Cassiopeia A is the expanding remainder of a massive star that exploded 340 years ago in he constellation of Cassiopeia (hence we call it Cas A for short).  As the star erupted, hot radioactive material was shot out in all directions, churning up the surrounding outer debris, before the star finally tore itself apart.  Simulations of supernova explosions have found it difficult to model the extreme conditions during this process, even when using the world’s best supercomputers.  So what are astronomers missing? By studying recent supernovae like Cas A, astronomers can study the processes that formed these massive expanding shock waves, leading...