How much Iron is required to kill a star?
When the mass of iron in a star's core reaches about 1.4 times the mass of the Sun, the star will effectively die.
Large stars over 8 solar masses fuse progressively heavier elements over their life. When the fusion process produces nickel and iron the star will come to the end of its life in a matter of days.
At the end of their life the star is layered. It has an outer layer of Hydrogen and then a layer of Helium. The heavier element Nickel and Iron are denser and are at the centre or core of the star. Iron and Nickel are produced by a fusion process involving Silicon.
As it requires additional energy to fuse iron, the core becomes inert as no fusion reactions can take place. When the core reaches 1.4 solar masses, the core collapses under gravity to form a neutron star or a black hole. This process can start fusion reactions in outer layers which causes the star to explode violently as a supernova.
Once the stellar core has collapsed into a neutron star or a black hole the star is effectively dead.