Definition
A star is a sphere of plasma held together by its own gravity, whose central temperature enables nuclear fusion of atomic nuclei. It emits electromagnetic radiation, primarily visible light, from the energy generated by these internal reactions. Stars are not solid objects but gaseous masses in dynamic equilibrium.
Structure
The structure of a star is organized into concentric layers: the core, where fusion reactions occur; the radiative zone, where energy is transported by radiation; the convective zone, where heat moves through fluid motion; the visible photosphere; the chromosphere; and finally the outer corona. Each layer is characterized by differences in temperature, pressure, and energy transport mechanisms.
Function
The functioning of a star is based on a balance between thermal pressure from nuclear fusion and gravitational force seeking collapse. In the core, light atoms like hydrogen fuse into heavier elements, producing energy in the form of photons. This process also releases neutrinos and maintains the star’s stability as long as fuel is available.
Evolution
Stars follow a life cycle determined by their initial mass. They are born from molecular clouds, evolve as main sequence stars, and then change depending on their mass: some become red giants or supergiants, others end as white dwarfs, neutron stars, or black holes. Their composition and dynamics evolve with nuclear fuel depletion and internal equilibrium shifts.
Variability and Classification
Stars differ in mass, surface temperature, luminosity, and chemical composition. They are classified by spectral types (O, B, A, F, G, K, M) based on their light spectra. Some stars exhibit natural variability (flares, pulsations, binary systems), affecting their brightness or orbital behavior.