Why does the formation of an iron core (iron-56) cause the star's internal furnace to shut down?

Answer

Fusing iron or elements heavier than iron consumes energy rather than releasing it.

Iron-56 possesses the maximum nuclear binding energy per nucleon; therefore, energy must be input rather than gained when attempting to fuse it further, eliminating the outward thermal pressure supporting the star.

Why does the formation of an iron core (iron-56) cause the star's internal furnace to shut down?
Elementastronomystarmassnucleosynthesis