We have heard much about black holes – those super-dense stars that swallow everything that gets trapped in its voracious gravitational pull. But neutron stars – close cousins of black holes – have received far less publicity despite the fact that they explain the origin of one of the most prized elements on earth – gold.
First, let’s recap what a black hole is. When a star runs out of hydrogen, its core implodes, then explodes in spectacular fashion in what is called a supernova. As the core undergoes runaway shrinkage, its gravity intensifies until it is so strong that nothing – not even light – can escape. A black hole is thus born.
By its very nature, a black hole is black and tiny. Like black holes, neutron stars are also spawned by supernova explosions. A typical neutron star is also super-dense due to the crushing force of gravity on a shrinking core. In fact, it is so dense that a sugar cube-sized volume of its material would weigh as much as the entire human population!
The crucial difference between a black hole and a neutron star is that it is not simply a bottomless pit in space-time but an object made of actual “stuff.” Whereas a black hole is black because all matter in the vicinity has long ago been sucked up, a neutron star is a fireball of blistering hot matter. Among these materials are gamma rays, a form of electromagnetic radiation arising from the decay of atomic nuclei.
As early as the 1960s, scientists detected weak bursts of gamma rays in space. But it did not occur to them that these gamma rays emerged from neutron stars – until 2017. For the first time, astronomers caught sight of a burst of gamma rays arising from the merger between two faraway neutron stars. What they saw was in fact the fingerprint of gold, or more precisely, gold isotopes forged in the fireball of the neutron star mergers. And it wasn’t an itsy-bitsy bit of gold; the amount of gold isotopes was about 20 times the mass of the Earth!
Astronomers have long known that all the elements heavier than hydrogen and helium were forged in the nuclear furnaces of stars, which, when they exploded, spewed them into space to be incorporated into successive generations of stars. But while they had successfully identified the origin of pretty much all of the 92 naturally-occurring elements on Earth, nobody knew where gold came from. Now we do. If you have a gold ring or a gold bar, look at them again with fresh eyes, knowing that their atoms were forged a long, long time ago before the Earth was born, in a furnace ignited by the merger of two distant neutron stars.