What Is a Black Hole? How They Form, What’s Inside and How We See Them

A black hole is a region of space where matter is packed so tightly that gravity stops anything, even light, from escaping. That simple definition hides some of the strangest physics in the universe: objects heavier than billions of Suns, boundaries where time appears to stand still, and shadows that astronomers have now photographed directly. This guide explains what black holes are, how they form, how big they really get and how scientists study something that, by definition, cannot be seen.

What Exactly Is a Black Hole?

Every object with mass bends space and time around it. Earth’s gravity holds the Moon in orbit; the Sun’s holds the planets. Gravity grows stronger the more mass you squeeze into a smaller space. If you compress enough mass into a small enough volume, the speed needed to escape its pull rises above the speed of light, about 300,000 kilometres per second. Nothing can travel faster than light, so nothing gets out. That is a black hole.

According to NASA, a black hole is “an astronomical object with a gravitational pull so strong that nothing, not even light, can escape it”. The idea comes from Albert Einstein’s general theory of relativity, published in 1915. Within months, the German physicist Karl Schwarzschild had worked out a solution to Einstein’s equations that described exactly this kind of object, although for decades many scientists, including Einstein himself, doubted such things could exist in nature.

What Is the Event Horizon?

The event horizon is the boundary of a black hole, the point of no return. Outside it, light and matter can still escape if they move fast enough. Once anything crosses it, every possible path leads further in. The event horizon is not a physical surface; an astronaut falling through a very large black hole would not feel any bump as they crossed it.

At the centre, general relativity predicts a singularity, a point of infinite density. Most physicists treat that as a sign that the theory breaks down there rather than a literal description, and a full explanation will probably need a theory that unites gravity with quantum mechanics.

The size of the event horizon depends only on mass. Its radius, known as the Schwarzschild radius, is roughly 3 kilometres for every solar mass. A black hole with ten times the Sun’s mass would have an event horizon about 60 kilometres across, smaller than many cities.

How Do Black Holes Form?

Astronomers recognise several kinds of black hole, sorted mainly by mass:

• Stellar-mass black holes form when very massive stars, roughly 20 times the Sun’s mass or more, run out of fuel. The core collapses under its own gravity, often in a supernova explosion, leaving a black hole typically between about 5 and a few dozen solar masses.
• Supermassive black holes contain millions to billions of solar masses and sit at the centres of most large galaxies, including our own. How they grew so large so early in cosmic history is one of astronomy’s big open questions.
• Intermediate-mass black holes, from hundreds to hundreds of thousands of solar masses, are thought to bridge the gap. Evidence for them is growing but is still thinner than for the other two types.
• Primordial black holes are hypothetical objects that may have formed in the first moments after the Big Bang. None has been confirmed.

Black holes can also grow. They swallow gas, dust and stars that stray too close, and two black holes can spiral together and merge into a single, heavier one.

Is a Black Hole Bigger Than the Sun?

It depends on what you mean by “bigger”. A typical stellar black hole is far heavier than the Sun but far smaller in size. The Sun is about 1.39 million kilometres across. A ten-solar-mass black hole’s event horizon would be only about 60 kilometres wide, so you could fit a vast number of them inside the Sun’s volume.

Supermassive black holes are another matter. Sagittarius A*, the black hole at the centre of the Milky Way, is about 4 million times the mass of the Sun. Its event horizon is roughly 24 million kilometres across, about 17 times wider than the Sun. M87*, in the galaxy Messier 87, has about 6.5 billion solar masses. Its event horizon spans tens of billions of kilometres, wider than the orbit of Neptune.

A useful rule of thumb: double the mass and you double the width of the event horizon.

Has Anyone Actually Photographed a Black Hole?

Yes, twice. On 10 April 2019 the Event Horizon Telescope (EHT) collaboration released the first image of a black hole: M87*, about 55 million light-years away. The picture, shown at the top of this article, is not of the black hole itself, which emits no light, but of its shadow surrounded by a glowing ring of superheated gas bent by gravity.

To get enough detail, the EHT linked radio telescopes across the planet into a virtual telescope the size of Earth. In May 2022 the same team released the first image of Sagittarius A*, the black hole at the heart of our own galaxy, about 27,000 light-years away. Despite being around 1,500 times less massive than M87*, it looks similar because it is so much closer.

How Do Scientists Find Black Holes If They Are Invisible?

Astronomers detect black holes from their effects on their surroundings. The main methods are:

• Orbiting stars. For decades, teams tracked stars whipping around an invisible point at the centre of the Milky Way. Their orbits revealed a mass of about 4 million Suns packed into a tiny space. Reinhard Genzel and Andrea Ghez shared half of the 2020 Nobel Prize in Physics for this work.
• X-rays from hot gas. Material falling towards a black hole forms a swirling disc that heats up to millions of degrees and glows in X-rays. Cygnus X-1, identified in the early 1970s, was among the first strong black hole candidates found this way.
• Gravitational waves. On 14 September 2015 the LIGO observatories detected ripples in space-time from two black holes, about 36 and 29 solar masses, merging around 1.3 billion light-years away. It was the first direct detection of gravitational waves and earned a Nobel Prize in 2017. Detectors have since recorded hundreds of mergers.
• Wobbling stars. The European Space Agency’s Gaia mission has found quiet black holes by spotting stars that wobble around unseen companions, including Gaia BH1, roughly 1,560 light-years away and among the closest known black holes to Earth.

What Would Happen If You Fell Into a Black Hole?

Near a small, stellar-mass black hole, gravity pulls much harder on your feet than your head. That difference stretches you into a thin strand long before you reach the event horizon, an effect physicists genuinely call “spaghettification”. Near a supermassive black hole, the stretching at the event horizon is far gentler, so in theory you could cross it intact, although you would never be able to come back or send a signal out.

To a distant observer, something different appears to happen. Because gravity slows time, they would see you slow down, redden and fade as you approached the horizon, never quite seeming to cross it.

Could a Black Hole Swallow Earth?

No. Black holes are not cosmic vacuum cleaners. At a distance, their gravity behaves like that of any other object with the same mass. If the Sun were somehow replaced by a black hole of exactly one solar mass, Earth would keep orbiting just as it does today, although it would become very cold and dark.

The nearest known black holes are more than a thousand light-years away, far too distant to affect the solar system. Fears that particle colliders could create dangerous black holes have also been examined and dismissed by physicists; any microscopic black hole, if one could form at all, would be expected to evaporate almost instantly.

Do Black Holes Last Forever?

Probably not. In 1974 Stephen Hawking showed that quantum effects should make black holes emit a faint glow, now called Hawking radiation, and slowly lose mass. For black holes of stellar mass or larger, the process is unimaginably slow, taking vastly longer than the current age of the universe. Hawking radiation has never been observed directly, but it remains one of the most important ideas in modern physics because it links gravity, quantum theory and thermodynamics.

Why Do Black Holes Matter?

Black holes are more than curiosities. Supermassive black holes appear to shape how galaxies grow, because the energy released by gas falling into them can heat or blow away the material that would otherwise form new stars. They are also natural laboratories for testing Einstein’s theory in the most extreme conditions known, and so far general relativity has passed every test.

If you enjoy space science, read our guide on how to see the northern lights, our report on NASA’s changing Artemis Moon landing plans and the story of the Euclid telescope’s first comet discovery.

Black Holes at a Glance

• Definition: a region where gravity is so strong that not even light can escape
• Boundary: the event horizon, about 3 km in radius per solar mass
• Milky Way’s black hole: Sagittarius A*, about 4 million solar masses, 27,000 light-years away
• First image: M87*, released 10 April 2019
• First gravitational waves detected: 14 September 2015, from two merging black holes
• Can one swallow Earth? No; the nearest known are more than 1,000 light-years away

Featured image: Black hole – Messier 87 crop max res, by Event Horizon Telescope collaboration / ESO / Wikimedia Commons, CC BY 4.0. Cropped, colour-graded and combined with MediaPavilion branding.

Sources

• NASA Science — Black Holes (accessed 27 October 2026).
• Event Horizon Telescope — Astronomers Capture First Image of a Black Hole (10 April 2019).
• Event Horizon Telescope — Astronomers Reveal First Image of the Black Hole at the Heart of Our Galaxy (12 May 2022).
• LIGO Caltech — Gravitational Waves Detected 100 Years After Einstein’s Prediction (11 February 2016).
• Nobel Prize — The Nobel Prize in Physics 2020 (6 October 2020).
• NOIRLab — Astronomers Discover Closest Black Hole to Earth (4 November 2022).