Intermediate · 1:49

How black holes are made

How the core of a giant star falls in on itself and closes off from the universe

Each is simulated from real physics and measurements; anything no eye could see, like magnetic fields and radio waves, is drawn and labelled as drawn.

Below are the 9 chapters of this simulation in Cosmic Scale: each one's caption, then the fuller explanation the app shows under “Read more”. Colours, arrows and lines mentioned in the text belong to the 3D scene in the app.

Open Cosmic Scale then the Phenomena tab

1. A doomed giant

A supergiant of 25 Suns, 1.3 billion km across, is nearing its end. Its core will collapse as a smaller star's does, but this time the explosion may fail.

Stars of more than about 20 times the Sun's mass end their lives with cores so big and dense that the usual supernova can struggle to get going. Computer models suggest that a good share of them fail to explode at all.

This one is modelled on N6946-BH1, a red supergiant of about 25 Suns in the galaxy NGC 6946, 22 million light-years away. Astronomers watching thousands of such stars for vanishing acts caught it in 2009.

From outside it looks like any other bloated, cool supergiant: nothing warns of what its core is about to do.

2. The core gives way

As in a supernova, the iron core collapses in a quarter of a second and bounces, forming a neutron star about 30 km across. A shock wave sets out, and stalls.

Everything starts as in an ordinary supernova: the iron core, too heavy for its electrons to hold up, falls in on itself and stiffens when it reaches the density of an atomic nucleus. The new neutron star is still swollen and hot, and pours out neutrinos.

The shock wave from the bounce runs out of energy within a few hundredths of a second and halts about 150 km out, as it does in every such collapse. What happens next decides whether the star explodes.

3. Too much to hold

Here the neutrinos cannot revive the shock. Gas keeps raining onto the neutron star, which grows from 1.8 to 2.3 times the Sun's mass in about a second.

In the densest cores, gas falls in so fast that the neutrinos' heating cannot drive the shock out again. The shock sinks back, and the star's inner layers pour onto the neutron star at close to half the speed of light.

A neutron star is held up by the pressure of its neutrons, squeezed together as tightly as the nuclei of atoms. That pressure has a limit: the heaviest neutron star known, PSR J0952-0607, has 2.35 times the Sun's mass, and none is expected to be much heavier.

Every tenth of a second adds more mass. The neutron star is running out of time.

4. Past the limit

No force can hold it now: it collapses in about a ten-thousandth of a second. A horizon appears at its centre and grows outwards, meeting the falling surface just as that passes the horizon's final size.

This is the simplest collapse general relativity describes exactly, worked out by Robert Oppenheimer and Hartland Snyder in 1939. The horizon is not something the star's matter makes at its surface: it is the edge of the region from which light can no longer get out, and it starts as a point at the centre.

Once the surface is inside, nothing of the star can ever be seen again. Its matter goes on falling towards the centre, where general relativity predicts a singularity: a point where our present laws of physics stop working.

For a neutron star of 2.3 Suns, the horizon ends up about 14 km across.

5. Frozen, reddened, gone

Seen from far away, the surface seems to slow as it nears the horizon and never quite reach it. Its light grows redder and fainter, and within a thousandth of a second there is nothing left to see.

Close to the horizon, time as seen from far away stretches without limit, and light climbing out loses more and more of its energy. So a distant watcher sees the collapse slow almost to a stop, the star frozen just outside its horizon. But the frozen star is invisible: its light fades by a factor of about three every sixty millionths of a second.

This is the truth behind a popular claim, that someone falling into a black hole would watch the whole future of the universe race by. They would not: a falling traveller crosses the horizon in a moment of their own time and sees nothing special there. It is only watchers far away who never see them cross.

6. A star vanishes

The rest of the core falls in within the hour, and the hole grows to about ten Suns. The star's loosely held outer layers drift off, glowing faintly for months; then there is nothing left to see.

The neutrinos carry off so much energy that the star suddenly weighs less, by a few tenths of a Sun. Its outer layers, barely held by gravity, feel the pull loosen and drift away, glowing dimly: the only sign from outside that anything has happened.

That is what astronomers saw in 2009: N6946-BH1 brightened to about a million Suns for a few months, then faded. By 2015 Hubble could not find it in visible light, where a star of 25 Suns had been. Only a faint infrared glow of warm dust remains.

It is the best case yet of a star collapsing straight into a black hole, though astronomers are still checking that nothing else could explain it.

7. A black hole

What is left is a black hole of about ten Suns: a horizon 59 km across, no bigger than a city, bending the light of the stars behind it.

The black disc is its shadow, about two and a half times wider than the horizon, because light that comes too close is captured. Just outside it, the light of the stars behind is bent into rings; the thin bright one hugging the shadow is light that circled the hole before escaping.

All that is left of the star is its mass, its spin and any electric charge: a black hole has no surface and nothing else to tell it apart. Two black holes with the same mass and spin are exactly alike.

Our galaxy should hold something like a hundred million black holes made this way. Most are alone and dark, and almost impossible to find.

8. Feeding on a neighbour

A black hole with a close companion can pull gas off it into a disk. Heated to millions of degrees, the disk shines in X-rays: this is how Cygnus X-1, the first black hole found, gave itself away.

Cygnus X-1 is a black hole of 21 Suns circling a blue supergiant of about 41, 7,200 light-years away. Its X-rays were found in 1964, and by the early 1970s the companion's orbit showed that the unseen partner was too heavy to be anything but a black hole.

Gas spiralling in heats up by rubbing against itself, and near the hole it moves at a good fraction of the speed of light. The side of the disk coming towards us looks brighter than the side moving away, and the hole's gravity bends the far side of the disk into view.

In 2024 the Gaia spacecraft found Gaia BH3, a black hole of 33 Suns only 1,926 light-years away, by the way it swings its companion star round. It is feeding on nothing, and dark.

9. From cities to giants

Black holes made by stars weigh a few to a few dozen Suns. At the centres of galaxies are giants: Sagittarius A* has four million Suns, M87* six and a half billion, with a horizon wider than our planetary system.

A horizon grows in step with its mass: about 6 km across for each Sun's worth. Sagittarius A*'s is about 25 million km across; M87*'s about 38 billion km, several times wider than Neptune's orbit.

Anything squeezed small enough would become a black hole. The Sun would have to be crushed into a ball 6 km across; Earth into one just 18 mm across, the size of a marble.

How the giants got so big is still being worked out. They grow by swallowing gas and by merging with one another, but the James Webb Space Telescope has found black holes of millions of Suns less than 500 million years after the Big Bang, which may have started out big: born from collapsing clouds of gas, with no star at all.

When two black holes merge they shake space itself, sending out gravitational waves. Since 2015 detectors on Earth have caught hundreds of such mergers.

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