The many-multiplet method applied to high redshift quasar absorption spectra has indicated a possible time variation of the fine structure constant .
Alternatively , a constant value of \alpha is consistent with the observational analysis if a non-solar isotopic ratio of ^ { 24 , 25 , 26 } Mg occurs at high redshift .
In particular , a higher abundance of the heavier isotopes ^ { 25 , 26 } Mg are required to explain the observed multiplet splitting .
We show that the synthesis of ^ { 25 , 26 } Mg at the base of the convective envelope in low-metallicity asymptotic giant branch stars , combined with a simple model of galactic chemical evolution , can produce the required isotopic ratios and is supported by recent observations of high abundances of the neutron-rich Mg isotopes in metal-poor globular-cluster stars .
We conclude that the present data based on high redshift quasar absorption spectra may be providing interesting information on the nucleosynthetic history of such systems , rather than a time variation of fundamental constants .