that point is a bubble-shaped boundary called the event horizon. Cross the event horizon and there is no way back, no matter how powerful your rocket is.
The size of a black hole usually means the size of its event horizon. The bigger the black hole, the bigger the bubble.
How do black holes form?
There are three main ways scientists think black holes form.
Stellar black holes are made when a giant star runs out of fuel and dies. Without the outward push of nuclear fusion, gravity wins and the star collapses. The outer layers fly off in a huge explosion called a supernova, and the heart of the star squashes down into a black hole. Only stars heavier than approx. 20 Suns end up this way. Smaller stars become neutron stars or quiet white dwarfs instead.
Supermassive black holes sit at the middle of nearly every big galaxy. Scientists are not sure how they got so big so quickly. The leading idea is that they started from giant gas clouds in the early universe and grew by eating gas, stars and other black holes for billions of years.
Primordial black holes are a guess. They may have formed in the first second after the Big Bang, when the universe was so hot and squashed that little patches could have collapsed straight into tiny black holes. Nobody has spotted one yet, but they might still be out there.
How do we know they exist?
For decades, black holes were just a prediction from Einstein's mathematics. We could not see one, so we could not prove they were real. That changed in three big ways.
First, we noticed stars and gas behaving strangely. In some places, stars zip around an invisible point at huge speed, as if pulled by something with the mass of millions of Suns. The simplest explanation is a supermassive black hole.
Second, we picked up gravitational waves. When two black holes crash into each other, they make ripples in space itself. The LIGO detector first felt those ripples in 2015. We have now spotted over 90 black hole mergers.
Third, in 2019 we took the first picture of a black hole. The Event Horizon Telescope linked eight radio dishes around the world into a single Earth-sized camera and captured the dark shadow of the black hole at the centre of galaxy M87.
What would happen if you fell in?
If you fell into a stellar black hole, you would not even make it to the event horizon. Gravity would pull on your feet much harder than your head, stretching you into a long thin strand of particles. Scientists call it spaghettification because you would end up looking like a strand of spaghetti.
For a giant supermassive black hole the stretching is much gentler. You could fall across the event horizon without feeling much at first. But once you are inside there is no way out, and time itself starts to behave strangely. To anyone watching from outside, you would seem to slow down and freeze forever at the edge.
Deeper dive: Einstein, singularities and the maths of nothing escaping
The idea of a black hole comes out of Albert Einstein's general theory of relativity, published in 1915. In Einstein's picture, mass and energy curve the fabric of space and time. The more mass packed into a small space, the more sharply space curves. Push enough mass into a small enough region and space curves so tightly that no path leads back out. That is a black hole.
The boundary where escape becomes impossible is called the event horizon. Its size is given by the Schwarzschild radius, after the German physicist Karl Schwarzschild who solved Einstein's equations for a spherical black hole in 1916 while serving in the First World War. The Schwarzschild radius is approx. 3 km for each solar mass. A 10-solar-mass black hole has an event horizon of 30 km across, while the 4-million-solar-mass black hole at the centre of our galaxy has one of 12 million km, roughly the size of the inner Solar System.
At the very centre, general relativity says the matter is crushed to a point of infinite density called a singularity. Most physicists think this is not really what happens, that a future theory combining gravity with quantum mechanics will replace the singularity with something less extreme. Working out what that something is, the so-called theory of quantum gravity, is one of the biggest open problems in physics today.
Even though no light escapes from inside the event horizon, the existence of black holes can be confirmed by their effects: the orbits of stars around them (used to track Sagittarius A* at our galaxy's centre), the X-rays from gas falling in, the gravitational waves from mergers (first detected by LIGO in 2015 and now an entire branch of astronomy), and the silhouette imaged by the Event Horizon Telescope in 2019. Each of these matches the predictions of Einstein's theory to extreme precision.
Read more about the three kinds of black hole on stellar black holes and supermassive black holes, or learn about the strange boundary called the event horizon.