What Is a Black Hole?

Key Takeaways

• A black hole is a region of spacetime where gravity is so strong that nothing, not even light, can escape it
• The first direct image of a black hole’s shadow was captured in 2019 by the Event Horizon Telescope
• Gravitational wave detectors have confirmed dozens of black hole mergers since 2015

Where the Laws of Physics Break Down

At the center of the Milky Way, 26,000 light-years from Earth, sits a compact mass of approximately 4 million times the Sun’s mass occupying a region smaller than our solar system. Astronomers call it Sagittarius A*. On May 12, 2022, the Event Horizon Telescope (EHT) collaboration published the first image ever captured of this object – a glowing ring of heated gas orbiting an area of significant darkness at its center. That darkness is a black hole.

A black hole is a region of spacetime where the curvature caused by concentrated mass becomes so extreme that the escape velocity – the speed required to leave its gravitational pull – exceeds the speed of light. Since nothing travels faster than light, nothing that crosses the boundary called the event horizon can ever return. Not light. Not information. Not matter of any kind. What lies beyond the event horizon is, in a strict sense, permanently cut off from the observable universe.

How Black Holes Form

The most common pathway to a black hole begins with a massive star reaching the end of its life. Stars produce energy by fusing hydrogen into helium in their cores, generating outward pressure that counterbalances gravity. When a star exhausts its nuclear fuel, that balance collapses. For stars with masses roughly eight times the Sun’s mass or greater, the collapse is sudden and catastrophic – a core-collapse supernova that can briefly outshine an entire galaxy. If the remaining core mass exceeds about three solar masses, gravity overwhelms every other known force, and the matter collapses into a singularity: a point of theoretically infinite density. The black hole forms around it.

Not all black holes come from dying stars. Primordial black holes may have formed in the dense, turbulent conditions of the very early universe, though their existence has not been confirmed. Supermassive black holes – those with masses ranging from millions to tens of billions of solar masses, found at the centers of most large galaxies – have formation histories that remain an open question. They were likely too large to form purely from individual stellar deaths and may have grown from smaller seed black holes that merged and accreted mass over billions of years, though the details are unresolved.

The Event Horizon and the Point of No Return

The event horizon is not a physical surface. There’s no wall or membrane there. A spaceship crossing the event horizon of a sufficiently large black hole would not immediately experience anything unusual – no dramatic boundary, no change in local physics. What makes the event horizon absolute is what happens afterward: every possible path forward, every direction in spacetime, curves inward. There is no trajectory that leads back out.

The radius of the event horizon – called the Schwarzschild radius – scales with mass. For a black hole with the Sun’s mass, the Schwarzschild radius would be about 3 kilometers. For Earth’s mass, it would be approximately 9 millimeters. The Sun and Earth are not black holes because their actual sizes vastly exceed their Schwarzschild radii. Only when mass is compressed below its Schwarzschild radius does a black hole form.

For supermassive black holes, the event horizons can be enormous. The black hole at the center of the galaxy Messier 87, designated M87*, has a mass of approximately 6.5 billion solar masses and an event horizon with a radius larger than the distance between Earth and its Sun by a factor of several hundred.

The First Black Hole Image and What It Showed

Before 2019, black holes had been inferred but never directly imaged. Everything known about them came from the behavior of surrounding gas, the orbits of nearby stars, and, beginning in 2015, gravitational waves produced by mergers. On April 10, 2019, the EHT collaboration published the first direct image of a black hole’s shadow – specifically, of M87* – revealing a bright, asymmetric ring of light surrounding a dark central region.

That image required a planet-spanning network of radio observatories coordinated to function as a single Earth-sized telescope using a technique called very long baseline interferometry (VLBI). Eight observatories on four continents participated, collecting data simultaneously over several nights in April 2017. Processing that data into an image took two years.

The follow-up image of Sagittarius A*, released in May 2022, was technically more difficult to capture because the Milky Way’s central black hole is smaller and its orbiting gas moves faster, causing the emission to change significantly over the hours-long observation window. The EHT team used new imaging techniques to reconstruct a stable average image from the variable data.

Gravitational Waves: Listening to Black Holes Merge

On September 14, 2015, detectors at the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Washington State and Louisiana registered the first direct detection of gravitational waves – ripples in spacetime produced by two black holes merging approximately 1.3 billion light-years away. The detection, designated GW150914, confirmed a prediction of Albert Einstein‘s general theory of relativity that had stood untested for a century.

Since then, LIGO and its European counterpart Virgo have published catalogues documenting dozens of confirmed gravitational wave events, the majority of them produced by merging black hole pairs. The KAGRA detector in Japan joined the network in 2020, and a fourth detector, LIGO-India, is in advanced construction as of 2026. Each new detector improves the network’s ability to localize the source of a gravitational wave signal on the sky.

The data from these mergers has revealed a population of black holes with masses in ranges that earlier models did not clearly predict, including so-called intermediate-mass black holes and “gap” events where the merging masses fall in regions between expected stellar-mass and supermassive categories.

What Happens Inside a Black Hole

This is where certainty ends and speculation begins – and where physics itself may break down. Einstein’s general relativity predicts that all matter falling into a black hole converges at a singularity, a point of infinite density where the equations of physics cease to produce meaningful results. Most physicists regard infinite density as a signal that general relativity is incomplete rather than a literal description of what exists inside.

Stephen Hawking introduced a further complication in 1974 with the concept of Hawking radiation. Applying quantum mechanics to the region just outside the event horizon, Hawking showed that black holes should emit a slow, steady stream of thermal radiation caused by quantum effects near the boundary. Over extremely long timescales – for stellar-mass black holes, timescales vastly longer than the current age of the universe – this radiation would cause a black hole to slowly lose mass and eventually evaporate entirely. Hawking radiation has not been directly observed because for any astrophysical black hole, the radiation is so faint as to be undetectable with current instruments.

The combination of Hawking’s prediction and general relativity produces a problem known as the black hole information paradox: quantum mechanics requires that information about the physical state of matter cannot be permanently destroyed, but if a black hole evaporates completely, what happens to the information about everything that fell in? This problem remains unresolved and is one of the most active areas of theoretical physics research.

Types of Black Holes

Astrophysicists recognize several categories based on mass:

Stellar-mass black holes typically range from about 3 to roughly 100 solar masses. They form from the collapse of massive stars and are the type detected most frequently by LIGO. The first confirmed stellar-mass black hole, Cygnus X-1, was identified through its X-ray emissions in 1964 and has an estimated mass of about 21 solar masses.

Intermediate-mass black holes, hypothesized to range from hundreds to hundreds of thousands of solar masses, have proven elusive to confirm. Several candidates have been identified, and some gravitational wave events suggest mergers involving objects in the intermediate range.

Supermassive black holes reside at the centers of most large galaxies. M87* at 6.5 billion solar masses and Sagittarius A* at 4 million solar masses bracket a range that extends up to TON 618, a quasar-hosting black hole estimated at 66 billion solar masses – the most massive confirmed black hole known as of April 2026.

Summary

A black hole is a region of spacetime defined by its event horizon, beyond which gravity is so extreme that escape is impossible. They form primarily from the collapse of massive stars, though supermassive black holes likely formed through a more complex combination of early-universe seeding and billions of years of accretion and merging. The 2019 and 2022 EHT images provided the first visual evidence of black holes’ structure, while LIGO and Virgo have confirmed dozens of mergers through gravitational wave detection. What happens inside the event horizon remains a frontier question that exposes the limits of current physics.

Appendix: Top 10 Questions Answered in This Article

What is a black hole? A black hole is a region of spacetime where gravity is so intense that the escape velocity exceeds the speed of light. Once matter or light crosses the event horizon, it cannot escape, making the interior permanently cut off from the observable universe.

How do black holes form? The most common pathway is the death of a massive star at least eight times the Sun’s mass. When the star’s nuclear fuel runs out, gravity crushes the core beyond the point where any force can halt the collapse, forming a singularity surrounded by an event horizon.

What is the event horizon? The event horizon is the boundary surrounding a black hole beyond which nothing can escape. It is not a physical surface but a mathematical boundary in spacetime. The event horizon’s radius is called the Schwarzschild radius and scales with the black hole’s mass.

When was the first black hole image captured? The first direct image of a black hole’s shadow was published on April 10, 2019, by the Event Horizon Telescope collaboration. It showed M87*, the black hole at the center of the Messier 87 galaxy, with a mass of approximately 6.5 billion solar masses.

What is Sagittarius A?* Sagittarius A* is the supermassive black hole at the center of the Milky Way galaxy, located approximately 26,000 light-years from Earth. It has a mass of roughly 4 million solar masses. The EHT published the first image of it in May 2022.

What are gravitational waves and how do they relate to black holes? Gravitational waves are ripples in spacetime produced by accelerating massive objects, most dramatically by merging black holes. LIGO detected the first gravitational waves on September 14, 2015, confirming a prediction of Einstein’s general relativity.

What is Hawking radiation? Hawking radiation is a theoretical form of thermal radiation predicted by Stephen Hawking in 1974 to be emitted by black holes due to quantum effects near the event horizon. It has not been directly detected but implies that black holes slowly lose mass and could eventually evaporate.

What is the black hole information paradox? The information paradox is the unresolved conflict between quantum mechanics – which requires that information cannot be permanently destroyed – and the apparent ability of a black hole, through Hawking radiation and eventual evaporation, to erase information about matter that fell into it.

What is the most massive known black hole? As of April 2026, the most massive confirmed black hole is TON 618, a quasar-hosting supermassive black hole with an estimated mass of approximately 66 billion solar masses.

How do scientists detect black holes they cannot see? Black holes are detected through their gravitational effects on surrounding matter and light, the X-ray emissions from heated gas falling into them, the orbital behavior of nearby stars, the gravitational lensing of background light, and directly through gravitational waves produced when two black holes merge.