  The accelerative expansion of the Crab nebula's outer envelope is a mystery
in dynamics as a conventional expanding blast wave decelerates when bumping
into the surrounding interstellar medium. Here we show that the strong
relativistic pulsar wind bumping into its surrounding nebula induces
energy-generating processes and initiates a detonation wave that propagates
outward to form the current outer edge, namely the shock front, of the nebula.
The resulting detonation wave, with a reactive downstream, then provides the
needed power to maintain propagation of the shock front. Furthermore,
relaxation of the curvature-induced reduction of the propagation velocity from
the initial state of formation to the asymptotic, planar state of
Chapman-Jouguet propagation explains the observed accelerative expansion. The
essential role of detonative propagation in the structure and dynamics of the
Crab nebula offers potential richness in incorporating reactive fronts in the
description of various astronomical phenomena.
