Fusion necessitates extremely high temperatures to overcome the electrostatic repulsion between atomic nuclei.

Fusion is the process in which two light atomic nuclei combine to create a heavier nucleus, releasing an immense amount of energy in the process. This reaction is the primary energy source for stars, including our Sun. However, for fusion to occur, very specific and extreme conditions must be met, particularly regarding temperature.

The need for high temperatures is rooted in the characteristics of atomic nuclei. Atomic nuclei possess a positive charge, which leads to electrostatic repulsion between them; like charges repel each other. Thus, for two nuclei to get close enough to fuse, they must overcome this repulsive force. This is where temperature plays a critical role.

Temperature is defined as a measure of the average kinetic energy of particles within a system. Essentially, it reflects the degree of motion of these particles: the higher the temperature, the faster the particles move. When temperatures reach sufficiently high levels, the particles gain enough kinetic energy to overcome their electrostatic repulsion and collide with sufficient force to initiate fusion. This explains why fusion requires such extreme temperatures.

For instance, in the core of the Sun, the temperature is estimated to be around $15$ million degrees Celsius. At these high temperatures, hydrogen nuclei, or protons, possess enough kinetic energy to surpass their mutual electrostatic repulsion and collide with enough force to fuse into helium, while simultaneously releasing energy.

In efforts to achieve controlled fusion on Earth, even higher temperatures are necessary. This increased requirement arises because the density of the fuel in a fusion reactor is significantly lower than that in the core of a star. Consequently, the fuel particles must attain even greater kinetic energy to collide and fuse effectively. As a result, the temperatures in fusion reactors need to reach ranges of hundreds of millions of degrees Celsius.

In summary, fusion requires extremely high temperatures to endow atomic nuclei with sufficient kinetic energy to overcome their mutual electrostatic repulsion and collide with enough force to achieve fusion, thereby releasing energy in the process.