The results of this paper rest on the use of stellar spectra from the SDSS SEGUE survey. Stellar spectra allow observers to calculate the spectral type, iron metallicity, and other chemical abundances of a population of stars. Two of the most common abundances to examine are iron metallicity [Fe/H] relative to the sun, and α-element abundance relative to iron [α/Fe].
The iron abundance [Fe/H] can give information about the age of the star, since older stars were born when the heavy elements in the universe had not yet been synthesised inside massive stars or supernovae. Therefore older stars will have lower metallicty and hence iron abundance.
Alpha elements such as C, N, O, and Ca, are produced in triple-alpha reactions in the cores of red giant stars. The α-element abundance can also give clues to the age and star formation history of a population. According to Wikipedia it is a characteristic of older Population II stars that despite their lower overall metallicity, they often have a higher ratio of alpha elements (O, Si, Ne, etc.) relative to Fe as compared to younger Population I stars.
Theory suggests “this is the result of Type II supernovae being more important contributors to the interstellar medium at the time of their formation, whereas Type Ia supernovae metal enrichment came later in the universe’s evolution”. \cite{wiki}