Which element is described as a fusion end point for massive stars?

Which element is described as a fusion end point for massive stars?

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

Answer: (A)

Fusion in massive stars proceeds up to iron because iron-56 has the highest binding energy per nucleon. That means fusing lighter elements releases energy and powers the star, but fusing iron or heavier elements would require energy input rather than releasing energy. Once a star’s core builds up an iron core, there’s no net energy available from further fusion to support it against gravity, so the core collapses and a supernova occurs. Earlier stages involve burning hydrogen into helium and helium into carbon and oxygen, but those steps release energy; iron marks the end point where fusion no longer provides outward pressure.

Fusion in massive stars proceeds up to iron because iron-56 has the highest binding energy per nucleon. That means fusing lighter elements releases energy and powers the star, but fusing iron or heavier elements would require energy input rather than releasing energy. Once a star’s core builds up an iron core, there’s no net energy available from further fusion to support it against gravity, so the core collapses and a supernova occurs. Earlier stages involve burning hydrogen into helium and helium into carbon and oxygen, but those steps release energy; iron marks the end point where fusion no longer provides outward pressure.