Forging the Elements

In the final phase of a massive star, the core burns successively heavier fuels — carbon, neon, oxygen, and silicon — each stage faster than the last, building an onion-like structure of shells. Silicon burning yields iron, which cannot release energy by fusing, ending the sequence and setting the stage for core collapse.

A second route from hydrogen to helium, in which carbon, nitrogen, and oxygen act as catalysts that are used and regenerated. Because it is extremely temperature-sensitive, the CNO cycle dominates hydrogen burning in stars more massive than roughly 1.3 solar masses, whose cores are hotter than the Sun's.

The dominant way low-mass stars like the Sun turn hydrogen into helium: through a chain of steps, four protons are fused into a single helium-4 nucleus, releasing energy and neutrinos. It powers the cores of the coolest main-sequence stars, where temperatures are too low for the competing CNO cycle.

The rapid capture of neutrons that forges the heaviest elements — gold, platinum, the lanthanides, and uranium. It demands an intense flood of neutrons in a fraction of a second, conditions found when two neutron stars merge, an origin confirmed by the kilonova that followed the 2017 gravitational-wave event GW170817.

The slow capture of neutrons that builds about half the elements heavier than iron, up to lead and bismuth. Neutrons are added one at a time, slowly enough that unstable nuclei usually decay before catching the next — a process that unfolds over thousands of years in the interiors of asymptotic-giant-branch stars.

How stars make carbon: three helium-4 nuclei (alpha particles) combine into carbon-12. It requires temperatures around a hundred million kelvin and proceeds through a finely-tuned nuclear resonance, so it only switches on in the helium-burning cores of evolved giant stars.