Stellar Evolution

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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
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The churning motion of the material in the convective core causes the nuclear ash of helium to be mixed with the surrounding hydrogen fuel. This motion ensures that virtually all the hydrogen will be available to the nuclear fires that heat the star.
Both high- and low-mass stars respond to the depletion of hydrogen fuel in a similar manner. In order to supply the heat to oppose its own self-gravity, the star 's core again responds by shrinking. In a sort of reflex reaction, the outer regions of the star expand, causing a great increase of its radiating surface area. Although the total energy output of the star increases during this phase, the greatly enhanced surface area results in a cooling of the surface and the star takes on a redder appearance. The size and color change lead to the name of red giant for these stars. If the star is very massive, it may become what is called a red supergiant.
For the low-mass stars, the expansion to the red giant phase will begin when about 90% of its hydrogen has been converted to helium. During the contraction of its core, a complicated sequence of events occurs. The shrinkage required to produce the energy radiated by the large giant causes the core to shrink to the dimensions of a white dwarf, while hydrogen continues to burn by nuclear fusion in a thin shell surrounding the core. This shell provides most of the energy that is radiated away by

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