nucleosynthesis

History

In the 1940s–1950s, scientists George Gamow, Ralph Alpher and others proposed that the elements originated in the hot early universe. But it later turned out that most elements were created in stars. In 1957, the famous B²FH paper was published, which described in detail the processes of stellar nucleosynthesis.

How it works

In the center of a star, temperatures of millions of degrees cause nuclei to collide and fuse, releasing energy, like compressed springs releasing their stored force when joined together. The star successively burns hydrogen, then helium, carbon and so on, until iron is formed. Iron is 'nuclear ash', further fusion yields no energy. Then the core collapses, and the star explodes, creating elements heavier than iron.

History

In the 1940s–1950s, scientists George Gamow, Ralph Alpher and others proposed that the elements originated in the hot early universe. But it later turned out that most elements were created in stars. In 1957, the famous B²FH paper was published, which described in detail the processes of stellar nucleosynthesis.

How it works

In the center of a star, temperatures of millions of degrees cause nuclei to collide and fuse, releasing energy, like compressed springs releasing their stored force when joined together. The star successively burns hydrogen, then helium, carbon and so on, until iron is formed. Iron is 'nuclear ash', further fusion yields no energy. Then the core collapses, and the star explodes, creating elements heavier than iron.

History

In the 1920s, Aston and Eddington proposed that the energy source of stars is nuclear reactions. In the 1940s, Bethe and von Weizsäcker discovered the CNO cycle. In 1948, Gamow and Alpher developed the hot universe model, predicting the cosmic microwave background and primordial nucleosynthesis. The key B²FH paper (Burbidge, Burbidge, Fowler, Hoyle) in 1957 systematized the processes of stellar nucleosynthesis, including s- and r-processes. For this research Fowler received the Nobel Prize in 1983.

How it works

Nuclear reactions are initiated by the tunnel effect (quantum penetration through the Coulomb barrier). In stars, the weak interaction is essential, converting protons into neutrons, enabling the synthesis chains. Reaction rates are computed through cross-sections averaged over the Maxwell-Boltzmann distribution (Gamow peak). The released energy counteracts gravity, creating hydrostatic equilibrium. As fuel is exhausted, the core contracts and heats up, triggering the next stage. For the synthesis of elements heavier than iron, an external source of neutrons is required; the s-process occurs at low neutron density (~10^8 cm⁻³), the r-process during an explosion, when the neutron flux is enormous (~10^20–10^30 cm⁻³). The equivalence of mass and energy E=mc^2 determines the energetics.

Open problems & fun facts

The cosmological lithium problem: the predicted abundance of ^7Li is 3–4 times higher than observed in old stars.; Precise rates of key reactions: for example, ^12C(α,γ)^16O, which determines the ratio of carbon to oxygen in the universe.; Astrophysical sites of the r-process: the contribution of supernovae and neutron star mergers is not yet fully clear.; Nucleosynthesis in the first generation of stars (Population III): how elements formed in the absence of metals.

💡 The most abundant isotope in the universe is hydrogen-1, but the second most abundant is helium-4, and carbon-12 owes its abundance to a fortuitous resonance in the triple-alpha process, predicted by Fred Hoyle (the Hoyle state).