In short
A black hole is a very compact object whose gravity is so strong that nothing, not even light, can escape once it crosses the event horizon, the boundary of no return.
Black holes usually form when massive stars collapse at the end of their life, and many grow by pulling in matter around them.
Scientists cannot see black holes directly, so they find them by watching how they affect nearby matter and light, and by detecting gravitational waves.
Why it matters
Black holes test our best theory of gravity and help explain how galaxies grow, including the one we live in.interpretation
Learning how they work also raises deep questions about information and physics that scientists have not yet solved.
All 13 key points are included (1 added in "Other important points")
- A black hole is an object so compact that its gravity stops anything, including light, from escaping.core
- General relativity predicts that any sufficiently compact mass will form a black hole, and that every black hole has a central singularity where spacetime curvature is infinite.core
- The boundary of no escape is the event horizon. Crossing it traps an object but causes no locally detectable change.
- Black holes typically form when massive stars collapse at the end of their life, often in a supernova. They can then grow by absorbing mass from their surroundings.core
- Supermassive black holes are thought to exist at the centres of most galaxies.core
- Hawking radiation makes black holes lose mass very slowly. Even stellar black holes gain more mass from the cosmic microwave background than they lose this way.
- Black holes are seen indirectly. Scientists infer them from falling matter, accretion disks, gravitational waves, and the motion of nearby stars.core
- The Milky Way's centre, Sagittarius A*, contains a supermassive black hole of about 4.3 million solar masses.
- Cygnus X-1 was the first strong black hole candidate. By 1974 it was widely considered a black hole, though 100% certainty may not be possible.
- In 2015 gravitational waves from a black hole merger were first detected. In 2019 the first direct image of a black hole, M87*, was published.
- The no-hair theorem says a stable black hole has only three properties: mass, electric charge, and angular momentum. Otherwise it is featureless.core
- Information about matter that formed a black hole seems lost as the hole evaporates. This is the black hole information paradox, which is still unsolved.
- Some scientists have proposed alternatives, such as quark stars and gravastars. None explains all observations, so black holes remain the leading explanation.
How the black hole idea fits together
A black hole is defined by its gravity and event horizon, forms from collapse, grows over time, and is found through indirect evidence.
Black hole
An object so compact that not even light can escape its gravity.
is bounded by
Event horizon
The boundary of no escape around a black hole.
forms through
Formation by collapse
Typically from massive stars collapsing at the end of their life.
Growth
Absorbs surrounding mass and merges with other black holes.
is revealed by
Indirect detection
Found through accretion disks, gravitational waves, and star motions.
is described by
Three properties
Mass, electric charge, and angular momentum.
How the others connect
- Formation by collapse then grows by Growth
Based on
Section by section: a short excerpt from the article, then what it means in plain words.
What a black hole is
From the article
“A black hole is an astronomical body so compact that its gravity prevents anything, including light, from escaping. Albert Einstein's theory of general relativity, which describes gravitation as the curvature of spacetime, predicts that any sufficiently compact mass will form a black hole.[4] The boundary of no escape is called the event horizon. In general relativity, crossing a black hole's event horizon traps an object inside but produces no locally…” ¶1
In plain words
- A black hole is a very compact object. Its gravity is so strong that nothing can escape it, not even light.
- Einstein's general relativity is his theory of gravity. It describes gravity as the bending of spacetime, which is space and time joined together. The theory predicts that any mass squeezed small enough will form a black hole.
- The event horizon is the boundary of no escape. Once something crosses it, it cannot get back out.
- Crossing the event horizon traps an object inside but causes no change that can be detected locally. You would notice nothing special at the edge itself.
- The theory also predicts that every black hole has a singularity at its center. A singularity is a point where spacetime curves infinitely.
How the main black hole ideas connect
This map shows how a compact mass, general relativity, the event horizon and the singularity fit together to describe a black hole.
Black hole
Gravity so strong that nothing, even light, can escape.
Very compact mass
Enough mass packed into a small space.
forms Black hole
is bounded by
Event horizon
The boundary of no escape.
has at its center
Singularity
A center where spacetime curves infinitely.
General relativity
Einstein's theory that gravity is the bending of spacetime.
predicts Black hole
Based on
Think of it like this: The event horizon works like a one-way door. You can walk through it going in, but no path leads back out.
How the idea developed
From the article
“Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century. In 1916, the first solution of general relativity that would characterise a black hole was found. By the late 1950s, this solution began to be interpreted physically as a region of space from which nothing can escape. Black holes were long considered a mathematical curiosity; it was not until the 1960s that…” ¶2
In plain words
- People first thought about objects whose gravity is too strong for light to escape in the 1700s.
- General relativity is Einstein's theory of gravity. In 1916, scientists found the first solution, meaning a set of mathematical answers to its equations, that described a black hole.
- By the late 1950s, that solution was read as a real place in space that nothing can escape from.
- For a long time, black holes seemed like a math oddity. In the 1960s, theory showed they follow naturally from general relativity.
- Cygnus X-1 became the first black hole most scientists accepted. Studies from 1971 to 1974 proposed it as a black hole paired with a star.
Key steps in the idea of black holes
The main steps in how the idea of black holes grew, from the 1700s to the early 1970s.
18th century
Objects that trap light first considered
Scientists first think about objects whose gravity is too strong for light to escape.
1916
First black hole solution found
The first solution of general relativity that would describe a black hole is found.
Late 1950s
Solution read as a region of no escape
The solution is understood as a region of space that nothing can escape from.
1960s
Black holes shown to be a normal prediction
Theory shows black holes follow from general relativity, not just a math oddity.
1971–1974
Cygnus X-1 proposed as a black hole
Several studies propose Cygnus X-1 as a black hole paired with a star.
Based on
Think of it like this: Like a detective's theory that starts as a guess and becomes accepted as clues pile up.
How black holes form and grow
From the article
“Black holes typically form as part of a supernova event when massive stars collapse at the end of their life cycle. After a black hole has formed, it can grow by absorbing mass from its surroundings. Supermassive black holes of millions of solar masses may form by absorbing stars and merging with other black holes, or via direct collapse of gas clouds. There is consensus that supermassive black holes exist…” ¶3
“Quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the rate of emission being inversely proportional to the mass. This causes the black hole to lose mass very slowly, provided it is not accreting matter. However, even the smallest class of black holes observed, stellar black holes, are gaining mass from the cosmic microwave background faster than they are losing mass via Hawking radiation.” ¶4
In plain words
- Most black holes form when a very massive star collapses at the end of its life. This collapse happens as part of a supernova, a huge explosion.
- Once a black hole exists, it can grow by pulling in matter from its surroundings.
- Supermassive black holes, with millions of times the Sun's mass, may grow by swallowing stars, merging with other black holes, or forming straight from huge gas clouds.
- Most galaxies are thought to have a supermassive black hole at their centre.
- Hawking radiation is a faint energy loss that quantum theory predicts from a black hole's edge. Smaller black holes lose it faster, so a black hole slowly shrinks if it is not pulling in matter.
- Even the smallest black holes we have seen gain mass from the faint background glow left over in space (the cosmic microwave background). That gain is larger than what they lose through Hawking radiation.
- In practice, the smallest known black holes are getting bigger, not smaller.
How a black hole forms and grows
A massive star collapses, forms a black hole, and then the black hole grows by taking in more matter.
Massive star reaches end of its life
Star collapses in a supernova
Black hole forms
Black hole grows by absorbing matter or merging
Based on
Think of it like this: A snowball rolling downhill picks up snow as it goes, so it keeps getting bigger.
How scientists detect black holes
From the article
“The presence of a black hole can be inferred through its interaction with matter and electromagnetic radiation such as visible light. Matter falling toward a black hole can form an accretion disk of infalling plasma, heated by friction and emitting light. In extreme cases, this creates a quasar, some of the brightest objects in the universe. Merging black holes can be detected by the gravitational waves they emit. If stars…” ¶5
“A black hole is generally defined as a region of spacetime from which no information-carrying signals or objects can escape.[51] However, verifying an object as a black hole by this definition would require waiting for an infinite time and at an infinite distance from the black hole to verify that nothing has escaped, and thus cannot be used to identify a physical black hole.[52] There are several other definitions that…” ¶41
In plain words
- Black holes give off no light of their own, so scientists look for indirect signs. A black hole's effect on nearby matter and radiation, such as visible light, is how its presence is inferred.
- Gas falling toward a black hole forms a spinning disk, called an accretion disk. Friction heats the disk so it glows. In extreme cases this glow becomes a quasar, one of the brightest objects in the universe.interpretation
- When two black holes merge, they send out ripples in spacetime called gravitational waves. Detectors can pick these up.interpretation
- If stars orbit a black hole, their movement shows how heavy the black hole is and where it is. Scientists use this to find black holes in double-star systems.
- A simple rule is used to label a black hole from observations. A very compact object heavier than about three Suns is treated as a black hole.
- Using these methods, astronomers have found many black hole candidates. They also concluded that the centre of the Milky Way, Sagittarius A*, holds a giant black hole of about 4.3 million times the mass of the Sun.
- In 2019, scientists published the first direct picture of a black hole's shadow, a dark circle inside a glowing ring of gas. It came from the Event Horizon Telescope, a network of radio telescopes linked across the world.interpretation
Clues that reveal a black hole
A black hole cannot be seen directly, but its effects on nearby matter give clues that scientists can measure.
Causes
Black hole with no visible light
Leads to Gas falls in and heats up (pulls in), Nearby stars move in orbits (pulls on), Two black holes merge (can merge)
Which lead to
Gas falls in and heats up
Leads to Glowing accretion disk is seen (heats and glows)
Nearby stars move in orbits
Leads to Mass and location are worked out (reveals)
Two black holes merge
Leads to Gravitational waves are detected (sends out)
Effects
Glowing accretion disk is seen
Mass and location are worked out
Gravitational waves are detected
Based on
What this shows: The first direct image of a black hole, taken by the Event Horizon Telescope of the supermassive black hole in galaxy Messier 87. It shows a glowing orange-red ring of gas around a dark central circle, which is the black hole's shadow. This image was published in 2019.
Think of it like this: It is like spotting an invisible wind by watching the leaves it blows around.
Properties and structure of black holes
From the article
“The no-hair theorem establishes that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, electric charge, and angular momentum; the black hole is otherwise featureless. Any two black holes that share the same values for these properties, or parameters, are indistinguishable from one another. A related no-hair conjecture proposes that dynamic gravitational collapse always results in an object characterized with only…” ¶43
“The defining feature of a black hole is the existence of an event horizon, a boundary in spacetime through which matter and light can pass only inward towards the centre of the black hole. Nothing, not even light, can escape from inside the event horizon.[138][139] The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach or affect an…” ¶124
In plain words
- Scientists describe a black hole with only three basic traits: its mass, its electric charge, and how fast it spins. Two black holes with the same three values cannot be told apart.
- The event horizon is the edge of no return. Anything that crosses it, including light, can only move inward. Events inside it cannot reach or affect an outside observer.
- All black holes spin, often very fast. The black hole at the centre of the Milky Way spins at about 90% of the fastest speed allowed.
- Size depends on mass. Stellar black holes weigh about 2 to 150 times the Sun. Supermassive black holes weigh more than a million times the Sun.
- Gas falling toward a black hole forms a spinning disk. The gas heats up and glows, and this glow is among the brightest light in the universe.interpretation
- At the centre of every black hole is a singularity, a point where the curvature of spacetime becomes infinite. All of the black hole's mass ends up there.interpretation
Main parts of a black hole
A black hole has a central singularity, an edge called the event horizon, and nearby regions such as the ergosphere, photon sphere, accretion disk and jets.
Black hole
Event horizon
The point of no return. Nothing inside can escape.
Singularity
A centre where spacetime curvature becomes infinite.
Photon sphere
A boundary where light can be bent around the black hole.
Ergosphere
A region where spacetime drags matter around a spinning black hole.
Accretion disk
Hot gas swirling inward before it falls in.
Relativistic jets
Thin streams of plasma shot away from the black hole.
Based on
| Class | Approx.mass | Approx.radius |
|---|---|---|
| Ultramassive black hole | 109–1011 M☉ | >1,000 AU |
| Supermassive black hole | 106–109 M☉ | 0.001–400 AU |
| Intermediate-mass black hole[75] | 102–105 M☉ | 103 km ≈ REarth |
| Stellar black hole | 2–150 M☉ | 30 km |
| Micro black hole | up to MMoon | up to 0.1 mm |
What this shows: A visualization of a black hole surrounded by a glowing orange accretion disk. The disk forms a ring across the middle, and the parts that appear above and below the hole are light from the far side of the disk, bent by gravitational lensing.
Think of it like this: The event horizon works like a one-way door. You can walk in, but nothing can walk back out.
Open questions and alternatives
From the article
“According to the no-hair theorem, a black hole is defined by only three parameters: its mass, charge, and angular momentum. This seems to mean that all other information about the matter that went into forming the black hole is lost, as there is no way to determine anything about the black hole from outside other than those three parameters. When black holes were thought to persist forever, this information loss…” ¶204
“While there is a strong case for supermassive black holes,[250][251] the dividing line between lighter black holes and neutron stars relies on theories of extremely dense matter. Direct observational tests are not available: objects observed to have mass higher than the predictions for neutron stars are assumed to be black holes. Recent evidence from gravitational wave events suggests modifications of these theories may be needed.[80] New exotic phases of matter…” ¶211
In plain words
- The no-hair theorem says a black hole can be described by only three things: mass, electric charge, and spin (angular momentum). Everything else about what fell in seems hidden.
- Black holes slowly lose mass through Hawking radiation, a faint glow that quantum theory predicts. That glow does not seem to carry any clues about what formed the black hole. This is the black hole information paradox: the information looks lost for good.
- Scientists do not agree on how to solve this puzzle. Solving it is expected to matter for a future theory that joins quantum mechanics and gravity.
- Quasars, which are very bright centres of galaxies powered by huge black holes, existed less than a billion years after the Big Bang. Scientists are still working out how those black holes grew so big so fast. Ideas include mergers of smaller black holes and direct collapse of huge gas clouds.
- The line between heavy neutron stars and black holes depends on theories of extremely dense matter. We cannot test those theories directly. So a very heavy object is assumed to be a black hole.
- Some other objects, such as gravastars and black stars, have been suggested. They could look almost the same as black holes but work in a different way, so black holes are not the only possible explanation.
- Black holes also appear in art and films. The film Interstellar used real relativity maths to draw its black hole, and fiction is sometimes used to teach science.interpretation
Why the information seems lost
Black holes slowly lose mass, and the radiation they give off seems to carry no clues about what formed them.
Black hole has only mass, charge, and spin
Its other details seem hidden from outside.
and
Black hole slowly loses mass
It emits Hawking radiation over time.
so
Radiation carries no clues about what formed it
Nothing extra about the infalling matter is visible.
so
Information seems gone forever
This is the black hole information paradox.
Based on
Think of it like this: Burning a book leaves ash, but the ash alone cannot tell you which book it was. A black hole's Hawking glow may be like that ash: it seems to hide the details of what fell in.
Other important points
- In 2015 gravitational waves from a black hole merger were first detected. In 2019 the first direct image of a black hole, M87*, was published.
Key takeaways
- Most black holes start when a very large star runs out of fuel and collapses. Some grow bigger by pulling in gas and stars, or by merging with other black holes.
- We cannot see a black hole directly, since no light escapes it. Scientists find them by watching what they do to nearby stars, gas, and light. A glowing gas disk, called an accretion disk, is one clue.
- The event horizon is a point of no return. Anything that crosses it cannot send information back out to an outside observer.
- Huge black holes, called supermassive black holes, are thought to sit at the centres of most large galaxies. The Milky Way's centre, Sagittarius A*, is one example.
- Hawking radiation is a tiny leak of energy that could make black holes shrink over time. For today's known black holes, though, they gain more mass from the cosmic microwave background (faint leftover light from the early universe) than they lose.
From the original article
More figures and tables from the article.
What this shows: A black-and-white rendering of a black hole, drawn as a dark semicircle with a bright glowing accretion disk wrapping around its top, bottom and sides. The disk is brightest on the left, showing Doppler beaming. This is the first simulated image of a black hole, made by Jean-Pierre Luminet in 1979, and it shows the early theory of how black holes would look.
What this shows: Graphs of the first gravitational wave detection, GW150914, recorded at two LIGO sites: Hanford, Washington and Livingston, Louisiana. Each site shows measured signal, theoretical prediction and residual noise as wavy lines, plus spectrograms below. The signal grows in strength over a fraction of a second and then cuts off, matching the merger of two black holes.
What this shows: A galaxy, Centaurus A, against a starry background, with glowing jets shooting out in opposite directions from its centre. The jets come from the supermassive black hole at the galaxy's core and can stretch for millions of light-years.
What this shows: Three artistic views of black holes with different spins: retrograde rotation, no rotation and prograde rotation, from top to bottom. The accretion disk sits closer to the hole when it orbits in the same direction as the hole spins. Graphs on the right show how the X-ray energy spectrum changes in each case, which lets astronomers estimate black hole spin.
What this shows: A dark, peanut-shaped chart of the ergosphere around a rapidly spinning black hole with 99% of maximum spin. The ergosphere touches the event horizon at the middle and bulges outward at the poles. Inside this region nothing can stay still, because spacetime itself is dragged around.
What this shows: A space-time diagram showing light cones near a black hole, with the event horizon on the right and the black hole shown as a dark blue region. Arrows fan out from a point far from the horizon to show a particle can move in any direction there. Closer to the horizon, the light cones tilt toward the hole, so motion is restricted.
What the article doesn't say
- The horizon itself cannot be photographed. The Event Horizon Telescope images show the black hole's shadow, a dark silhouette, not the edge.
- The information loss paradox is still unsolved. Scientists do not agree on whether information that falls into a black hole is truly lost.
- Some objects, like Sagittarius A*, are not proven to be black holes. Their measured size is larger than the Schwarzschild radius, so other exotic explanations remain possible.interpretation