Final answer:
The internal structure of a star is determined by its total mass and composition. These two principles uniquely determine the temperature, pressure, density, and energy production within a star.
Step-by-step explanation:
The internal structure of a star is determined by two main principles: the total mass and the composition of the star. Astrophysicists have shown that stars in equilibrium, deriving their energy from nuclear fusion, have their structure completely and uniquely determined by these two factors.
The total mass of a star determines its gravitational pull, which in turn affects the temperature, pressure, and density within the star. The composition of a star, particularly the abundance of elements like hydrogen, helium, and heavier elements, influences the nuclear reactions that occur and the overall energy production within the star.