SOLAR SYSTEM

Detailed explanation-1: -Fusion reaction takes place at high temperature because kinetic energy is high enough to overcome repulsion between nuclei.

Detailed explanation-2: -Since the density of the nuclei in a given space is increased the pressure in that space will also increase. Hence, high pressure is also essential for nuclear fusion to take place. From the above conditions we can conclude that both high temperature and pressure is necessary for nuclear fusion to take place.

Detailed explanation-3: -Fusion reaction takes place at temperatures around 107k. It requires this high temperature so that nucleus start moving at rapidly speed, which in turn increases their kinetic, so that they overcome the repulsion between them and can come together.

Detailed explanation-4: -Very high temperature of fuel. Very high kinetic energy / speed of nuclei to overcome repulsion. Very high density / pressure to increase the possibility of suitable collisions.