Alpha Enhancement, Global Metallicity, and the Resulting Shift in Cluster Ages

Globular cluster stars are not scaled copies of the Sun's mixture. Their alpha elements — oxygen, magnesium, silicon, calcium — are enhanced relative to iron by factors of two or more, and it is the total metal content, not iron alone, that governs opacity and therefore evolution. This study applies the standard global-metallicity equivalence to five clusters and converts the result into age terms. The equivalence raises the effective metallicity above the iron abundance by up to 0.29 dex, and dating an alpha-enhanced population with scaled-solar models misstates its age by up to 0.88 billion years, in NGC 6397. Modern age analyses use alpha-enhanced models and avoid the error; older scaled-solar ages, still quoted in secondary literature, carry it in full. The computation follows a published equivalence formula applied to published abundances; it introduces no new observation.