  In a non-spherical stellar explosion, non-radial motions become important
near the stellar surface. For realistic deviations from spherical symmetry,
non-radial flow dramatically alters the dynamics and emission of shock
emergence on a significant fraction of the surface. The breakout flash is
stifled, ejecta speeds are limited, and matter is cast sideways. Non-radial
ejection allows for collisions outside the star, which may engender a new type
of transient. Strongly oblique breakouts are most easily produced in compact
stellar progenitors, such as white dwarfs and stripped-envelope core collapse
supernovae.
  We study the shock structure and post-shock acceleration using conservation
laws, a similarity analysis, and an approximate theory for oblique shocks. The
shock is likely to extend vertically from the stellar surface, then kink before
joining a deep asymptotic solution. Outflow from the region crossed by an
oblique shock is probably unsteady and may affect the surface ahead of the main
shock. We comment on the implications for several notable explosions in which
the non-spherical dynamics described in this paper are likely to play an
important role. We also briefly consider relativistic and superluminal pattern
speeds.
