A young star cluster in a star forming region, consisting of stars of a huge variety of masses. Some of them will someday undergo silicon-burning, producing iron and many other elements in the process. The origin of the very heaviest elements, however, requires a different process. (ESO / T. PREIBISCH)

What Was It Like When The Universe Made Its Heaviest Elements?

The heaviest elements in the periodic table have their own unique story. No, they don’t come from a supernova.

When it comes to the elements of the Universe, every one of them has its own unique story. Hydrogen and helium were created in the earliest stages of the Big Bang; light elements like carbon and oxygen are created in Sun-like stars; heavier elements like silicon, sulfur and iron are created in more massive stars; elements beyond iron are made when those massive stars explode in supernovae.

But the most massive elements of all at the very high end of the periodic table — including platinum, gold, radon, and even uranium — owe their origins to an even rarer, more energetic process. The heaviest elements of all come from merging neutron stars, a fact that was long suspected but only confirmed in 2017. Here’s the cosmic story of how the Universe got there.

The elements of the periodic table, and where they originate, are detailed in this image above. While most elements originate primarily in supernovae or merging neutron stars, many vitally important elements are created, in part or even mostly, in planetary nebulae, which do not arise from the first generation of stars. (NASA/CXC/SAO/K. DIVONA)

Whenever you form stars, they arise from a large molecular cloud of gas that contracts down into a variety of clumps. The clumps grow more and more massive over time, as the atoms and…