Final answer:
Fusion primarily takes place in the center of stars where temperatures are highest, helping to overcome the Coulomb repulsion between nuclei. Quantum mechanical tunneling lowers the kinetic energy needed for fusion, and when a star's core is depleted of hydrogen, fusion then occurs in a surrounding hydrogen shell.
Step-by-step explanation:
Fusion occurs primarily at the center of the Sun and other stars, where the conditions are most conducive for overcoming the Coulomb repulsion between positively charged atomic nuclei. The process of quantum mechanical tunneling is essential to fusion because it allows nuclei to merge at lower temperatures than would be required if they were to come into direct contact.
The core of a star is where temperatures are highest, and it is here that fusion is most likely to occur. When core hydrogen is depleted and fusion ceases there, the core contracts and heats up, leading to fusion in a surrounding shell of hydrogen, a process that marks the beginning of the end of the star's life on the main sequence.
The principle that governs this phenomenon is that as atoms are heated, their kinetic energy increases, allowing them to overcome the electrostatic repulsion force more readily.
Quantum mechanical tunneling then allows the nuclei to fuse at a lower kinetic energy than would otherwise be necessary, which is why increased temperature is correlated with an increased rate of fusion. Hence, the central region of a star, with its extreme temperatures, provides the optimal conditions for fusion to take place, making it the primary fusion layer in a star.