Question

1. why does nuclear fusion happen only in the sun’s core?

Explanation:

Nuclear fusion requires extremely high temperature and pressure to overcome the electrostatic repulsion between positively - charged atomic nuclei. The Sun's core has a temperature of about 15 million Kelvin and extremely high pressure due to the huge mass of the Sun pressing down on the core. These extreme conditions are only present in the Sun's core, allowing hydrogen nuclei to fuse into helium nuclei.

Answer:

Nuclear fusion occurs only in the Sun’s core because the core has extremely high temperature (about 15 million K) and pressure. These conditions are needed to overcome the electrostatic repulsion between positively - charged atomic nuclei, allowing hydrogen nuclei to fuse into helium nuclei.