Dynamic Nucleosynthesis
Last updated: February 2026
Stars are not quiet nuclear furnaces. They are dynamic, turbulent systems in which nuclear reactions and fluid motion interact in subtle and sometimes dramatic ways. Most of the time, nuclear burning proceeds slowly compared to the movement of stellar material. Mixing smooths out composition differences, and the star evolves in a relatively orderly fashion. But in certain critical phases of stellar evolution, this separation breaks down.
Dynamic nucleosynthesis describes situations where nuclear reactions and fluid motion occur on comparable timescales. In these events, fresh fuel is transported into hot burning regions at just the right speed for reactions to ignite while mixing is still ongoing. The flow influences where and how nuclei are transformed — and the energy released by those reactions can, in turn, reshape the flow. Small changes in mixing efficiency can lead to large differences in the elements that are produced.
Hydrogen ingestion and the i process
One striking example is hydrogen ingestion in evolved stars. When hydrogen-rich material is mixed into a helium-burning convection zone, rapid nuclear reactions produce high neutron densities and trigger the so-called intermediate neutron-capture process (the i-process). Three-dimensional hydrodynamic simulations show that the flow becomes highly asymmetric and time-dependent, with large-scale oscillations driven by the energy release from burning. Advective two-stream post-processing methods applied to these simulations produce abundance patterns that match observations of metal-poor stars, providing evidence that this dynamic regime operates in nature (see also the Quanta Magazine feature).
O-C shell mergers in massive stars
Another example occurs in massive stars shortly before they explode as supernovae. When different nuclear burning shells interact — for instance, oxygen- and carbon-burning layers — turbulent entrainment can transport fresh material into hotter regions. The resulting nucleosynthesis can vary dramatically depending on the details of the mixing. Isotopes such as titanium-44, important for interpreting supernova remnants, are especially sensitive to these dynamic processes. This work also reveals that O-C shell mergers may be significant sources of p-nuclei and light odd-Z elements including potassium-40 — with implications for radiogenic heating in rocky exoplanets.
Nuclear physics in astrophysics
Dynamic nucleosynthesis environments produce nuclear reaction pathways that pass through unstable, short-lived isotopes away from the valley of stability. The nuclear properties of these species — reaction rates, decay half-lives, neutron-capture cross sections — are often poorly known or entirely unmeasured. This makes close collaboration with experimental and theoretical nuclear physicists essential. A comprehensive review in Nature Reviews Physics (see news) describes how bridging stellar astrophysics and nuclear physics is key to unlocking i-process nucleosynthesis. Targeted measurements, such as the first 139Ba(n,γ)140Ba reaction rate directly constrain the astrophysical conditions under which the i-process operates. We help organize the interdisciplinary aspects of this nuclear astrophysics research through the Canadian Nuclear Physics or Astrophysics Network (canpan.ca).
Between the extremes
At the opposite extremes lie more familiar regimes: in main-sequence stars like the Sun, nuclear reactions are so slow that mixing dominates and composition remains nearly uniform. In explosive events such as supernovae or neutron star mergers, reactions proceed so rapidly that they outpace fluid motion. Dynamic nucleosynthesis occupies the fascinating middle ground, where mixing and burning compete and co-evolve.
Understanding this interplay requires going beyond traditional one-dimensional stellar models. It demands three-dimensional hydrodynamic simulations that capture turbulent flows and their feedback on nuclear burning. By studying dynamic nucleosynthesis, we connect fluid dynamics, nuclear physics, and astronomical observations — revealing how the restless interiors of stars shape the chemical history of the Universe.