Tracing gold’s cosmic origin story

Abstract

Astronomers thought they had finally figured out where the gold, platinum, and other heavy elements in the universe came from. In light of recent results, they’re not so sure. Desperate phone calls made in the dead of night rarely convey good news, much less first word of a major scientific discovery. Alex Ji made such a call in 2015 from atop a mountain in Chile, where he was using one of the world’s largest telescopes. “This was actually the first time that I had taken data on a telescope ever,” says Ji, then a graduate student at the Massachusetts Institute of Technology in Cambridge. “I was absolutely convinced that I had done something wrong.” How does the universe make heavy elements such as gold? It’s a question astronomers have wrestled with for more than half a century—and the answers are only starting to emerge. Image credit: Science Source/Tom McHugh. So, just after midnight, Ji called his advisor Anna Frebel to describe what he had just seen and could barely believe: A faint star in a dim nearby galaxy named Reticulum II seemed to have extraordinary amounts of europium, an element that’s even rarer in the cosmos than gold. Frebel told Ji to observe another star in the same small galaxy. That star also turned out to be rich in europium, as did a third. In the end, seven of the nine stars Ji observed had extreme levels of the rare element. It would take some time for the astronomers to interpret their surprising result, but when they did, it bore on one of the most pressing questions in astronomy: How does the universe make its heaviest elements? It’s a question astronomers have wrestled with for more than half a century. Elements such as gold, silver, and platinum don’t normally form …