A Concise Treatise on Converting Stellar Mass Fractions to Abundances to Molar Ratios

Understanding stellar composition is fundamental not only to our comprehension of the Galaxy, especially chemical evolution, but it can also shed light on the interior structure and mineralogy of exoplanets, which are formed from the same material as their host stars. Unfortunately, the underlying mathematics describing stellar mass fractions and stellar elemental abundances is difficult to parse, is fragmented across the literature, and contains vexing omissions that makes any calculation far from trivial, especially for nonexperts. In this treatise, we present a clear mathematical formalism and clarification of inherent assumptions and normalizations within stellar composition measurements, which facilitates the conversion from stellar mass fractions to elemental abundances to molar ratios, including error propagation. We also provide an example case study of HIP 544 to further illustrate the provided equations. Given the important chemical association between stars, as well as the interdisciplinary relationship between stars and their planets, it is vital that stellar mass fractions and abundance data be more transparent and accessible to people within different subfields and scientific disciplines.