Solar Structure: Foundations

The core occupies the central region where temperature and density permit nuclear fusion. Energy then moves outward through the radiative zone and convective zone before escaping from the photosphere. Above the visible surface, the chromosphere and transition region lead into the tenuous million-degree corona.

Gravity compresses the solar interior while gas and radiation pressure resist collapse, producing hydrostatic equilibrium. In the radiative zone photons exchange energy repeatedly with matter, while in the convective zone bulk plasma motions carry energy outward. Temperature, density, pressure, and ionization change strongly with radius.

Solar models are constrained by the Sun's mass, radius, luminosity, composition, spectrum, and age. Helioseismology probes internal sound-speed structure, while spectroscopy identifies atmospheric composition and temperature. Imaging at different wavelengths isolates features in the photosphere, chromosphere, and corona.

The Sun is a self-gravitating plasma sphere with distinct regions defined by how energy is generated, transported, and released. Its visible surface is only a thin photospheric layer; most solar mass lies deep inside. Understanding the Sun begins with hydrostatic balance and a radial temperature-density gradient rather than with a solid-surface analogy.