Stars are gravitationally bound astronomical objects whose energy output is generated primarily by processes occurring in their interiors, with nuclear fusion providing the dominant long term energy source for ordinary main sequence stars. They consist mainly of plasma composed of hydrogen and helium, with heavier elements present in smaller amounts. Gravity compresses stellar material while pressure produced by thermal motion, radiation and other physical effects acts against gravitational contraction. A star reaches a state of approximate hydrostatic equilibrium when inward gravitational forces are balanced by outward pressure gradients. Stars vary enormously in mass, radius, temperature, luminosity, chemical composition and evolutionary stage.
The basic structure of a Star is determined by the balance among gravity, pressure, energy transport and nuclear reactions. The core is generally the region where temperatures and densities are highest and nuclear reactions occur most rapidly. Energy then moves outward through radiation, convection or combinations of both depending on the star's mass, composition and internal conditions. The photosphere is the visible layer from which much of the observed electromagnetic radiation escapes, although it is not a sharp physical surface. Above it lies a complex atmosphere that can include chromospheres, coronae, winds and magnetic activity. Stellar interiors cannot normally be observed directly, so astronomers infer their properties through spectroscopy, oscillations, neutrinos, luminosity and evolutionary modelling.
Stellar mass is one of the most important parameters controlling a Star's evolution. More massive stars have much higher central temperatures and luminosities and generally consume their nuclear fuel much more rapidly. Low mass stars have cooler interiors and much longer main sequence lifetimes. The relationship between mass and luminosity is not a simple universal law across all stellar types, but on the main sequence luminosity rises strongly with mass. Because massive stars evolve rapidly, the most massive stars observed today must have formed relatively recently on astronomical timescales. Their short lifetimes also make them important contributors to stellar feedback and chemical enrichment.
Stars produce energy through nuclear reactions that transform lighter nuclei into heavier nuclei and release energy because the resulting products have lower total rest mass than the initial configuration. In Sun like stars, hydrogen fusion proceeds primarily through the proton proton chain. In more massive stars, the carbon nitrogen oxygen cycle becomes increasingly important for hydrogen burning. Later evolutionary stages can involve helium burning and, in sufficiently massive stars, the fusion of heavier nuclei. The exact sequence depends strongly on initial mass and composition. Nuclear reactions also produce neutrinos, which can escape from stellar interiors and provide direct information about core processes.
A Star's spectrum provides information about its surface temperature, chemical composition, radial velocity and other physical properties. The classification system based on spectral types O, B, A, F, G, K and M broadly follows decreasing surface temperature, although modern stellar classification includes more detailed categories and additional spectral types for very cool or unusual objects. Absorption lines arise when atoms and ions in cooler atmospheric layers absorb specific wavelengths from the underlying radiation field. Spectral line strengths depend on temperature, density, composition and ionisation. Stellar spectroscopy therefore provides one of the most important observational methods for determining fundamental stellar properties.
Stars also exhibit magnetic activity, rotation, variability and mass loss. Magnetic fields can generate starspots, flares and other forms of activity, while stellar winds carry charged particles away from the surface. Rotation can influence internal mixing and angular momentum evolution. Some stars pulsate because their internal structures allow oscillation modes to become unstable. Binary interactions can dramatically alter stellar evolution through mass transfer, tidal effects and mergers. Consequently, a Star should not be treated as a static ball of gas. Its observed properties can change on timescales ranging from seconds for oscillations to billions of years for evolutionary changes.
Stars have several major endpoints depending primarily on their initial masses and subsequent interactions. Low and intermediate mass stars eventually lose their outer envelopes and leave behind compact white dwarf remnants. Massive stars can undergo core collapse after exhausting their nuclear fuel and may produce neutron stars or black holes, with the explosion often generating a supernova. Binary interactions can alter these pathways and produce outcomes that would not occur for isolated stars. Stellar remnants can continue to influence their surroundings through radiation, magnetic fields, accretion and gravitational interactions. The material returned to the Interstellar Medium enriches later generations of stars and planetary systems.
Stars are fundamental to astronomy because they provide the main sources of visible light and many other forms of electromagnetic radiation in galaxies. Their masses, ages, compositions and motions allow astronomers to investigate the structure and history of galaxies. Stellar populations are also used to estimate distances and study cosmic evolution. The Sun is a typical example of a relatively low mass main sequence star but is close enough for detailed observations unavailable for most other stars. Understanding Stars therefore requires integrating nuclear physics, plasma physics, gravity, radiation transport, spectroscopy and stellar evolution into a coherent physical description.
