Nuclear fusion is at the center of stellar evolution. Much of a star’s life is a struggle between gravity and nuclear energy. Despite our broad understanding of this process, many details still escape our notice. We cannot dive into a star to see its nuclear furnace, so we rely on sophisticated computer simulations. A recent study has taken a big step forward by modeling the entire synthesis cycle of a single element.

Although stars begin their lives by converting hydrogen to helium, later in their lives, when hydrogen is depleted, they begin to fuse heavier elements. Toward the end of their lives, massive stars will combine helium with carbon, carbon with neon, neon with oxygen, and climb up the periodic table in a desperate attempt to avoid gravitational collapse. Although different fusion chains can form in different layers of the star, this is a complex process involving turbulent flows, heat transfer and mixing.

This process has several simulation models that simplify things by modeling a single piece of star. These one-dimensional simulations are powerful, but they fall short of simulating the volume of real stars. This new work produced a three-dimensional simulation of the neon combustion cycle of a ring in the thermonuclear chain, specifically a star with a mass of 20 solar masses.

Modeling of the neon fusion layer. Image credit: Rizzuti

To create the initial conditions for the simulation, the team used a so-called 1D model driven by the 321D. Essentially, they used hydrodynamic equations to project a 3D model into a one-dimensional simulation. They can then run this simulation from the star’s initial formation to its initial collapse and the onset of the neon glow. Taking this as an initial condition, they performed a full 3D simulation of the neon combustion stage from the beginning to the point of extinction of neon.

One of the things they discovered was that fusion in the neon layer was extremely efficient. Neon fusion is rapid and without mixing consumes pockets inside the star, creating nuclear dead zones. The simulations show that hydrodynamic mixing is sufficient to ensure a stable synthesis rate. Another discovery was that the size of the convection zones in the melt layer significantly influenced its structure and evolution. This has not been seen in 1D simulations and could change our understanding of the lifetimes of massive stars.

This study does not cover the general structure and evolution of the interior of the star, but it is an important step forward. Researchers can simulate other layers with a fully 3D model of one synthesis layer and eventually combine the simulations into a coherent 3D model of the entire synthesis chain. This will help us understand not only massive stars but also stars similar to our Sun. Source