A burst of gamma rays more than 10 times brighter than any detected before struck Earth in October 2022, searing the atmosphere, wowing astronomers, and earning the nickname of the BOAT—the brightest of all time. Now, astronomers using NASA’s JWST orbiting observatory have identified the source of the blast—and stumbled on a new puzzle.

The big bang endowed the universe with abundant hydrogen and helium. But all the other 92 natural elements are forged in stars as nuclei are fused together into bigger ones. Ordinary stars produce lighter elements, but the ones heavier than iron are thought to require the explosive conditions of a supernova or some other extreme event.

Half of the heavier elements—those richest in neutrons—require special conditions, in which neutrons bombard a seed nucleus so quickly that it doesn’t have time to decay before it absorbs more—a process known as rapid neutron capture or the r-process. Stars dating back to the early universe contain r-process elements such as europium, suggesting the process began early.