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A [α/Fe] versus [Fe/H] diagram (aka Tinsley-Wallerstein diagram) is a graph plotting the positions of stars of interest according to two particular stellar abundance-ratio measures. The two axes are for the abundance ratio of alpha elements to iron [α/Fe] (i.e., in bracket notation) and for the abundance ratio of iron to hydrogen [Fe/H]. The plot shows stars' range of alpha enhancement, summarizing their alpha-element characteristics. Alpha enhancement indicates stars that formed before Type Ia supernovae produce and spread more iron and iron peak elements. On the order of twenty million years after star formation, the OB stars undergo core collapse supernova, distributing alpha elements, which show up in subsequent star formation. Later, a gigayear after that initial star formation, smaller stars have evolved into white dwarfs, some of which undergo Type Ia supernovae, which form and distribute iron peak elements, which appear in stars formed subsequently, giving them a higher ratio of iron to alpha elements.