The s-process works as long as the decay time for unstable isotopes is longer than the capture time. The second process, the r-process, is what is used to produce very heavy, neutron rich nuclei. Here the capture of neutrons happens in such a dense environment that the unstable isotopes do not have time to decay. The supernova explosion also has the side benefit of propelling the new created elements into space to seed molecular clouds which will form new stars and solar systems. Constant impacts by photons knock electrons off of atoms which is called ionization.
Lower temperatures mean photons with less energy and fewer collisions.
Thus, atoms become stable at about 15 minutes after the Big Bang. These atoms are now free to bond together to form simple compounds, molecules, etc. And these are the building blocks for galaxies and stars. Radiation, in the form of photons, and matter, in the form of protons, neutrons and electron, can interact by the process of scattering. Photons bounce off of elementary particles, much like billiard balls.