A pot of porridge boiling over a high flame: bubbles carry grains from the bottom to the surface. A similar mechanism works on a neutron star. As it accumulates hydrogen and helium from a companion star, it occasionally triggers a powerful nuclear explosion — an X-ray burst. In a new study, astrophysicists simulated how, during such a burst, radiation strips off the outer layers and carries freshly synthesized elements into space.

The NICER telescope aboard the ISS spotted absorption lines in the spectra of these bursts, indicating the presence of elements like silicon, sulfur, and even iron. The authors tested the idea that this is "ash" from thermonuclear burning, blown outward by a powerful wind. Using the stellar evolution code MESA, they ran dozens of scenarios, varying the depth at which the fuel ignites and how the layers mix.

It turned out that the wind can indeed pick up nuclei from carbon to iron. But the outcome strongly depends on how we describe convection — the turbulent mixing of hot plasma. If convection penetrates deep, it brings more heavy elements to the surface. If it is confined by special "boundaries," the wind remains almost pure helium. This mechanism acts as a window into the neutron star's interior: the composition of the ejected material reveals what is happening hundreds of meters beneath the dazzling surface. Thus, nuclear "ash" thrown into space tells us about the violent processes hidden from direct observation.