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Published online by Cambridge University Press: 12 April 2016
We summarize here some of the results that we have obtained from chemical evolution models. In particular, we discuss those aspects related to stellar mass loss, helium production, a varying heavy element yield and the very light stars.
The observed value of the helium to heavy element abundance ratio ΔY/ΔZ is about 3 (e.g. Lequeux et al. 1979). First theoretical estimates gave ΔY/ΔZ= 0.4 (Hacyan et al. 1976).