  Astrophysical explosions such as supernovae are fascinating events that
require sophisticated algorithms and substantial computational power to model.
Castro and MAESTROeX are nuclear astrophysics codes that simulate thermonuclear
fusion in the context of supernovae and X-ray bursts. Examining these nuclear
burning processes using high resolution simulations is critical for
understanding how these astrophysical explosions occur. In this paper we
describe the changes that have been made to these codes to transform them from
standard MPI + OpenMP codes targeted at petascale CPU-based systems into a form
compatible with the pre-exascale systems now online and the exascale systems
coming soon. We then discuss what new science is possible to run on systems
such as Summit and Perlmutter that could not have been achieved on the previous
generation of supercomputers.
