A supernova (SN) is the explosive end of a massive star (more than 8 times the mass of our Sun) which is the culmination of massive stellar evolution – a fascinating yet complicated topic. Massive stars live for only a few million years, during which they go through several different nucleosynthesis stages creating progressively more heavy elements in their cores. Upon achieving silicon burning, they will create a core made of predominantly iron and they run into a problem: no further fusion can take place to generate more energy. This invariably leads to the collapse of the core onto itself; the contracting core will eventually reach nuclear densities, after which it will bounce back and launch a shock wave,which will disrupt the outer layers of the star back into the cosmos.

The disruption of a massive star in such an event is called a core-collapse supernova (CCSN). During the explosion, more nucleosynthesis occurs which includes the synthesis of radioactive isotopes which will decay slowly to release energy at later times. This radioactive decay can take months or even years, creating an energy source long after the explosive event. The material which is ejected during a SN can reach velocities of several thousands of km s⁻¹, and, after a few months, has grown into a diffuse nebula of the expelled material (the “nebular phase”).

By studying the nebular phase, we can learn a lot about SNe: what elements are produced, in what quantities, and where in the ejecta do they end up? Which mechanisms are important to consider for the explosion mechanism itself, and can current models match observations when brought into the nebular phase? What can we learn about stellar evolution from the differences between the various supernova types? Answers for questions such as these can be found by processing modern, 3D explosion simulations with advanced spectral synthesis codes, which is the focus in this work. A new, 3D spectral synthesis code (ExTraSS; EXplosive TRAnsient Spectral Simulator) is presented to achieve this.

The development of ExTraSS and using it to generate synthetic spectra from modern 3D explosion models was the main goal for my thesis. In Paper I, the code is introduced and applied to one model as test case, a 3.3M_⊙ He-star which is evolved homologously to the nebular phase. ExTraSS was then used to determine the radioactive energy deposition and subsequently the temperatures and level populations across the nebula in a non-local thermodynamic equilibrium (NLTE) setting. From these level populations emission was then generated to create UVOIR spectra (UltraViolet, Optical and InfraRed), to gain insight in how observers at different viewing angles would see this nebula. In Paper II, the code is updated with an approximate “on-the-spot” photoionization treatment and then applied to a grid of nine different He-stars, to compare the line profile properties of these models against observed SNe, and to investigate the near-IR spectra of such SNe in more detail. In Paper III, the approximate photoionization treatment is upgraded to a full radiative transfer calculation and applied to a low-energy, 9.0M_⊙ H-rich model, with a focus to investigate a subset of low-luminosity SNe whose explosion mechanism is uncertain.