What Is a Black Hole? How They Form and What the First Images Show
Black holes are the leftovers of giant stars and the anchors of galaxies. Here is how they form, what the event horizon and "spaghettification" mean, what the Event Horizon Telescope photographed, and whether one could ever threaten Earth.
A black hole is a region of space where matter is packed so tightly that its gravity lets nothing escape, not even light. NASA describes black holes as huge concentrations of matter squeezed into very small spaces. Most form when a very massive star collapses at the end of its life, and the largest, millions to billions of times the Sun’s mass, sit at the centers of galaxies, including our own.
Black holes cannot be seen directly, but they can be found by their effects on nearby gas and stars. In 2019 and 2022, astronomers released the first images of the glowing material around two of them.
How do black holes form?
Stellar-mass black holes form when a giant star runs out of fuel and its core collapses under its own gravity. According to NASA’s guide to black hole types, a star with more than about eight times the Sun’s mass ends with a core collapse, often in a supernova explosion. Stars of about 20 solar masses or more leave behind a black hole; those just above the threshold leave a neutron star instead.
Supermassive black holes are a bigger puzzle. Every large galaxy seems to have one, but how they grew so big so early in the universe’s history is still an active research question. The James Webb Space Telescope is now spotting large black holes in very distant, young galaxies; our James Webb Space Telescope explainer covers how it sees that far back.
What types of black holes are there?
Astronomers group black holes by mass. The ranges below are approximate and come from NASA.
| Type | Mass (Suns) | Where found |
|---|---|---|
| Stellar-mass | A few to hundreds | Throughout galaxies, often in pairs with a star |
| Intermediate-mass | About 100 to hundreds of thousands | Hard to confirm; a few candidates |
| Supermassive | Hundreds of thousands to billions | Centers of large galaxies |
| Primordial (theoretical) | From tiny up to about 100,000 | Not yet found |
NASA says about 50 stellar-mass black holes in the Milky Way have been found or suspected in binary systems, but scientists think the galaxy may hold as many as 100 million. Most are invisible because nothing is falling into them.
What is an event horizon?
The event horizon is the boundary around a black hole beyond which nothing can return. It is not a solid surface; it is the distance at which the speed needed to escape becomes greater than the speed of light. Anything that crosses it, light included, cannot get back out.
The size of the event horizon grows in proportion to the black hole’s mass. A black hole with a few times the Sun’s mass has a horizon only tens of kilometers across, while the supermassive one in galaxy M87 has one larger than our solar system’s planetary orbits.
Outside the horizon, a black hole’s gravity behaves like that of any object with the same mass. If the Sun were replaced by a black hole of equal mass, the solar system would get much colder, but the planets would stay in their orbits.
What is spaghettification?
Spaghettification is the stretching of an object that falls toward a black hole. Gravity pulls much harder on the near end of the object than the far end, so it is squeezed sideways and stretched lengthwise, which NASA compares to a noodle.
The effect is strongest near small black holes, where gravity changes sharply over short distances. Near a stellar-mass black hole, an object would be torn apart well before reaching the horizon. Around a very large supermassive black hole, the change is gentler at the horizon, so an object could in principle cross it before being stretched apart. Astronomers do see stars torn up by supermassive black holes, in flares called tidal disruption events.
What did the Event Horizon Telescope photograph?
The Event Horizon Telescope (EHT) linked radio observatories around the world into one Earth-sized virtual telescope, observing at a wavelength of 1.3 mm. It produced the first two images of black holes. Each shows a dark central shadow surrounded by a bright ring of hot gas bent by gravity.
| Black hole | Image released | Mass | Distance from Earth |
|---|---|---|---|
| M87*, in galaxy Messier 87 | April 10, 2019 | 6.5 billion Suns | About 55 million light-years |
| Sagittarius A*, center of the Milky Way | May 12, 2022 | About 4 million Suns | About 27,000 light-years |
Data from the European Southern Observatory’s 2019 release and 2022 release. ESO says the event horizon is about 2.5 times smaller than the shadow it casts. Sagittarius A* was harder to image even though it is much closer, because gas around it orbits in minutes, so its appearance changed during the observations.
Could a black hole harm Earth?
No known black hole is a threat to Earth. The nearest known one, Gaia BH1, is about 1,560 light-years away, according to the 2022 study that found it, far too distant to affect the solar system. Sagittarius A*, the Milky Way’s central black hole, is about 27,000 light-years away.
The Sun will never become a black hole, because it is far below the eight-solar-mass threshold for a core collapse. A wandering black hole passing close to the solar system is possible in principle, but nothing like it has been detected, and such an encounter would be extremely unlikely on human timescales. Particle physics experiments, such as those at CERN, also cannot make dangerous black holes; our article on antimatter explains some of what such colliders do produce.
Bottom line
A black hole is matter so compressed that nothing, not even light, escapes from inside its event horizon. Most form from collapsing giant stars, and supermassive ones sit at galaxy centers. The Event Horizon Telescope imaged M87* in 2019 and our galaxy’s Sagittarius A* in 2022. The nearest known black hole is about 1,560 light-years away and poses no danger.
FAQ
Is a black hole a hole?
No. It is an extremely dense concentration of mass, not an empty hole. The name refers to the fact that no light comes back out.
What happens if you fall into a black hole?
Near a small black hole, tidal forces would stretch and tear an object apart before it reached the horizon. Near a very large one, it could cross the horizon intact, but nothing that crosses can send a signal back out.
How do astronomers find black holes if they are invisible?
They watch for stars orbiting an unseen heavy companion, X-rays from hot gas falling in, and ripples in space called gravitational waves from black holes colliding. The Event Horizon Telescope imaged the glowing gas right at the edge of two of them.
Is there a black hole at the center of the Milky Way?
Yes. Sagittarius A* has about 4 million times the Sun’s mass and lies about 27,000 light-years from Earth. Its image was released on May 12, 2022.
