A neutron star is what is left when a massive star’s core collapses: more than a Sun’s worth of matter packed into a ball a city across. Sometimes two of them orbit each other, slowly spiralling inward as they lose energy, until they collide. The flash of light from that collision is a kilonova.
The name comes from its brightness: about a thousand times that of a classical nova, and a tenth to a hundredth of a typical supernova. Li and Paczyński described the idea in 1998, and Brian Metzger and colleagues gave it its name in 2010. For years it was only a prediction.
17 August 2017
At 12:41 UTC on 17 August 2017 the LIGO and Virgo detectors recorded a gravitational-wave signal unlike any before: not two black holes, but two lighter objects, together about 2.8 times the Sun’s mass. They were neutron stars. 1.7 seconds after the merger, NASA’s Fermi satellite caught a short gamma-ray burst from the same direction.
Within hours telescopes found a new point of light in the galaxy NGC 4993, roughly 130 to 140 million light-years away. More than 70 observatories on seven continents and in space followed it. It was the first event ever seen in both gravitational waves and light, and the kilonova, named AT 2017gfo, behaved as predicted. It started blue and turned red as the debris expanded and cooled.
Where gold comes from
The glow of a kilonova is powered by radioactivity. In the collision, neutron-rich matter is thrown out and rapidly builds up heavy atoms, a chain called the r-process. Astronomers picked out the elements strontium and yttrium in the spectrum of AT 2017gfo. In total, the merger is thought to have made heavy elements weighing about 16 000 times as much as Earth, including about ten Earth masses of gold and platinum. Collisions like this are one of the main places where the universe makes its heaviest elements.
The same event also measured gravity itself. Because the gravitational waves and the gamma rays arrived only seconds apart after travelling for over a hundred million years, the speed of gravity must equal the speed of light to within about one part in a thousand million million.
Since then
Other bursts have since been linked to mergers. GRB 211211A lasted 51 seconds, far longer than a typical short burst, yet it has been argued to come from a merger too. In 2023, GRB 230307A was associated with tellurium and other heavy elements. Each new one is a chance to watch the universe forging the atoms that end up in rings and circuit boards on Earth.