The theory of the internal structure of the sun is treated in three sections. The first section is a general introduction to the nature of stars and treat how they form, the relationship between macroscopic and microscopic scales, and how evolution depend on mass. The second section describes the equations that describe the internal structure of stars during their quasistationary state during their main sequence stage. The third section starts with a brief review of the Standard Solar Model (SSM), BP2000, which is considered to be the best solar model, at present. Thereafter I present my own, simplified, phenomenological solar model, which I call "a modified Schwarzschild model". I describe relevant theory and my assumptions. In particular, I use SI-units and a simplified equation of state for the opacity, κ = k₀ρ^bT^a with a=-3.5, and with k₀ and b as parameters, which I call "a modified Kramers opacity." Matlab 6.0 was used for numerical calculations. The numerical results were compared with the SSM, BP2000, and were in fairly good agreement for k0=1x10²² and b=0.41884 for 0<x<0.3 and b=0.447892 for 0.3<x<1, where x = R/R_Θ.