什么是恒星

质量巨大的气体天体，由内部产生的辐射能而发光。银河系拥有数千亿颗恒星，仅有极少的比例肉眼可见。最接近的恒星约距离太阳4.3光年；最遥远星系里的恒星则有数十亿光年之遥。单一的恒星如太阳是少数，大多数恒星以成对、多重系统或星团出现（参阅binary star、globular cluster、open cluster）。星座并不是由这种组成构成，而只是一些从地球上看起来方向相同的恒星。恒星的亮度（星等）、颜色、温度、质量、大小、化学组成与年龄差异极大。几乎所有的恒星，氢是最多的元素。恒星以光谱（参阅spectrum）来分类，从蓝白到红分为O，B，A，F，G，K，M，太阳是G型恒星。归纳恒星的性质和演化可以用特性之间的相关性与统计结果（参阅Hertzsprung-Russell diagram）。当氢与微尘颗粒构成致密恒星际云一部分因为自身的重力向内塌陷，就形成恒星。随着云气压缩，密度和内部温度增加，直到足以在核心造成核融合（若不成功就成为棕矮星）。在核心的氢耗尽之後，核心收缩并加温，恒星外层明显膨胀并冷却，恒星成为红巨星。恒星演化的最後阶段，不再能产生足够的能量对抗本身的重力，就看其质量大小以及是否是密近双星的成员（参阅black hole、neutron star、nova、Supernova、white dwarf star）。亦请参阅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.