A star is a luminous astronomical object held together by its own gravity, composed mainly of hydrogen and helium. During most of its active lifetime, nuclear fusion in its interior supplies the energy it radiates into space. The Sun is the nearest star and the central body of the Solar System. Stars differ greatly in size, brightness, temperature, and lifetime; their initial mass strongly influences their evolution and eventual fate. (science.nasa.gov)
Physical structure and energy
Ordinary stars consist largely of hot gas, much of it plasma, in which electrons are separated from atomic nuclei. Their interiors are much hotter and denser than their visible outer layers. A stable star maintains approximate hydrostatic equilibrium: the outward force arising from a pressure gradient balances the inward pull of gravity. This balance can persist while the star’s structure and composition change slowly. (openstax.org)
Fusion releases energy because the products have less total mass than the reacting particles, with the difference accounted for by mass–energy equivalence. In the Sun, hydrogen is converted into helium mainly through the proton–proton chain. In hotter stellar cores, the carbon–nitrogen–oxygen cycle can dominate hydrogen fusion, using carbon, nitrogen, and oxygen nuclei as intermediaries. Both mechanisms have the same net result: four hydrogen nuclei become one helium nucleus, with energy released. (openstax.org)
Energy moves outward principally through radiation and convection. Radiative transport involves repeated interactions between matter and photons; convection carries heat through circulating material. The relative importance and location of these processes vary among stars. The visible “surface” is not solid but an atmospheric layer from which much of the observed radiation escapes. (openstax.org)
Properties and classification
A star’s principal measurable properties include mass, radius, temperature, chemical composition, and luminosity—the total energy it emits per unit time. Luminosity must be distinguished from apparent brightness: a very luminous distant star can appear fainter than a less luminous nearby one. Establishing distance is therefore essential to interpreting observed brightness. (openstax.org)
Spectroscopy separates starlight into its constituent wavelengths. Absorption lines reveal substances in stellar atmospheres, but their strengths also depend strongly on temperature and ionization. The principal spectral classes, from hottest to coolest, are O, B, A, F, G, K, and M. Hot stars generally appear blue or blue-white, whereas cooler stars appear orange or red. The Sun belongs to class G. Spectral classes L, T, and Y extend classification to cooler objects, including brown dwarfs. (openstax.org)
The Hertzsprung–Russell diagram plots luminosity against surface temperature or spectral class. Most ordinary stars occupy the main sequence, a band extending from hot, luminous stars to cool, faint ones. Giants and supergiants occupy high-luminosity regions, while white dwarfs combine relatively high temperatures with low luminosities because of their small radii. A star’s position changes as it evolves; the diagram is not simply a sequence of increasing age. (openstax.org)
Formation and main-sequence life
Stars form within cold molecular clouds of interstellar gas and dust. Gravity causes sufficiently dense regions to collapse and fragment. A developing protostar gains material and heats as it contracts; initially, gravitational contraction supplies much of its energy. Sustained hydrogen fusion in the core establishes the main-sequence phase. Stars often form in groups rather than individually. (science.nasa.gov)
Objects that never acquire enough mass to sustain ordinary hydrogen fusion are brown dwarfs, rather than main-sequence stars. They can resemble faint, cool stars but have a different long-term energy supply. Red dwarfs, by contrast, are genuine low-mass main-sequence stars. Their slow fuel consumption gives them exceptionally long predicted lifetimes, potentially trillions of years. (science.nasa.gov)
Massive main-sequence stars have larger fuel supplies but consume them disproportionately rapidly. Consequently, they may live only a few million years, whereas stars resembling the Sun remain on the main sequence for billions of years. Stellar lifetimes also depend on composition and on how material is mixed within the interior. (science.nasa.gov)
Evolution and remnants
When hydrogen becomes depleted in the core, the star’s structure changes. In a Sun-like star, the core contracts while hydrogen fusion continues in a surrounding shell, and the outer layers expand into a red giant. Later, helium fusion produces carbon and oxygen. The star eventually sheds its outer layers, which can form a planetary nebula, leaving a white dwarf that gradually cools. (science.nasa.gov)
More massive stars undergo additional burning stages that produce increasingly heavy nuclei. Once an iron-rich core can no longer support itself, collapse may produce a supernova. Depending on the remaining core and its subsequent evolution, the remnant can be a neutron star or a black hole. Stellar fusion and the return of material to interstellar space contribute to nucleosynthesis and the chemical enrichment of later generations of stars. (science.nasa.gov)
Observation and stellar systems
Astronomers infer stellar properties by combining brightness measurements, spectra, and positional observations. Stellar parallax measures the apparent displacement of a nearby star against distant background objects as Earth changes position around the Sun, providing a geometrical distance measurement. (openstax.org)
Many stars belong to binary or multiple systems. Measurements of orbital motion provide especially important determinations of stellar masses. Stars also occur in star clusters: open clusters are relatively loosely bound groupings, while globular clusters are dense, roughly spherical systems containing many old stars. Comparing stars within a cluster helps astronomers investigate populations that share a formation environment. (openstax.org)