Neutron Star Mergers: Gravitational Waves, Kilonovae and the Origin of Gold

Two city-sized stars, spiralling together

A neutron star packs more mass than the Sun into a sphere only about twenty kilometres across, dense enough that a teaspoon of its material would weigh roughly as much as a mountain on Earth. When two neutron stars orbit each other in a close binary system, they are not gravitationally stable forever: general relativity predicts that the pair continuously radiates energy away as gravitational waves, ripples in spacetime itself, and that steady energy loss slowly shrinks the orbit. Over millions to billions of years the inspiral accelerates, and in the final seconds before collision the two stars are whipping around each other hundreds of times per second.

The chirp that gravitational-wave detectors listen for

As the orbit shrinks and speeds up, the frequency and amplitude of the emitted gravitational waves both rise together, producing a distinctive rising tone that detectors such as LIGO and Virgo call a chirp. On 17 August 2017, this chirp was recorded from a neutron star merger for the first time, an event catalogued as GW170817, and crucially it was also seen across the electromagnetic spectrum within seconds by an independent gamma-ray burst detection, followed by weeks of observation across optical, infrared, radio and X-ray telescopes - the first time a single cosmic event had ever been confirmed simultaneously through both gravitational waves and light.

gravitational-wave chirp (schematic), final seconds of inspiral:

  frequency:  rises as the orbit shrinks and speeds up
  amplitude:  rises as the stars accelerate toward merger

  ...low, quiet  ->  rising, louder  ->  peak at merger  ->  ringdown

matched against GW170817:
  ~100 seconds of signal in LIGO's sensitive band before merger

The kilonova: where heavy elements are forged

At the moment of collision, tidal forces shred a fraction of the neutron-rich material off the merging stars and fling it outward at a meaningful fraction of the speed of light. This ejected material is so overwhelmingly rich in free neutrons that it undergoes rapid neutron capture, or the r-process - atomic nuclei absorb neutrons far faster than they can beta-decay away, building up extremely heavy, neutron-rich isotopes that then decay toward stability, releasing a burst of light called a kilonova. This process is now understood to be a dominant source of the universe's gold, platinum, and other elements heavier than iron, elements that ordinary stellar fusion cannot produce because fusion stops being energetically favourable once a nucleus reaches iron.

What is left behind

The immediate aftermath of the merger depends sensitively on the combined mass of the two neutron stars. If the remnant's mass stays below the neutron star maximum-mass limit, it can briefly or even indefinitely persist as a single, more massive neutron star; more often, for typical observed masses, the remnant exceeds that limit and collapses promptly or within milliseconds into a black hole, sometimes still surrounded by a hot disk of leftover debris that can power a relativistic jet and an accompanying short gamma-ray burst, matching what was observed alongside GW170817.

Frequently asked questions

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Everything above runs in your browser — open Neutron Star Merger & Kilonova and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.