Subgiant-Branch Luminosity as a Lower-Systematics Age Indicator: An Error Budget Comparison

The classic turnoff age is read partly from colour, and colour is where stellar models are least trustworthy — it depends on atmosphere models, convection near the surface, and reddening. The subgiant branch offers a different observable: the luminosity of stars just leaving the main sequence, which changes rapidly with age while depending far less on surface physics. This study assembles an explicit error budget for three dating methods from published method comparisons. The subgiant luminosity method totals 0.40 billion years against 0.69 billion years for the classic turnoff-colour method, an improvement of 42 percent, with the gains concentrated exactly where theory is weakest. The method does not escape distance or composition errors, and the budget says so. The comparison is an accounting from published terms, not a new age determination for any cluster.