質量巨大的氣體天體，由內部產生的輻射能而發光。銀河系擁有數千億顆恆星，僅有極少的比例肉眼可見。最接近的恆星約距離太陽4.3光年；最遙遠星系裡的恆星則有數十億光年之遙。單一的恆星如太陽是少數，大多數恆星以成對、多重系統或星團出現（參閱binary star、globular cluster、open cluster）。星座並不是由這種組成構成，而只是一些從地球上看起來方向相同的恆星。恆星的亮度（星等）、顏色、溫度、質量、大小、化學組成與年齡差異極大。幾乎所有的恆星，氫是最多的元素。恆星以光譜（參閱spectrum）來分類，從藍白到紅分為O，B，A，F，G，K，M，太陽是G型恆星。歸納恆星的性質和演化可以用特性之間的相關性與統計結果（參閱Hertzsprung-Russell diagram）。當氫與微塵顆粒構成緻密恆星際雲一部分因為自身的重力向內塌陷，就形成恆星。隨著雲氣壓縮，密度和內部溫度增加，直到足以在核心造成核融合（若不成功就成為棕矮星）。在核心的氫耗盡之後，核心收縮並加溫，恆星外層明顯膨脹並冷卻，恆星成為紅巨星。恆星演化的最後階段，不再能產生足夠的能量對抗本身的重力，就看其質量大小以及是否是密近雙星的成員（參閱black hole、neutron star、nova、Supernova、white dwarf star）。亦請參閱Cepheid variable、dwarf star、eclipsing variable star、flare star、giant star、supergiant star、T Tauri star、variable star、Populations I and II。
star
Any massive, celestial body of gas that shines by radiant energy generated inside it. The Milky Way contains hundreds of billions of stars; only a very small fraction are visible to the unaided eye. The closest star is about 4.3 light-years from the sun; the most distant are in galaxies billions of light-years away. Single stars such as the sun are the minority; most stars occur in pairs, multiple systems, or clusters (see binary star, globular cluster, open cluster). Constellations do not consist of such groupings but of stars in the same direction as seen from earth. Stars vary greatly in brightness (magnitude), color, temperature, mass, size, chemical composition, and age. In nearly all, hydrogen is the most abundant element. Stars are classified by their spectra (see spectrum), from blue-white to red, as O, B, A, F, G, K, or M; the sun is a G-type star. Generalizations on the nature and evolution of stars can be made from correlations between certain properties and from statistical results (see Hertzsprung-Russell diagram). A star forms when a portion of a dense interstellar cloud of hydrogen and dust grains collapses inward from its own gravity. As the cloud condenses, its density and internal temperature increase until it is hot enough to trigger nuclear fusion in its core (if not, it becomes a brown dwarf). After hydrogen is exhausted in the core, the latter shrinks and heats up while the star's outer layers expand significantly and cool, making the star a red giant. The final stages of a star's evolution, when it no longer produces enough energy to counteract its own gravity, depend largely on its mass and whether it is a component of a close binary system (see black hole, neutron star, nova, supernova, white dwarf star). See also Cepheid variable, dwarf star, eclipsing variable star, flare star, giant star, supergiant star, T Tauri star, variable star, Populations I and II.
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