Summary

A fundamental observation is that the abundance of heavy elements in the interstellar medium has increased through time. Generally astrophysicists use, as a proxy for time, the time-calibrated solar-normalized iron-to-hydrogen metallicity index [Fe/H], which is a relatively easy quantity to measure in stars. However, Fe is produced in different types of stars and its evolution is complicated. Another promising "chronometer" is [O/H], because O is the most abundant element produced in a galaxy and its chemical evolution is straightforward: except for the O-excess in the very early Galaxy, supernovae II are by far the most important source of oxygen.

The [CNO/Fe] abundance has its largest values at extremely low metallicities. These and some other observations point to early nucleosynthesis in supermassive stars with M > 100 M0. Such stars have not yet been observed; modelling suggests that they could form at very low metallicities, would have a very short life span and could explode at an early stage, ejecting dominantly light (below Fe-peak) elements into the ISM.

Very high abundances of heavy r-process elements observed in ultra-metal-poor stars indicate an early contribution from supernovae II, the only sources of these elements. The pattern of — 14-Gyr-old r-elements is indistinguishable from that in the solar system, suggesting a great uniformity of the r-process in different stars.

Trace s-process elements are underabundant at low metallicities; their abundance, along with the [s-process/r-process] value, increases smoothly with time. This is explained by the generation of s-process elements in low-mass stars, which live longer than massive stars.

Grains in the interstellar medium have preserved a high-resolution record of the evolution of stable isotopes as well as vestiges of the decay of short-lived nuclides. The stable isotopes provide constraints for models of red-giant, AGB and supernova nucleosynthesis processes. The radionuclides can potentially highlight details of galactic chemical evolution occurring on a short time scale, but interpretation of the data depends on the relevant nucleosynthetic processes, which are a subject of ongoing discussion.

Not only the evolution trends for the elements but also many other observations, such as the present-day mass of the Galaxy, the mass/gas ratio and the relative abundance of stars of different masses, constitute the basis for galactic chemical evolution models. Such modelling allows some important parameters to be quantified. Thus, the formation rate of stars appears to be proportional to the square of the gas density in the Galaxy. Further, the number of stars being formed with mass m within an internal Am decreases exponentially as m increases from > 0.1 to ~ 100 M0. A constant initial mass function over galactic time appears to be a reasonable approximation. Inflow and outflow are important for complicated models envisaging galaxy heterogeneity in space and time, which is indeed the case for our Galaxy.

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