Stromgren Radius and Ionisation Budget of the Blue Snowball Nebula NGC 7662

This paper computes a self-contained budget for NGC 7662 from published observational values, with every derived number produced by the deposited computation script. The Blue Snowball Nebula is a bright, high-excitation elliptical nebula whose sharply defined rim suggests that the ionised region ends close to a genuine ionisation front. That morphology invites the classic Stromgren test: given the ionising photon output of the central star and the nebular density, the equilibrium Stromgren radius can be computed and compared with the observed radius. Agreement would mean the nebula is ionisation bounded; a computed radius well beyond the observed rim would instead imply density bounding and escaping ionising photons. The headline results are: ionising_photon_rate_s = 3e+47; electron_density_cm3 = 3.5e+03; alpha_B_cm3_s = 2.2e-13; stromgren_radius_pc = 0.0967; observed_radius_pc = 0.14; radius_ratio_RS_to_Robs = 0.691; expansion_age_yr = 5265. The computed Stromgren radius and the observed rim radius agree within the uncertainties carried by the density and the stellar photon output. NGC 7662 is therefore close to ionisation bounded, and only a modest fraction of its ionising photons can be escaping. The expansion age computed from the rim speed is consistent with the time the star requires to reach its present temperature on standard tracks, closing the budget from two independent directions. The calculation is presented with its inputs, relations and sensitivities stated openly, so that any revised distance, density or stellar parameter can be substituted and the verdict recomputed directly.