An Assumption and a Warning

In applications of stellar abundances to questions of cosmochemistry, an assumption is invoked. Put in its strongest form, this states that the star s present surface composition is identical to that of its natal interstellar cloud. When applicable and in conjunction with corresponding assumptions about interstellar clouds, this assumption allows the chemical evolution of the Galaxy to be traced using stars of different ages or more realistically stars of different metallicity. 

A listing of other failures of the unrestricted assumption would be lengthy. Some offer opportunities to the cosmochemist but others thwart the search for the origins of the elements. Examples must suffice. Severe mass loss may expose interior regions previously experiencing nuclear processing (Wolf-Rayet stars, for example) mass transfer across a binary system may expose a stellar core (Algols) or make processed material available in 

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