Which element is a "dead end" for massive stars?

• A. Iron
• B. Hydrogen
• C. Carbon
• D. Neon

Answer: (A)

In stars, energy comes from nuclear fusion that fuses lighter elements into heavier ones, releasing energy that pushes outward against gravity. Iron is where this process reaches a limit: fusing iron into heavier elements does not release energy; it actually requires energy input. Because iron-56 has one of the highest binding energies per nucleon, turning iron into heavier nuclei would absorb energy instead of liberating it, so the core can no longer generate the outward pressure needed to support the star.

Once an iron core forms and grows, there’s no energy-producing fusion left to counter gravity, so the core collapses and the star ends its life in a core-collapse supernova. Earlier fusion stages (like hydrogen to helium, helium to carbon, and later steps) release energy and keep the star going, but those steps stop being energetically favorable once iron is produced.