Stellar nucleosynthesis describes the set of processes by which nucleosynthesis inside stars creates new chemical elements. These elements are formed when nuclei undergo nuclear fusion and other reactions under the high temperatures and pressures found in stellar interiors.

Overview

As a star burns its nuclear fuel, lighter nuclei combine to form heavier ones; this changes the composition of the star over time. The internal rearrangement of elements alters the star’s structure and energy generation, a key part of why stars evolve. However, the elements produced remain locked inside the star unless some of the stellar material is returned to space.

Principal fusion stages

  • Hydrogen burning — fusion of hydrogen into helium, via chains such as the proton–proton chain and the CNO cycle.
  • Helium burning — conversion of helium into carbon and oxygen (for example, by the triple‑alpha reaction).
  • Advanced burning — in more massive stars, successive stages burn carbon, neon, oxygen and silicon, producing elements up to iron.
  • Neutron-capture processes — slow (s‑process) and rapid (r‑process) neutron captures create many of the isotopes heavier than iron, typically in late stellar phases or explosive environments.

Enriching the cosmos

For the composition of the universe to change appreciably, newly formed elements must leave the star and mix into the interstellar medium. This requires the loss of mass from the star through winds or violent ejection events; otherwise the elements remain confined to the stellar interior.

How elements are returned to space

Low- and intermediate-mass stars remove their outer layers slowly and at the end of their lives shed much of their envelope through strong stellar outflows — analogous to the solar wind — and often leave behind a glowing planetary nebula. Massive stars expel large amounts of material in powerful explosions, such as a supernova, distributing heavy elements produced by fusion and neutron-capture processes back into their surroundings.

Historical note

The idea that stars build the elements was articulated and developed in the mid‑20th century, most prominently by Fred Hoyle in 1946, and it has since been refined with observational and theoretical advances in stellar physics and nuclear astrophysics.