The late stages of stellar evolution have great importance for the synthesis and dispersal of the elements heavier than helium. We focus on the helium shell flash in low mass stars, where incorporation of hydrogen into the convection zone above the helium burning shell can result in production of carbon-13 with tremendous release of energy. The need for detailed 3-D simulations in understanding this process is explained. To make simulations of the entire helium flash event practical, models of turbulent multimaterial mixing and nuclear burning must be constructed and validated. As an example of the modeling and validation process, our recent work on modeling subgrid-scale turbulence in 3-D compressible gas dynamics simulations is described and a new turbulence model presented along with supporting results. Finally, the potential impact of petascale computing hardware on this problem is explored.