What is a black hole?

A black hole forms when a lot of mass is squeezed into a tiny space. The boundary you can't come back from is called the event horizon. Outside it, a black hole pulls on things just like any other object of the same mass. If the Sun were swapped for a black hole of the same mass, the Earth would keep orbiting exactly as it does now, only in the dark.

How big is a black hole? The event horizon

The size of a black hole means the radius of its event horizon, called the Schwarzschild radius. It grows in step with the mass:

So the biggest black holes are larger than our whole Solar System: Neptune orbits at about 30 times the Earth-Sun distance. Work out the horizon for any mass in our Black Hole Calculator, or try one here and press Mark to check it in our Black Holes course.

What is the biggest black hole?

There is no settled answer, because the masses of the very biggest black holes are hard to measure. The best measurements watch stars or gas moving around the black hole and work out how much mass that motion needs.

• Holm 15A: a 2019 study tracked the motion of stars in the core of this giant galaxy, about 700 million light years away, and found a black hole of about 40 billion solar masses. It is one of the most massive black holes measured this direct way.
• TON 618 and other quasars: quasars are extremely bright galaxy centres where a black hole is swallowing gas. Indirect estimates put some, such as TON 618, at tens of billions of solar masses, but they rely on assumptions and carry large uncertainties.
• Phoenix A and a few other giants at the centres of galaxy clusters have also been suggested as record holders, with estimates that are even less certain.

Types of black holes

Only stars much heavier than the Sun end as black holes. The Sun isn't massive enough: it will end as a white dwarf.

How do black holes form?

1. A star many times heavier than the Sun burns through its fuel in a few million years.
2. When fusion in its core stops, nothing holds the core up against its own gravity, and it collapses in a fraction of a second.
3. The outer layers are blasted away in a supernova. If the leftover core is light enough, it becomes a neutron star. If it is heavier, nothing can stop the collapse, and it becomes a black hole.

Black holes can then grow by swallowing gas and stars, and by merging with other black holes. How supermassive black holes got so big so early in the universe's history is still an open question, and the James Webb Space Telescope is finding large black holes surprisingly soon after the Big Bang. The Black Holes course follows a star's life from birth to collapse.

What is the closest black hole to Earth?

The closest known is Gaia BH1, about 1,560 light years away in the constellation Ophiuchus, with about 10 times the mass of the Sun. It was found in 2022 from the wobble of a Sun-like star orbiting it, measured by ESA's Gaia spacecraft. In 2024, Gaia data revealed Gaia BH3, about 1,926 light years away and 33 times the Sun's mass, the most massive stellar black hole known in our galaxy, according to ESA. Closer, undiscovered ones probably exist, because a black hole with nothing near it is almost invisible.

When will a black hole hit Earth?

There is no known black hole on a path towards Earth, and none is expected. The nearest known one is over 1,500 light years away, and black holes don't roam around swallowing things. They move through space like stars do, and pull on things only as strongly as any star of the same mass. Our own galaxy's central black hole, Sagittarius A*, is about 27,000 light years away. Space is so empty that a close encounter with any black hole is extraordinarily unlikely.

What does a black hole look like?

A black hole itself gives off no light, so you can't see it directly. You can see what is around it: gas swirling in at enormous speed, heated until it glows, in a disc called an accretion disc. Against that glow, the black hole shows up as a dark shadow. Gravity bends light around it, so the shadow looks about two and a half times as wide as the event horizon.

The Event Horizon Telescope, a network of radio dishes around the world working as one Earth-sized telescope, made the first pictures. In April 2019 it released an image of M87*, 6.5 billion solar masses and 55 million light years away: a bright, lopsided ring around a dark centre. In May 2022 it released an image of Sagittarius A*, the black hole at the heart of the Milky Way.

What is inside a black hole?

Nobody knows for sure, because no information can get back out. Einstein's general relativity predicts that everything that falls in is crushed towards a point of infinite density at the centre, called a singularity. Most physicists think that prediction means general relativity breaks down there, and a theory of quantum gravity, which doesn't exist yet, is needed to say what really happens.

• Spaghettification: falling into a stellar-mass black hole, the pull on your feet would be much stronger than on your head, stretching you out long before you reached the horizon.
• Time slows down: to a distant watcher, a clock near the horizon ticks slower and slower, and something falling in seems to freeze at the edge and fade.
• A big one is gentler at the edge: near a supermassive black hole the stretching at the horizon is mild, so you could cross it without noticing, though you could never get out.

How do we know black holes are real?

• Orbiting stars: stars near the centre of our galaxy whip around an invisible object of about 4 million solar masses packed into a tiny space.
• Gravitational waves: on 14 September 2015, the LIGO detectors picked up ripples in space from two black holes merging over a billion light years away, as LIGO announced in February 2016.
• Pictures: the Event Horizon Telescope images of M87* and Sagittarius A* match the shadows general relativity predicts.
• X-rays: gas falling onto black holes in binary star systems gets hot enough to shine in X-rays.

Worked example: how small would the Sun have to be?

The Sun's mass is about 1.99 × 10³⁰ kg. Using r = 2GM ÷ c², with G = 6.67 × 10⁻¹¹ and c = 3.00 × 10⁸ m/s:

About 3 km. The Sun is about 1.4 million km across, so it would have to be squeezed to a ball smaller than a town. That is why only the collapsed cores of very massive stars become black holes.

Common mistakes

• Thinking black holes suck everything in like a vacuum cleaner. From far away, their gravity is the same as any object of the same mass.
• Thinking the Sun will become a black hole. It is far too light. It will become a white dwarf.
• Calling the event horizon a surface. It is a boundary in space, not something solid.
• Saying the EHT photographed the black hole itself. It imaged the glowing gas and the shadow around it.
• Treating record masses as exact. The biggest black holes' masses carry large uncertainties.

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