Astrophysics context: The Sun’s prodigious energy output is generated primarily by which class of nuclear reactions occurring in its core?

Introduction / Context:
The Sun shines because mass is converted to energy in its core. Understanding which nuclear process dominates clarifies stellar lifecycles, neutrino production, and elemental synthesis in main-sequence stars like the Sun.

Given Data / Assumptions:

• Core conditions: ~15 million K, immense pressure and density.
• Hydrogen is the primary fuel; helium is the principal product.
• General relativity and quantum tunneling enable reactions at sub-Coulomb energies.

Concept / Approach:
Nuclear fusion combines light nuclei (primarily protons) to form heavier nuclei (helium), releasing energy due to mass deficit (E = m * c^2). In the Sun, the proton–proton chain is dominant, with side branches and neutrino emission. Fission breaks heavy nuclei apart, relevant in reactors and supernova nucleosynthesis contexts, not in the Sun. Combustion is chemical, orders of magnitude too weak for stellar luminosities.

Step-by-Step Solution:

Verification / Alternative check:
Solar neutrino measurements and helioseismology are consistent with fusion-based models of solar energy production.

Why Other Options Are Wrong:

• Fission: Occurs in heavy nuclei; not the Sun’s primary energy source.
• Combustion: Chemical, inadequate energy density.
• None: Incorrect since fusion is established.

Common Pitfalls:
Assuming the CNO cycle dominates in the Sun; it is more important in heavier, hotter stars.

Final Answer:
fusion