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Energy Production in Stars Stars are at enormous distances, yet we can see thousands => extremely luminous What is the source of their enormous energy? o Thermonuclear reactions (fusion) that convert mass to energy In the special theory of relativity Einstein demonstrated that the total mass-energy was conserved => e = mc2 Speed of light squared is a very large number => small amount of mass corresponds to a huge amount of energy The conversion of mass to energy accounts for the enormous energy output of the stars What physical mechanisms can cause this? Nuclear fission - splitting of an atom's nucleus Nuclear fusion - sticking two nuclei together Nuclear fusion is favored because: o The most stable nuclei in the universe is Fe o Demonstrated by the Curve of Binding Energy o o Stars are made of light elements (mostly H and He) and favor a reaction that makes them closer to the structure of Fe To combine, nuclei must overcome the coulomb barrier => provides a strong resistance to nuclear fusion in stars The greater the density and temperature of a star, the greater probability (and the actual amount) of fusion Interior of Stars Information on a star's interior gathered from observations of the Sun Core o Dense (12 times that of solid lead) => 1/3 the total mass of the star o Nuclear fusion takes place here o Ends ¼ of the distance to the surface Radiation Zone o Less dense than the core (~ density of H2O) o Energy from the core is absorbed and reemitted many times (photons move through like a drunken sailor) => can take millions of years o Ends ½ of the distance to the surface Convection Zone o Density much less (~ 1% the density of H2O) o Gases heated at the bottom of zone, rise to the surface (photosphere), release energy, cool, and sink back to the bottom