the interface of inner and outer core. First, the shock moves outward dynamically. It then quickly loses energy by work done breaking up infalling iron-group nuclei into neutrons, protons, and alpha particles. The copious emission of neutrinos from the hot ($T \sim 10 \,\mathrm{MeV} \simeq
  10^{11}\,\mathrm{K}$) gas further reduces energy and pressure behind the shock. The shock stalls and turns into an accretion shock: the ram pressure of accretion of the star’s outer core balances the pressure behind the shock.

The *supernova mechanism* must revive the stalled shock to drive a successful core-collapse supernova explosion. Depending on the structure of the progenitor star, this must occur within one to a few seconds of core bounce. Otherwise, continuing accretion pushes the protoneutron star over its maximum mass ($\sim$$2-3\,M_\odot$), which results in the formation of a black hole and no supernova explosion.

If the shock is successfully revived, it must travel through the outer core and the stellar envelope before it breaks out of the star and creates the spectacular explosive display observed by astronomers on Earth. This may take more than a day for a red supergiant star (e.g., like Betelgeuse, a $\sim$$20\,M_\odot$ star in the constellation Orion) or just tens 