Abstract
Enhancements of the ratios He/H and N/O in planetary nebulae are shown to be consistent with enhancements in surface abundances predicted by the theory of stellar evolution. From a quantitative comparison between observation and theory we infer that: (1) hydrogen is converted into helium, and both carbon and oxygen are converted into nitrogen in progenitor stars; (2) the primordial ratio of helium to hydrogen in the Galaxy is approx.0.08 and is augmented by approximately 20% during galactic history; (3) the initial main-sequence masses of stars which ultimately eject planetaries with the largest enhancements of He/H and N/O are in excess of 3--5 M/sub sun/; and (4) considerable mass loss by stellar winds precedes the ejection of a planetary shell.
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