The Age Spread of Galactic Globular Clusters as a Measure of the Halo Formation Duration

If globular clusters had all formed in a single burst, their ages would scatter only by measurement error. They do not. This study deconvolves published errors from the observed age dispersions of three metallicity groups and measures how long cluster formation actually lasted. The metal-poor halo group centres on 13.00 billion years with an intrinsic dispersion of 0.24 billion years, while the metal-rich group centres 1.8 billion years younger. Cluster formation in and around the young Milky Way was therefore an extended process spanning roughly two billion years, not an event. The old edge of the distribution — the quantity cosmology cares about — is carried by the metal-poor group alone, and using the full cluster population's mean age in a cosmological argument understates the age of the oldest systems by more than a billion years. Dispersions are computed from published compilations; no new photometry is presented.