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The adjective alpha-enhanced (or alpha rich or high-alpha) indicates metallicity that has a high ratio of alpha elements to iron peak elements, and alpha poor or low-alpha indicates the reverse. These terms may be used to describe populations of stars, such as those of a stellar cluster or those of a structure within a disk galaxy (e.g., spiral galaxy) such as its bulge, thin disk, thick disk or some substructure, e.g., an alpha rich disk (a disk defined by its alpha-enhancement) or alpha poor disk. An alpha-enhanced metallicity is taken as a clue to the history of star formation. Following a burst of star formation, core collapse supernovae begin on the order of 20 million years later (a very short time), occurring in the most massive of these newly formed stars. Such supernovae tend to produce alpha elements (as opposed to iron peak elements) and the generation of stars that immediately follows, that includes gas from these supernovae, are alpha-enhanced. Type Ia supernovae additionally produce iron peak elements, but don't occur until less massive stars evolve into white dwarfs, on the order of a gigayear after the initial burst of star formation, and having the lower ratio of alpha elements to iron peak elements, are relatively alpha poor. While metallicity itself gives some clue to the star formation history, knowing the portion of the metallicity attributable to alpha elements provides more detail.
A measure showing the amount of alpha enhancement is alpha abundance, calculated as an alpha-to-iron ratio ([α/Fe], i.e., in bracket notation) where α indicates alpha elements. A useful graph to make this clear is the [α/Fe] versus [Fe/H] diagram. Rather than all alpha elements, comparison of the abundance of a specific element with that of iron is sometimes used as a proxy, e.g., the oxygen-to-iron ratio ([O/Fe]).