  We consider a strong ultrarelativistic shock moving through a star whose
envelope has a polytrope-like density profile. When the shock is close to the
star's outer boundary, its behavior follows the self-similar solution given by
Sari (2005) for implosions in planar geometry. Here we outline this solution
and find the asymptotic solution as the shock reaches the star's edge. We then
show that the motion after the shock breaks out of the star is described by a
self-similar solution remarkably like the solution for the motion inside the
star. In particular, the characteristic Lorentz factor, pressure, and density
vary with time according to the same power laws both before and after the shock
breaks out of the star. After emergence from the star, however, the
self-similar solution's characteristic position corresponds to a point behind
the leading edge of the flow rather than at the shock front, and the relevant
range of values for the similarity variable changes. Our numerical integrations
agree well with the analytic results both before and after the shock reaches
the star's edge.
