The total power output of the Sun is about 3.8 × 1026 W, and the power-producing core has a radius of about 1.4 × 105 km (about 20% of the full radius of the Sun).

Question

Using these figures and your knowledge of the energy release in the proton– proton chain estimate the power density (i.e. the number of watts per cubic metre) in the core of the Sun, and the number of helium nuclei formed per second per cubic metre. Give your answer in scientific notation and to two significant figures
The second step in the proton–proton chain is the reaction:
2 1 3
H+ H → He
1 1 2
State which nucleons are present in the
3
He nucleus and use the fact that it is
2
stable to estimate the strength of the strong nuclear force between nucleons
In stars greater than about nine times that of the Sun, the production of heavy elements and release of energy by nuclear fusion occurs until iron is formed. Explain why fusion of elements heavier than iron cannot power the star beyond this point

Answer 1

(1) From the energy output of the sun and the volume of the power-producing core, determine the power released per volume in that core.

(2) From the energy release per He-atom creation in the proton-proton chain, and using the answer from (1), compute the number of He atoms created per unit volume per unit time.

(3) Fusion of elements heavier that iron is not possible in stars because the "curve of binding energy" has a minimum there. Google that phrase for more details, if needed. It can happen endothermically in supernova explosions. That is where it is believed all elements heavier than iron were created.