Supernova nucleosynthesis

Supernova Nucleosynthesis: Forging the Stars’ Building Blocks

Introduction

In the grand cosmic symphony, supernovae play a vital role as the crescendo in the synthesis of chemical elements. This process, known as supernova nucleosynthesis, unfolds within the spectacular explosions of stars that possess sufficient mass.

During their life cycle, these massive stars undergo sequential stages of hydrostatic burning, where lighter elements are transformed into heavier ones through nuclear fusion. The phases include helium burning, carbon burning, neon burning, oxygen burning, and silicon burning. In this context, “burning” refers to nuclear fusion, not chemical reactions.

During these hydrostatic stages, the fuels predominantly synthesize alpha nuclides (atomic mass equals twice the atomic number), which are nuclei composed of integer numbers of helium-4 nuclei. This process begins with two helium-4 nuclei combining to form a beryllium-8 nucleus. The addition of another helium 4 nucleus results in carbon-12, followed by oxygen-16, neon-20, and so on, with each step adding 2 protons and 2 neutrons to the growing nucleus.

A rapid final explosive burning stage is initiated by the sudden temperature surge caused by the passage of a radially moving shock wave, which originates from the gravitational collapse of the core. This final burning stage, demonstrated by W. D. Arnett and his colleagues at Rice University, is more effective in synthesizing non-alpha nucleus isotopes than hydrostatic burning . As a result, elements like carbon (Z = 6), oxygen (Z = 8), and those with Z = 10 to 28 (from neon to nickel) are primarily produced by this combined process of shock-wave nucleosynthesis and hydrostatic burning .

The ejection of these newly created isotopes by supernova explosions gradually increased their abundance within interstellar gas, a pattern astronomers have observed since the initial abundances in newly born stars surpassed those in earlier-born stars.

Beyond Nickel: Synthesizing Heavy Elements

Elements heavier than nickel are less common due to the decrease with atomic weight of their nuclear binding energies per nucleon, but they too are produced partially within supernovae. Of particular historical interest is their synthesis through the rapid capture of neutrons during the r-process . However, recent research has proposed alternative methods for heavy element synthesis, as discussed below.

The r-process isotopes are about 100,000 times less abundant than the primary elements fused in supernova shells above. Additionally, other nucleosynthesis processes within supernovae, such as the rapid proton capture process (rp-process), the slow neutron capture (s-process) in helium- and carbon-burning shells of massive stars, and the photodisintegration process (γ-process or gamma-process), are believed to contribute to the synthesis of other heavy elements. The latter primarily produces lightweight, neutron-poor isotopes of elements heavier than iron from existing heavier isotopes .

Conclusion

Supernova nucleosynthesis is a breathtaking cosmic event that reshapes the universe, forging the building blocks of life and the cosmos. Understanding this process offers insights into the evolution of stars, the birth of elements, and even our own origins. The intricate dance between various nuclear processes within supernovae continues to fascinate astronomers, shedding light on the mysteries that lie at the heart of our universe.