A star is a self gravitating astronomical object composed primarily of hot plasma in which energy is produced or has been produced through nuclear processes. Most stars spend the longest portion of their lives on the main sequence, where hydrogen nuclei are fused into helium in their cores and the released nuclear energy supports the star against gravitational contraction. The Sun is a typical example of a star, although stars span enormous ranges of mass, temperature, luminosity, colour and lifetime.
Stars form from dense regions of gas and dust within molecular clouds, evolve through stages determined primarily by their initial mass and composition, and eventually end as white dwarfs, neutron stars or black holes depending largely on mass and subsequent evolution. The word star has ancient Germanic roots and has been used in many languages to describe luminous points in the night sky. Scientifically, however, a star is defined by physical properties and processes rather than by its appearance as a point of light.
A star is held together by gravity and supported against collapse by internal pressure. In a stable main sequence star, the inward pull of gravity is balanced approximately by the outward pressure gradient produced by hot gas and radiation. This condition is called hydrostatic equilibrium. Energy generated in the core is transported outward through processes such as radiation and convection.
In the Sun, energy travels outward through a radiative zone for much of the interior and through convection in the outer region. The precise structure depends on stellar mass, composition and evolutionary state. Nuclear fusion in the core changes the chemical composition and releases energy because the products have slightly less mass than the initial nuclei, with the mass difference appearing as energy according to the relationship between mass and energy represented by Einsteins famous equation. The energy eventually emerges from the surface as electromagnetic radiation and contributes to the stars luminosity.
The primary source of energy in most main sequence stars is hydrogen fusion. In stars similar to the Sun, hydrogen nuclei ultimately combine through the proton proton chain, producing helium and releasing energy along with neutrinos and other particles. In more massive stars, the carbon nitrogen oxygen cycle becomes increasingly important for hydrogen fusion because carbon, nitrogen and oxygen nuclei act as catalysts in the reaction sequence.
As a star evolves and its core hydrogen becomes depleted, other nuclear burning stages can occur if the core reaches sufficient temperature and density. Helium can fuse into carbon and oxygen, and massive stars can subsequently burn progressively heavier elements through increasingly complex stages. Very massive stars can eventually develop cores dominated by elements near iron. Fusion beyond iron generally does not provide net energy under ordinary stellar conditions, so the internal energy balance changes dramatically at that stage. Nuclear physics therefore determines much of a stars structure, lifetime and eventual fate.
Stars differ greatly in mass and this is one of the most important factors determining their evolution. Low mass stars consume nuclear fuel relatively slowly and can remain on the main sequence for extremely long periods. Massive stars have much higher core temperatures and pressures and consume their fuel far more rapidly, so they have much shorter lifetimes despite containing much more fuel.
A massive star may live for only millions of years, while a low mass star can remain active for tens or hundreds of billions of years under suitable conditions. Red dwarfs, the smallest hydrogen burning stars, are extremely common and can have theoretical lifetimes longer than the present age of the universe. Massive blue stars are much rarer but extraordinarily luminous. This relationship explains why the visible stellar population is not simply a direct reflection of the total number of stars. Bright massive stars are easier to see across enormous distances but represent a small fraction of the underlying population.
A stars colour provides information about its surface temperature. Hotter stars emit proportionally more light at shorter wavelengths and can appear blue or blue white, while cooler stars appear orange or red. Astronomers describe stellar temperatures using effective temperature, which is the temperature of an idealised blackbody producing approximately the same emitted flux per unit surface area. Stellar spectra contain absorption and sometimes emission lines produced by atoms and ions in stellar atmospheres.
These spectral features reveal chemical composition, temperature, pressure, magnetic activity, surface gravity and motion. The classification system commonly called the OBAFGKM sequence arranges stars broadly by surface temperature, with O stars hottest and M stars coolest among the principal classes. Additional subclasses and luminosity classes provide more detailed information. Spectroscopy is therefore one of the most powerful tools for determining stellar properties beyond simple brightness and colour.
Stars also have magnetic fields and can exhibit a wide range of activity. Magnetic fields are generated by processes involving electrically conducting plasma and internal rotation, often described through stellar dynamo theory. Magnetic activity can produce starspots, flares, prominences and energetic particle emission. Young and rapidly rotating stars can be particularly magnetically active. Stellar winds carry charged particles away from stars and can influence surrounding planetary systems.
The Sun produces the solar wind, which interacts with Earths magnetosphere and contributes to space weather. Other stars can produce much stronger flares and winds, potentially affecting the atmospheres and radiation environments of planets. Stellar activity is therefore important not only for understanding stars themselves but also for studying the habitability and evolution of planets orbiting other stars.
Stars are major sources of chemical elements. The Big Bang produced primarily hydrogen and helium with small quantities of lithium, while stars manufacture many heavier elements through nuclear reactions. When stars lose material through stellar winds or when massive stars explode as supernovae, newly produced elements are returned to the interstellar medium. Neutron star mergers and certain other explosive events produce additional heavy elements, including some of the elements heavier than iron.
Later generations of stars form from increasingly enriched gas. The carbon, oxygen, nitrogen, silicon, calcium and iron present in planets and living organisms ultimately derive from astrophysical processes that occurred before and during earlier generations of stars. This is why the statement that material in living organisms is connected to stellar nucleosynthesis has a literal physical basis. Stellar evolution is a major component of the chemical evolution of galaxies.
The final fate of a star depends strongly on its initial mass and the processes occurring during its evolution. Stars with relatively low or intermediate initial masses eventually lose their outer layers and leave behind dense white dwarfs composed largely of carbon and oxygen in many cases. Massive stars can undergo core collapse after exhausting their nuclear fuel, producing neutron stars or black holes depending on the core mass and the details of the collapse.
Supernova explosions can distribute newly synthesised elements into surrounding space and trigger or influence later star formation. Some stars also evolve through binary interactions, in which mass transfer, mergers or common envelope evolution can dramatically alter the expected single star pathway. Stars are therefore not static balls of burning gas. They are evolving physical systems whose lifetimes, structures, chemical products and remnants depend on mass, composition, rotation, magnetic fields and interactions with companion objects.
