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Two neutron stars collide

How two dead stars collide, forge gold, and were both heard and seen from Earth 130 million years later

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 11 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. Two neutron stars

Two neutron stars, of 1.46 and 1.27 Suns but each only about 24 km across, circle each other 130 million light-years away. They are 1,000 km apart and go round 3 times a second, shown here at their real speed.

Each is what was left when a massive star's core collapsed in a supernova: more mass than the Sun, crushed into a ball the size of a city and made mostly of neutrons. Their gravity is so strong that light from their far sides bends round towards us, so they look about a quarter larger than they are.

Each was born in its own supernova, yet the two stayed bound to each other. Ever since, their orbit has been shrinking, and here they have a little over an hour left. They lie in NGC 4993, a galaxy in the constellation Hydra.

2. Ripples in space

As they circle, they shake space itself. Gravitational waves spread out at the speed of light, stretching space one way and squeezing it the other, twice each orbit.

The waves carry energy away, and it comes out of the orbit, so the stars draw closer and circle faster. Neutron stars are far lighter than the black holes whose merger was heard first, so their waves are weaker and their spiral far slower.

Here the waves are drawn on the plane of the orbit, gold where space is stretched and blue where it is squeezed. At this stage they rise and fall six times a second, and each is about 50,000 km long, four times as wide as Earth.

3. Faster and faster

Time sped up: over the next hour the stars close from 1,000 km to 400 km apart and speed up to 12 turns a second. Their waves are now fast enough for LIGO and Virgo to hear.

The closer they are, the faster they move and the stronger their waves, so the orbit shrinks ever faster. From here the stars have 100 seconds left, and in that time they will go round nearly 2,000 more times.

The waves' pitch is twice the orbit's turns a second. At 24 cycles a second the stars came within the hearing of LIGO's two detectors in the United States and Virgo in Italy, which catch waves from about ten to a few thousand cycles a second.

4. The last 100 seconds

Time sped up, then slowed: in their last 100 seconds the stars close from 400 km apart to almost touching, and the waves' pitch climbs from 24 to over a thousand cycles a second.

This is what LIGO and Virgo heard: thousands of cycles over about 100 seconds, rising in pitch and strength, a far longer chirp than the fraction of a second black hole mergers had given. Turned into sound, it sweeps up like a bird's call.

The long chirp let scientists weigh the stars precisely. The blend of their masses that sets how fast the pitch rises came out at 1.19 Suns, to within a thousandth of a Sun; the masses themselves less exactly, about 1.46 and 1.27 Suns if, as is likely, the stars spin slowly.

5. Torn apart

Slowed down about 2,000 times: stretched by each other's pull, the stars touch at 40% of the speed of light. Tides tear arms of matter out of them, flung off at up to a quarter of light's speed.

Each star's near side is pulled harder by the other than its far side, and in their last turns the tides stretch them out of shape. As they collide, their outer layers are torn off and flung out in two curving arms, like water from a spinning garden sprinkler.

Where they meet, the crash squeezes and heats matter to hundreds of billions of degrees, and more of it squirts out, fastest towards the poles. What really happens here is known only from supercomputer simulations, and the shapes shown follow them only roughly.

6. Too heavy to last

The two become one star of about 2.7 Suns, spinning hundreds of times a second. Too heavy to hold itself up for long, it collapses into a black hole, ringed by a disk of debris.

At first only its fast spin and its heat hold the merged star up. The heaviest neutron stars ever found have little more than two Suns, and the light of this collision suggests the merged star collapsed within a fraction of a second, into a black hole about 15 km across.

Gas left circling it forms a hot, thick disk, which the black hole swallows or blows away within about a second. Here time is slowed down at first, then sped up, and the cloud of matter flung out is drawn see-through, so the star and the black hole show.

7. A jet of gamma rays

Along its axis a narrow jet bursts out at nearly the speed of light. Its gamma rays, drawn here in violet, reached Earth 1.7 seconds after the waves.

The jet is probably driven by the black hole's spin and the disk's magnetic field. It had to bore through the matter already flung out. Its gamma rays, far too energetic for any eye to see, are the brief flash astronomers call a short gamma-ray burst.

NASA's Fermi satellite caught this one, GRB 170817A, about two seconds long. It was unusually faint because the jet did not point at us: later radio images showed it aimed about 20 degrees away. Arriving 1.7 seconds after the waves, after 130 million years on the way, it showed that gravity travels at the speed of light, to within one part in 10¹⁵.

8. A kilonova

Time sped up: the matter flung out, some 20,000 Earths' worth, glows as the new heavy elements in it decay. This glow, a kilonova, shone like hundreds of millions of Suns.

The newly made atoms are unstable and break down over hours and days, heating the cloud from within. It shines about a thousand times brighter than a nova, an eruption on a white dwarf's surface, hence the name kilonova, given in 2010. This was the first one ever seen.

When first seen, eleven hours after the collision, it shone like about 270 million Suns and was already fading. Its first hours were not seen, so here they follow models. It was blue-white, about 10,000 degrees, and swelling at almost a third of the speed of light.

9. Gold and platinum

Days sped up: it cools from blue-white to red and fades. Its light shows elements heavier than iron being made: strontium was found in it, and it formed gold and platinum too.

In the first second after the collision, the flung-out matter was so rich in neutrons that atoms swallowed them faster than they could decay, building up elements far heavier than iron. This is the rapid process, or r-process; about half of the elements heavier than iron are thought to be made this way.

Heavy atoms called lanthanides, made alongside gold and platinum, block blue light, so as the cloud thins its light turns red and then infrared. Strontium was identified in its light in 2019. How much gold formed is less certain: estimates run from several to a few hundred times Earth's mass.

10. Seen from Earth

On 17 August 2017 at 12:41 UTC the waves swept across Earth, and 1.7 seconds later the gamma rays. Eleven hours on, a telescope in Chile found the new light in the galaxy NGC 4993.

LIGO's two detectors and Virgo together narrowed the source to a patch of sky about 130 times the Moon's area, in the constellation Hydra, some 130 million light-years away. Telescopes around the world searched the galaxies there as soon as darkness fell.

The Swope telescope at Las Campanas in Chile found it first, a new point of light near the centre of NGC 4993. Within days some 70 observatories, on all seven continents and in space, had watched it, from gamma rays to radio waves. The galaxy here is drawn from a simple model of it.

11. Heard and seen

It was the first time the same event was both heard in gravitational waves and seen in light. It showed where much of the universe's gold is made.

It proved that colliding neutron stars cause short gamma-ray bursts and forge heavy elements, and that gravitational waves travel at the speed of light. The waves' strength gave the distance, and the galaxy's speed away from us gave a new way to measure how fast the universe expands.

Days later X-rays appeared, then radio waves: the afterglow of the jet, which kept brightening for months. In 2019 detectors heard another pair of neutron stars collide, but no light was found from it. By 2025 this was still the only collision both heard and seen.

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