Any massive celestial body of gas that shines by radiant energy generated inside it. The Milky Way Galaxy contains hundreds of billions of stars; only a very small fraction are visible to the unaided eye. The closest star to Earth is the Sun. The closest star to the Sun is about 4.2 light-years away; 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 and multiple systems (seebinary star). Stars also associate by their mutual gravity in larger assemblages called clusters (seeglobular cluster; open cluster). Constellations consist not of such groupings but of stars in the same direction as seen from Earth. Stars vary greatly in brightness (magnitude), colour, temperature, mass, size, chemical composition, and age. In nearly all, hydrogen is the most abundant element. Stars are classified by their spectra (seespectrum), from blue-white to red, as O, B, A, F, G, K, or M; the Sun is a spectral type G star. Generalizations on the nature and evolution of stars can be made from correlations between certain properties and from statistical results (seeHertzsprung-Russell diagram). A star forms when a portion of a dense interstellar cloud of hydrogen and dust grains collapses 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 from nuclear burning, the core shrinks and heats up while the star's outer layers expand significantly and cool, and the star becomes 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 (seeblack hole; neutron star; nova; pulsar; supernova; white dwarf star). Some stars other than the Sun are known to have one or more planets (seeextrasolar planet). See alsoCepheid variable; dwarf star; eclipsing variable star; flare star; giant star; Populations I and II; supergiant star; T Tauri star; variable star.