Essentially all the elements heavier than iron in our galaxy were formed:

• A. by radioactive decay of lighter elements
• B. during supernova explosions
• C. through chemical reactions in interstellar clouds
• D. in the cores of main-sequence stars

Answer: (B)

Verified step by step guidance

1. Understand the context: Elements heavier than iron are not typically formed through normal stellar fusion processes because fusing elements heavier than iron requires energy input rather than releasing energy.
2. Recall that lighter elements up to iron are formed in the cores of main-sequence stars through nuclear fusion, but heavier elements require more energetic events.
3. Recognize that radioactive decay of lighter elements does not create heavier elements; it only transforms one element into another, usually lighter or more stable isotopes.
4. Consider the environments where extremely high energy and neutron fluxes are available, such as during supernova explosions, which provide the conditions necessary for rapid neutron capture (r-process) to form heavy elements.
5. Conclude that the formation of elements heavier than iron primarily occurs during supernova explosions, where the intense energy and neutron environment enable the synthesis of these heavy elements.