All stellar evolution models for nucleosynthesis require an initial isotopic abundance set to use as a starting point . Generally , our knowledge of isotopic abundances of stars is fairly incomplete except for the Sun . We present a first model for a complete average isotopic decomposition as a function of metallicity . Our model is based on the underlying nuclear astrophysics processes , and is fitted to observational data , rather than traditional forward galactic chemical evolution modeling which integrates stellar yields beginning from big bang nucleosynthesis . We first decompose the isotopic solar abundance pattern into contributions from astrophysical sources . Each contribution is then assumed to scale as a function of metallicity . The resulting total isotopic abundances are summed into elemental abundances and fitted to available halo and disk stellar data to constrain the model ’ s free parameter values . This procedure allows us to use available elemental observational data to reconstruct and constrain both the much needed complete isotopic evolution that is not accessible to current observations , and the underlying astrophysical processes . As an example , our model finds a best fit for Type Ia contributing \simeq . 7 to the solar Fe abundance , and Type Ia onset occurring at [ \mathrm { Fe / H } ] \simeq - 1.1 , in agreement with typical values .