sentences:  Fusion of iron or nickel produces no net energy output, so no further fusion can take place, leaving the nickel–iron core inert.Due to the lack of energy output creating outward thermal pressure, the core contracts due to gravity until the overlying weight of the star can be supported largely by electron degeneracy pressure.When the compacted mass of the inert core exceeds the Chandrasekhar limit of about , electron degeneracy is no longer sufficient to counter the gravitational compression.