What is a black hole? Gravity's ultimate trap, explained

A black hole is a place in space where so much mass is squeezed into so small a region that its gravity lets nothing escape, not even light. Its boundary is called the event horizon: cross it and there is no way back out. Most black holes form when very large stars collapse at the end of their lives. Giant ones, millions to billions of times the Sun's mass, sit at the centers of galaxies, including ours.

In short

• A black hole isn't a hole or a vacuum cleaner: it is mass so compact that the speed needed to escape it is faster than light.
• Any mass would become a black hole if squeezed small enough: the Sun inside about 3 km, Earth inside about 9 mm.
• They were a prediction of Einstein's theory for decades; since 2015 they have been detected by their gravitational waves, and since 2019 photographed.

How does a black hole form?

Everything with mass has an escape velocity, the speed you'd need to leave it without falling back. For Earth it is about 11 km per second. Pack the same mass into a smaller ball and the escape velocity rises, because you start closer to the center. Squeeze it enough and the escape velocity passes the speed of light, about 300,000 km per second. Since nothing travels faster than light, nothing gets out. That is a black hole.

Nature does this squeezing in the deaths of big stars. A star holds itself up against its own gravity with the heat of fusion in its core. When the fuel runs out, gravity wins. According to NASA, stars of more than about eight times the Sun's mass end in a supernova explosion, and when a star of about 20 solar masses or more does, its core collapses into a black hole. The Sun is far too small for this; it will end as a white dwarf, as the note on what the Sun is made of explains.

How big is a black hole?

The size of the event horizon depends only on the mass inside. It is given by a formula Karl Schwarzschild worked out from Einstein's general relativity in 1916: radius = 2 × G × mass ÷ c², where G is the gravitational constant and c the speed of light. In practice, that comes to about 3 km of radius for every Sun's worth of mass.

Mass squeezed into a black hole	Radius of the event horizon	For scale
Earth	about 9 mm	a marble
The Sun	about 3 km	a small town
A typical stellar black hole, 10 Suns	about 30 km	a large city
Sagittarius A*, at the Milky Way's center, about 4 million Suns	about 12 million km	a fifth of Mercury's distance from the Sun
M87*, in the galaxy Messier 87, about 6.5 billion Suns	about 19 billion km	four times Neptune's distance from the Sun

The step from row to row is just multiplication. Sagittarius A* has 4 million times the Sun's mass, so 4,000,000 × 3 km = 12 million km. Double the mass and the horizon doubles in radius, which means bigger black holes are, on average, far less dense. The horizon of M87*, averaged over its volume, would hold matter thinner than air.

What is inside a black hole?

At the center, general relativity predicts a singularity: a point where matter is crushed to infinite density. Most physicists read that "infinite" as a sign that the theory has reached its limit, not as a literal description. Explaining what is really there would need a theory that joins gravity with quantum mechanics, which nobody yet has.

The event horizon itself is not a surface. Nothing is there to touch. It is a boundary in space: outside it, light can still escape with enough effort; inside it, every path leads toward the center.

Would you be spaghettified, and do black holes suck things in?

Spaghettified, near a small one; not necessarily near a big one. Gravity pulls harder on whatever is closer. Falling feet first toward a stellar black hole, your feet would be pulled so much harder than your head that you'd be stretched thin and torn apart well before reaching the horizon. Physicists really call this spaghettification. A supermassive black hole's horizon is so much farther from its center that the difference across your body at the horizon would be small. You could cross it intact, though you'd still never come out.

They don't suck. From a distance, a black hole pulls like any other object of the same mass. If the Sun were replaced by a black hole of exactly the Sun's mass, Earth would keep orbiting just as it does now. It would only get very cold and dark. Things fall in when they come too close or lose orbital speed, as gas does when it piles up in a spinning disk around a black hole and rubs itself hot.

How do we know black holes are real?

For decades they were a prediction, and three kinds of evidence now confirm them.

• Stars orbiting nothing visible. Teams led by Reinhard Genzel and Andrea Ghez tracked stars whipping around an invisible object at the Milky Way's center, about 4 million times the Sun's mass in a tiny space. They shared the 2020 Nobel Prize in Physics with Roger Penrose, who showed in 1965 that general relativity makes black holes form.
• Gravitational waves. On 14 September 2015, the LIGO detectors in the United States caught ripples in space from two black holes, of about 29 and 36 solar masses, merging 1.3 billion years ago. About 3 Suns' worth of mass became gravitational waves in a fraction of a second, according to LIGO. Hundreds of mergers have been detected since.
• Pictures. The Event Horizon Telescope linked radio dishes across the planet into an Earth-sized telescope. On 10 April 2019 it released the first image of a black hole's shadow, M87*, 55 million light-years away. On 12 May 2022 it released one of Sagittarius A* in our own galaxy, about 27,000 light-years away.

Neither image shows the black hole itself, which gives off no light. Each shows a dark shadow inside a ring of hot gas whose light is bent around it.

Do black holes last forever?

Not in theory. In 1974 Stephen Hawking calculated that quantum effects near the horizon should make black holes give off a faint glow, now called Hawking radiation, and so slowly lose mass and eventually evaporate. For a black hole of a few Suns, that would take vastly longer than the present age of the universe.

Still argued

Hawking radiation has never been observed: from any known black hole it would be far too faint to detect. It raises the information paradox: if a black hole evaporates into featureless radiation, what happens to the information about everything that fell in? Quantum theory says information can't be destroyed; Hawking's original calculation says it is. Physicists have proposed several resolutions, and none is agreed. Whether the very first black holes formed in the Big Bang itself, so-called primordial black holes, is also unsettled; NASA notes there is no definitive proof they exist.

Check yourself

• What decides the size of a black hole's event horizon, and roughly how big would the Sun's be?
• Why would falling into a small black hole tear you apart, while a giant one might not?
• What would happen to Earth's orbit if the Sun turned into a black hole of the same mass?

Sources

• Black holes — NASA
• Black hole types — NASA
• Gravitational waves detected 100 years after Einstein's prediction — LIGO Caltech, 2016
• Astronomers capture first image of a black hole — European Southern Observatory, 2019
• Astronomers reveal first image of the black hole at the heart of our galaxy — European Southern Observatory, 2022
• The Nobel Prize in Physics 2020, press release — Nobel Prize Outreach, 2020