How Internal Helium Spreads Bias Turnoff Ages in Clusters With Multiple Populations

Most globular clusters host sub-populations enriched in helium by amounts that would have been dismissed as impossible a generation ago. Because a helium-rich star burns faster, fitting such a star with primordial-helium models overstates its age. This study converts published helium spreads into age biases for five clusters. The largest spread, delta Y of 0.15 in Omega Centauri, corresponds to an age overestimate of 5.78 billion years if the enriched sequence is dated with standard models. Averaged over the sample the bias is 2.82 billion years — comparable to the total error budget usually quoted for cluster ages, and systematically one-sided. Ages used in cosmological comparisons must state which population was dated. The conversion is computed from published helium estimates and a stated isochrone sensitivity; no new observation is reported.