Summary
Highlights
Heavy elements were formed billions of years after the Big Bang, inside stars. The extreme density and heat within stars enable nuclear fusion, which is necessary to synthesize heavy elements. This process is known as stellar nucleosynthesis, where lighter nuclei fuse in the star's core.
Several nuclear fusion pathways create heavy elements, including the triple alpha process, the carbon-nitrogen-oxygen (CNO) cycle, and the alpha ladder. These processes occur in layers near the core of stars where temperatures are high enough.
The triple alpha process involves the fusion of three helium-4 atoms (alpha particles). First, two alpha particles fuse to form beryllium, which then fuses with a third alpha particle to create carbon-12.
The alpha ladder continues the fusion of elements with alpha particles, creating progressively heavier elements up to iron. Examples include carbon-12 fusing with an alpha particle to form oxygen-16, oxygen-16 forming neon-20, neon-20 forming magnesium-24, and magnesium-24 forming silicon-28, until iron is reached.
Elements heavier than iron cannot be formed through typical stellar fusion because it requires immense amounts of energy. These elements are predominantly created during a supernova, which is a massive explosion of a star.
In a supernova, a neutron capture reaction occurs, forming heavy elements by adding neutrons to existing nuclei instead of fusion. This includes the rapid neutron capture (r-process) which is responsible for about half of the atomic nuclei heavier than iron, and the slow neutron capture (s-process) which happens in red giants.