The abundance of total alpha elements in stars is usually expressed in terms of logarithms, with astronomers customarily using a square bracket notation:
where  is the number of alpha elements per unit volume, and  is the number of iron nuclei per unit volume. It is for the purpose of calculating the number  that which elements are to be considered "alpha elements" becomes contentious. Theoretical galactic evolution models predict that early in the universe there were more alpha elements relative to iron.