Chemical Evolution of Stellar Populations in the Milky Way

The chemical evolution of stars within the Milky Way traces the complex interplay between nucleosynthesis, gas inflows and outflows, and dynamical processes that shape the Galaxy’s structure over cosmic time. Elements heavier than helium, collectively termed ‘metals’, are forged in the cores of successive generations of stars and dispersed into the interstellar medium through stellar winds, supernova explosions and mergers. As gas cools and collapses to form new stars, it inherits the enrichment history imprinted by preceding populations, creating spatial and temporal abundance patterns across the thin and thick discs, bulge and halo. Observations of elemental ratios—particularly those of α-elements relative to iron—and stellar ages provide a chrono-chemodynamic record of star formation rates, merger events and radial migration. Gradients of metallicity and α-enhancement reveal an early, rapid formation of a kinematically hot thick disc followed by more quiescent growth of the thin disc, punctuated by accretion episodes and secular processes. Advances in large-scale spectroscopic surveys and astrometric missions have yielded high-precision maps of abundance distributions, enabling chemical tagging of stellar populations and comparisons with cosmological simulations. This synergy has refined our understanding of Galactic accretion history, the role of radial flows in smoothing abundance gradients and the diversity of enrichment pathways among Milky Way-like galaxies.

Research from Nature Portfolio

Recent studies have reconstructed the integrated stellar metallicity profile of the Milky Way from light-weighted measurements, uncovering a distinct broken-profile shape. Within a Galactocentric radius of approximately 7 kpc, the metallicity gradient is mildly positive, transitioning to a steep negative gradient at larger radii. This pattern differs from the profiles observed in a representative sample of spiral galaxies and in cosmological simulations, suggesting that the Milky Way’s enrichment history may not be typical for its mass class. The findings highlight how early star formation intensity and subsequent gas accretion have sculpted a unique chemical architecture, offering new constraints on models of galaxy growth and the efficiency of mixing processes in disc galaxies.

Chemical Evolution of Stellar Populations in the Milky Way publication trend

The graph below shows the total number of articles in chemical evolution of stellar populations in the milky way across all publications each year (not limited to Nature Index journals).

Technical terms

Metallicity ([Fe/H]): The logarithmic measure of a star’s iron abundance relative to hydrogen, indicating its overall heavy-element content compared to the Sun.

α-elements: Elements such as oxygen, magnesium and silicon produced predominantly in Type II supernovae; their abundance relative to iron traces the timescale of star formation.

Thin disc: The kinematically cold component of the Galactic disc, characterised by younger stars with lower α-enhancements and a narrow vertical scaleheight.

Thick disc: An older, kinematically hotter component with higher α-enhancement and larger vertical scaleheight, reflecting rapid early enrichment.

Radial abundance gradient: The change in chemical abundance as a function of distance from the Galactic centre, shaped by star formation and gas flows.

References

1. The integrated metallicity profile of the Milky Way. Nature Astronomy (2023).
2. StarHorse results for spectroscopic surveys and Gaia DR3: Chrono-chemical populations in the solar vicinity, the genuine thick disk, and young alpha-rich stars⋆. Astronomy & Astrophysics (2023).
3. A Tale of Two Disks: Mapping the Milky Way with the Final Data Release of APOGEE. The Astrophysical Journal (2023).