The silicon-burning sequence lasts about one day before being struck by the shock wave that was launched by the core collapse. Burning then becomes much more rapid at the elevated temperature and stops only when the rearrangement chain has been converted to nickel-56 or is stopped by supernova ejection and cooling. The star can no longer release energy via nuclear fusion because a nucleus with 56 nucleons has the lowest mass per nucleon of all the elements in the alpha process sequence. Only minutes are available for the nickel-56 to decay within the core of a massive star, and only seconds if in the ejecta. The star has run out of nuclear fuel and within minutes its core begins to contract.