C
fusion of protons during synthesis of heavier elements
D
gravitational contraction
The answer is C.
Solution
The core of the sun is at a temperature of about $1.5 \times 10^7$ K. At such high temperatures, protons (hydrogen nuclei) fuse together to form helium (proton-proton cycle): $4\,{}^{1}_{1}H \rightarrow {}^{4}_{2}He + 2e^{+} + 2\nu + 26.7\ \text{MeV}$ The mass of the helium nucleus formed is less than that of four protons, and the mass difference is released as energy ($E = \Delta mc^2$). Chemical burning, fission of uranium or gravitational contraction cannot account for the enormous energy output of the sun over billions of years. So solar energy is mainly due to fusion of protons during synthesis of heavier elements (helium).
ConceptFusion as source of solar energy
Nuclei in past papers
77 questions from this chapter have appeared across 26 exam years.
