The changes that occur during a star's life are called stellar evolution. The mass of a star determines the ultimate fate of a star. Stars that are more massive burn their fuel quicker and lead shorter lives. Because stars shine, they must change. The energy they lose by emitting light must come from the matter of which the star is made. This will lead to a change in its composition. Stars are formed from the material between stars, shine until they exhaust their fuel, and then die a predictable death based upon their initial mass. From atoms to stars
Understanding of the processes of stellar evolution came as a result of twentieth century advances in both astronomy and atomic physics. Advances in quantum theory and improved models of atomic structure made it clear to astronomers that deeper understanding of the life cycle of stars and of cosmological theories explaining the vastness of space was to be forever tied to advances in understanding inner workings of the universe on an atomic scale. In addition, a complete understanding of the energies of mass conversion in stars was provided by German-American physicist Albert Einstein's (1879-1955) special theory of relativity and his relation of mass to energy (E = mc2, or energy [E ] equals mass [m] times the speed of light [c] squared). Indian-born American astrophysicist Subrahmanyan Chandrasekhar (1910-1995) first articulated the evolution of stars into supernovae, white dwarfs, and neutron stars; and predicted the conditions required for the formation of black holes, which were subsequently confirmed by observation in the last years of the twentieth century. Stellar mechanics
The material between stars occurs in clouds of varying mass. By processes that are still not completely clear, but involve cooling of the cloud-center with the formation of molecules, and the squeezing of the cloud by outside starlight or perhaps astellar explosion, the cloud begins to collapse under its own self-gravity. The collapse of the cloud results in the material becoming hotter simply from the squeezing of the collapse. At this point, the interior of the star churns. This churning process is called convection. Its rate of collapse is determined by the rate at which it can lose energy from its surface. Atomic processes keep the surface near a constant temperature so that a rapid collapse is slowed by the radiating surface area shrinking during the collapse. The star simply gets fainter while the interior gets progressively hotter. Finally, the internal temperature rises to the point where atoms located at the center of the star, where the temperature is the hottest, are moving so fast from the heat generated that they begin to stick together. This process is called nuclear fusion, and it results in an additional production of energy. Thus, the star has a new source of heat. The subsequent evolution of the star will be largely determined by its mass. If the mass of the star is about equal to that of the Sun or less, the nuclear fires that now provide the energy for the star to shine will determine its internal structure. A central radiative core is surrounded by a convective envelope. In the radiative core the material remains quiescent, while energy generated by nuclear fusion of hydrogen to helium simply diffuses through it like the light from automobile headlights shines through a fog. It is at the very center of this radiative core that the helium ash of the nuclear fires accumulates as the star ages. Beyond the radiative core lies the churning convective envelope through which the energy is carried by blobs of hot matter rising past returning cooler blobs. At the atmospheric surface, the energy again flows as it did in the core until it physically leaves the star as starlight. The structure of stars more than twice the mass of the Sun is essentially the reverse of the low-mass stars. The cores of these stars are fully convective so that the energy produced by nuclear fusion is carried...
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