Abstract
The N/O, 0/H diagram derived from galactic and extragalactic H II regions can be explained under the assumptions that most of the N is of secondary origin, that most of the N is produced by intermediate mass stars (1 M/M® S), and that accretion plays an important role in the chemical evolution of galaxies. Several observational and theoretical arguments are presented in favor of these hypotheses. Given that constant-yield models can never reach 12 + log 0/H % 9.0 because the instantaneous recycling for 0, breaks down it is an essential component of the new model that the yield increases with metallicity.
