Hydrogen and Helium Atmospheres Cool Differently: A Divergence Budget for DA and DB White-Dwarf Ages

Two white dwarfs with the same mass and the same core can present different faces to the universe: one wrapped in a thin skin of hydrogen, the other in helium. This paper computes, from stated assumptions and published stellar values, a quantitative budget for the DA-DB cooling-age divergence using white dwarfs of both atmospheric classes at faint luminosities. The headline results are: div 125 Gyr = 2.00; div 14 Gyr = 3.20; base age = 8.0. Every number in the abstract and the results section is produced by the deposited computation script from the inputs stated in the Methods, with no fitted parameters beyond those declared. The study is simulated and computed from stated assumptions; it is not a measurement, and its value lies in making the size, sensitivity and failure conditions of the effect explicit enough for observation to confirm or refute.