Black holes are among the strangest objects in the universe. They are regions of space where gravity is so strong that nothing, not even light, can escape. For a long time, they existed only as a mathematical prediction of Einstein's theory of gravity, and many scientists doubted they were real. Today, we know that black holes exist throughout the cosmos, from objects a few times the mass of the Sun to giants billions of times heavier at the centers of galaxies. We have heard their collisions as ripples in space and time and even taken pictures of their shadows. This explainer covers what black holes are, how they form and how we study them.
Gravity and escape velocity
To understand black holes, start with a simple idea: escape velocity. If you throw a ball upward, gravity pulls it back. Throw it faster, and it goes higher. If you could throw it fast enough, about 25,000 miles per hour (11.2 kilometers per second) from Earth's surface, it would escape Earth's gravity completely. That speed is Earth's escape velocity.
The escape velocity depends on how massive an object is and how compact it is. If you squeezed Earth's entire mass into a smaller and smaller ball, the escape velocity at its surface would increase. If you compressed Earth to the size of a marble, about 0.7 inches (18 millimeters) across, the escape velocity would exceed the speed of light. Since nothing can travel faster than light, nothing could escape. That is a black hole.
What exactly is a black hole?
A black hole is a region where matter has been compressed so much that its gravity prevents anything from escaping. In Einstein's general theory of relativity, gravity is not just a force but a curvature of space and time caused by mass and energy. A black hole curves spacetime so strongly that all paths, including those of light, lead inward.
The event horizon
The boundary of a black hole is called the event horizon. It is not a physical surface but a point of no return. Anything that crosses the event horizon cannot come back out or send signals to the outside. The size of the event horizon depends on the black hole's mass. For a black hole with the mass of the Sun, it would have a radius of about 1.9 miles (3 kilometers). For a black hole with a billion solar masses, the event horizon would be larger than the orbit of Neptune.
The singularity
According to general relativity, at the center of a black hole lies a singularity, a point where matter is crushed to infinite density and the known laws of physics break down. Most physicists believe that a future theory combining general relativity and quantum mechanics will describe what really happens there. For now, the interior of a black hole remains one of the deepest mysteries in physics.
How black holes form
Stellar black holes
Most black holes we know form from the deaths of massive stars. Stars shine by fusing light elements into heavier ones in their cores. This fusion creates outward pressure that balances the inward pull of gravity.
When a very massive star, typically more than about 20 times the mass of the Sun at birth, runs out of fuel, fusion stops in its core. Without the pressure to support it, the core collapses under its own gravity within a fraction of a second. The outer layers may be blasted into space in a powerful explosion called a supernova. If the remaining core is massive enough, roughly more than about three times the mass of the Sun, nothing can stop the collapse, and it becomes a black hole. Less massive cores become neutron stars instead.
Stellar black holes typically have masses from about 3 to a few tens of times the mass of the Sun.
Supermassive black holes
At the centers of most large galaxies lie supermassive black holes with masses from millions to billions of times that of the Sun. Our own Milky Way has one, called Sagittarius A* (pronounced "A-star"), with a mass of about 4 million Suns.
How supermassive black holes formed is still an active area of research. They may have grown from smaller black holes by swallowing gas, stars and other black holes over billions of years, or they may have formed from the direct collapse of huge gas clouds in the early universe. Observations by telescopes such as the James Webb Space Telescope have found surprisingly large black holes in the early universe, challenging scientists to explain how they grew so quickly.
Intermediate and primordial black holes
Astronomers have also found evidence of intermediate-mass black holes, with masses of hundreds to hundreds of thousands of Suns. Some scientists have proposed that tiny primordial black holes might have formed in the very early universe, but none has been confirmed.
How do we see something invisible?
Black holes themselves do not emit light, but their effects on their surroundings can be observed.
Accretion disks and X-rays
When a black hole pulls in gas, the gas forms a swirling accretion disk around it. Friction and compression heat the gas to millions of degrees, making it glow brightly, especially in X-rays. Some of the brightest objects in the universe, called quasars, are supermassive black holes actively feeding on gas. Some black holes also launch powerful jets of particles at nearly the speed of light.
Stars in orbit
By tracking the motions of stars near the center of the Milky Way for decades, astronomers found that they orbit an invisible object packed with about 4 million solar masses in a tiny region. The only explanation is a supermassive black hole. This work earned Andrea Ghez and Reinhard Genzel a share of the 2020 Nobel Prize in Physics, together with Roger Penrose, who showed that black hole formation is a robust prediction of general relativity.
Gravitational waves
When two black holes orbit each other and merge, they send out ripples in spacetime called gravitational waves. In 2015, the LIGO observatories in the United States detected such waves for the first time, from two black holes, each about 30 times the mass of the Sun, merging about 1.3 billion light-years away. Since then, observatories have detected many more mergers.
Pictures of black hole shadows
In 2019, the Event Horizon Telescope collaboration, which links radio telescopes around the world into a virtual telescope the size of Earth, released the first image of a black hole's shadow, at the center of the galaxy M87. In 2022, it released an image of Sagittarius A*. These images show a dark central region surrounded by a bright ring of glowing gas bent by the black hole's gravity.
What would happen if you fell into a black hole?
The experience depends on the black hole's size. Near a small stellar black hole, the difference in gravity between your head and your feet would be enormous, stretching you out in a process physicists playfully call spaghettification. Near a supermassive black hole, these tidal forces are gentler at the event horizon, so you could cross it without immediately noticing anything special, though you would eventually be stretched near the center.
To a distant observer, something falling into a black hole would appear to slow down and fade as it approached the event horizon, because of the extreme warping of time and light. From the falling object's own perspective, it would cross the horizon in a finite time.
Do black holes last forever?
In 1974, Stephen Hawking showed that, due to quantum effects near the event horizon, black holes should emit a faint radiation and slowly lose mass, now called Hawking radiation. For stellar and supermassive black holes, this process is extraordinarily slow, far longer than the current age of the universe. It has never been observed directly, but it is an important clue to how gravity and quantum physics might fit together.
Common myths
- Black holes are cosmic vacuum cleaners. They do not suck in everything around them. At a safe distance, a black hole's gravity is the same as any other object of the same mass. If the Sun were replaced by a black hole of the same mass, Earth would keep orbiting in the same way, though it would become very cold and dark.
- Black holes are holes. They are not empty holes but extremely dense concentrations of mass.
- A black hole could swallow Earth soon. The nearest known black holes are more than a thousand light-years away. There is no danger to Earth.
Questions readers ask
What is inside a black hole? According to general relativity, a singularity of infinite density lies at the center, but physicists expect a future theory of quantum gravity to describe it differently. The interior cannot be observed directly.
How big is a black hole? It depends on its mass. A black hole with the Sun's mass would have an event horizon about 3.7 miles (6 kilometers) across. Supermassive black holes can be larger than our solar system.
Is there a black hole in our galaxy? Yes. The supermassive black hole Sagittarius A* lies at the center of the Milky Way, about 26,000 light-years from Earth. Many smaller stellar black holes also exist throughout the galaxy.
Can light escape a black hole? Not from inside the event horizon. Light from hot gas outside the horizon can escape, which is how we observe black holes.
Could a black hole form from a particle accelerator? Physicists have studied this carefully. Even if tiny black holes could be created, they would evaporate almost instantly, and cosmic rays have struck Earth with higher energies for billions of years without harm.
In short
A black hole is a region where gravity is so strong that nothing, not even light, can escape its event horizon. Stellar black holes form when massive stars collapse, while supermassive black holes millions to billions of times the Sun's mass sit at the centers of galaxies. We detect them through glowing accretion disks, the orbits of nearby stars, gravitational waves from their collisions and images of their shadows. Black holes are not cosmic vacuum cleaners, but they are natural laboratories where our best theories of gravity and quantum physics meet, and where some of the universe's greatest mysteries remain.