Stellar structure refers to the internal composition and physical organization of a star, which is a massive body of plasma held together by its own gravitational force. The energy source powering a star is typically thermonuclear fusion occurring in the core, where lighter atomic nuclei combine to form heavier ones, releasing immense amounts of energy. Depending on the star's mass and evolutionary stage, the internal structure can consist of distinct layers, such as a high-density core, surrounding radiative zones, and more diffuse outer envelopes. The stability and lifespan of a star are directly related to the balance between the outward pressure generated by fusion and the inward pull of gravity.