star Self-luminous ball of gas, the radiant energy of which is produced by fusion reactions, mainly the conversion of hydrogen into helium. The temperatures and luminosities of stars are prescribed by their masses. The most massive stars are about 100 solar masses (a mass a hundred times greater than the Sun). Large stars are luminous and hot, and therefore appear blue. Medium-sized stars are yellow, while small stars are a dull red. The smallest stars contain less than one-twentieth of a solar mass. Stars form when a cloud of gas and dust collapses under its own gravity. As the cloud collapses, atoms collide and generate heat. This process continues until the heat generated causes nuclear fusion reactions, converting hydrogen to helium. The reactions from the core throw out radiation, which prevents further collapse. This stage (the main sequence phase) is the longest in a star's lifetime. Eventually the mainly hydrogen core of the star is depleted, and fusion can no longer occur. With the central energy source removed, the core collapses under gravity, and heats itself further until hydrogen fusion is able to occur in a spherical shell surrounding the core. As this change takes place, the outer layers of the star expand considerably, and the star becomes either a red giant, or in the most massive stars, a supergiant. During this phase, the core can reach temperatures of 100 million K, hot enough for fusion of helium to carbon. When this second process of fusion finishes, the core collapses again and heats up. In low-mass stars, the temperature will not rise sufficiently for carbon fusion to take place and the red giant loses its outer layers, leaving a white dwarf. In high-mass stars, carbon fusion is initiated, converting the carbon into elements with relative atomic masses close to that of iron. At this stage, no further fusion is possible and the core collapses explosively, throwing off outer layers in a supernova explosion. The resulting super-dense core forms either a neutron star or black hole. See also binary star