"a high-mass star near the end of its life undergoes successive cycles of energy generation within its core in which gravitational collapse increases the temperature to the point where a new nuclear fusion cycle generates sufficient energy to stop the collapse. this process does not work beyond the silicon-fusion cycle that produces iron. why is this? a. electrostatic forces between the highly charged iron nuclei are sufficient to overcome the collapse and stabilize the stellar core. b. iron nuclei are so large that they occupy all remaining space and so the collapse cannot continue. c. fusion of iron nuclei into heavier nuclei requires energy rather than producing excess energy and therefore will not produce the additional gas pressure to halt the collapse. d. the pressure from high-energy photons and neutrinos at the very high core temperatures reached at this stage of development is finally sufficient to halt the collapse.