Malinda and I got engaged right at the end of last year, and we are in the thick of planning our wedding. There’s a lot to think about, but we’re getting there. We have our wedding rings now: two matching bands of white gold. Our jeweler, Josh, told us that this means that they are made from 75% yellow gold, mixed with some copper, nickel and zinc, and with a hard rhodium coating to make them shiny, white and durable. But I wanted to know: where do all these elements come from? How is gold made?
It turns out that the story of our wedding rings began ten billion years ago, in two massive stars that formed as a cloud of gas gently collapsed under its own weight to become a star cluster. As the pressure inside these two stars rose, eventually their centers will have become hot enough and dense enough for nuclear fusion to start. The hydrogen nuclei will have started to fuse together to make helium, and a lot of light. The high energy jostling of these nuclei, plus the light itself, will have kept the stars stable - for a while.
Stars like our Sun last for billions of years, but massive stars are different: the gravitational force within them is higher, and so they collapse faster when they form, and they get much hotter and much denser. Their fusion reactions are more energetic, making carbon, and oxygen, and then silicon, sulphur, calcium, and titanium. Massive stars are the forges of the Universe, creating metal out of lighter elements. Each successive reaction needs higher and higher temperatures to happen, and so they take place closer and closer to the end of the star’s life, as it succumbs to its own gravitational pull. Each of our two massive stars will have lived for just 10 million years, and in its final collapse as its fuel ran out, each star will have rushed, head over heels, into a blaze of creativity, making all of its copper and zinc on its very last day.
What happened next? Well, there’s a limit to how much pressure ordinary matter can withstand, and each star’s metal core will have morphed into a neutron star, from which the rest of the falling gas will have bounced back in an enormous nuclear explosion: a supernova, a cosmic fireworks display throwing out neutrons and hot gas that fuse together to make even heavier elements, like nickel.
And the two neutron stars that remained? With each one having recently had its own supernova epiphany, our couple will have found itself bound together in a gravitational dance, with just enough friction to slow them into an inevitable inspiral. As the music reached its climax, and the two stars merged into a single object, the different kind of supernova that resulted will have been truly spectacular. It’s flash of light would have been visible halfway across the Universe, and its outpouring of neutrons so rapid, so generous, that precious gold and rhodium will have been able to form.
The atoms that were created in the falling of these two stars will have continued on their journey, blown out into space to form new gas clouds, including the one in which our Sun was born, surrounded by a cloud of metal-rich smoke that then fell into shape as a clumpy disk of grains, rocks and eventually planets.
And the metals, compressed together by gravity into the Earth itself, lay there underground for 4 and a half billion years, patiently awaiting the clink of first the miners’ pick, and soon, the champagne glasses. Two bright young stars, a merger, and two rings, whose being arose in a rush of energy, an unforgettable cosmic spark, an outpouring of new material fused from old, of hope, and of love, and then: quiet - traveling, settling, enduring, lasting.