The Sun formed when an unstable gas pocket that was one light year in diameter began to contract due to its huge gravitation. As the gas was compressed, the work being done by gravity was converted to heat. Once there was enough heat, the Sun ignited, entering its main sequence, where it remains to this day. The Sun's central region is 18 million Kelvin. At this extremely high density, the gas is still ideal, meaning that the energy carried by ions and electrons are so high that the forces between them are insignificant.
Larger stars burn their fuel faster.
On the human time scale these fluctuations are not significant, because it takes a million years for the energy to travel from the core to the surface. Therefore, any variation is not critical to our generation. Once the energy reaches the surface, however, it takes less than 10 minutes for it to travel to Earth. The surface of the Sun is in violent agitation due to convection. Sunspots, cooler darker regions of the photosphere, are evidence of this convection because they vary in number over an eleven year cycle.