Study of the chemical composition and chemical processes in stars
Internal structure of a massive star shortly before core collapse, showing concentric shells of fusion products formed during successive stages of stellar nucleosynthesis.
Understanding stellar chemical composition is essential for reconstructing the chemical evolution of galaxies, determining stellar ages, identifying distinct stellar populations, and constraining the conditions under which planetary systems form. Chemical abundances also provide key diagnostics for processes such as stellar convection, diffusion, mass loss, and supernova enrichment, linking the life cycles of stars to the broader evolution of the Universe.
Chemical composition of stars
Hertzsprung–Russell diagram for the open clusters M 67 (yellow) and NGC 188 (cyan). The distribution of stars along the main sequence and turnoff reflects differences in age and chemical composition between the clusters.
Stars are composed primarily of hydrogen and helium, with heavier elements—collectively referred to as metals in astronomy—constituting only a small fraction of their mass. The relative abundance of these heavier elements is expressed as a star’s metallicity, commonly measured through the logarithmic iron abundance ratio [[[Fe/H]]].
Population III stars are hypothetical first‑generation stars composed almost entirely of hydrogen and helium, formed before significant stellar nucleosynthesis enriched the Universe.
Chemical abundances provide key information about the environments in which stars formed, the interstellar medium from which they condensed, and the nucleosynthetic contributions of earlier stellar generations. Variations in metallicity across stellar populations underpin studies of galactic chemical evolution, stellar age determination, and the formation of planetary systems.
Nuclear processes and element formation
The chemical composition of a star evolves over time as nuclear fusion reactions in its core convert lighter elements into heavier ones. The dominant fusion pathways depend on the star’s mass, temperature, and evolutionary stage. Major processes include:
The s-process (slow neutron capture), occurring primarily in asymptotic giant branch (AGB) stars and responsible for producing many elements beyond iron.
Together, these processes generate most of the elements heavier than helium and drive the chemical enrichment of the interstellar medium. The products of stellar nucleosynthesis are later incorporated into new generations of stars and planetary systems, linking stellar evolution to the broader cycle of galactic chemical evolution.
Stellar atmospheres and abundance measurements
Solar irradiance spectrum showing the theoretical blackbody curve (yellow), the solar spectrum at the top of Earth’s atmosphere (orange), and the spectrum at sea level (blue). Atmospheric absorption by gases such as water vapor, oxygen, and carbon dioxide produces dips in the observed spectrum.
Ground‑based observations of stellar spectra must account for absorption by Earth’s atmosphere. Molecules such as water vapor, oxygen, and carbon dioxide introduce wavelength‑dependent attenuation—particularly in the infrared—that alters the apparent depth and shape of spectral features. Accurate abundance measurements therefore require correction using atmospheric models, telluric calibration, or observations from space telescopes. The visible portion of the spectrum is least affected by atmospheric absorption and remains the primary window for high‑precision optical spectroscopy.
Chemical abundances in stars are determined primarily through spectroscopy. Absorption lines in stellar spectra reveal the presence and relative abundance of elements in the stellar atmosphere, and their strengths depend on temperature, pressure, and ionization state. Modern abundance analysis combines:
Abundance ratios such as [Fe/H] (iron relative to hydrogen) and [α/Fe] (alpha‑elements relative to iron) are widely used to classify stellar populations, trace galactic chemical evolution, and identify stars with distinct formation histories.
As stars evolve, internal mixing and mass‑loss processes alter their observable chemical composition. Key mechanisms include:
Convective mixing in red giants
Dredge‑up events in AGB stars, which bring carbon and s‑process elements to the surface
Rotational mixing in massive stars
Mass loss through stellar winds, which enriches the surrounding medium with newly synthesised elements
These processes explain the chemical diversity observed among evolved stars.
Chemically peculiar stars
Echelle spectrum of the carbon star UU Aurigae, showing prominent molecular absorption bands from species such as C₂ and CN. These features reflect the star’s chemically enriched atmosphere and carbon‑rich composition.
A number of stellar classes exhibit unusual or anomalous chemical signatures in their spectra. These chemically peculiar stars provide important diagnostics of internal mixing, magnetic fields, binary interactions, and late‑stage nucleosynthesis. Major categories include:
Carbon stars – giants enriched in carbon due to third dredge‑up of helium‑burning products during the AGB phase, producing strong molecular bands of C₂, CN, and CH.
These chemical peculiarities offer insight into stellar interiors, magnetic and rotational processes, binary evolution pathways, and the nucleosynthetic origins of heavy elements.
Stellar chemistry and planetary habitability
Stellar chemical composition influences the formation and long‑term stability of planetary systems. Some studies suggest that higher abundances of elements such as carbon, magnesium, sodium, and silicon may affect stellar evolution rates and the duration of a star’s habitable zone.[1][2] Oxygen abundance may also influence how long a planet remains within a star’s habitable zone.[2]
Effects of stellar activity on planetary atmospheres
Stellar activity—including stellar flares, coronal mass ejections, and high‑energy ultraviolet and X-ray radiation—can strongly influence the atmospheric chemistry of orbiting exoplanets. These energetic events drive photochemical reactions, alter ozone abundance, and can modify the long‑term stability of planetary atmospheres.
A 2010 study modeling the impact of a strong flare from the active M‑dwarf AD Leonis on an Earth‑like planet found that such events do not necessarily sterilize planetary surfaces, even around highly active stars, although they can induce significant short‑term chemical perturbations.[4]
↑Oliveira, J. M.; van Loon, J. T.; Chen, C. H. R.; Tielens, A.; Sloan, G. C.; Woods, P. M.; Kemper, F.; Indebetouw, R.; Gordon, K. D.; Boyer, M. L.; Shiao, B.; Madden, S.; Speck, A. K.; Meixner, M.; Marengo, M., ICE CHEMISTRY IN EMBEDDED YOUNG STELLAR OBJECTS IN THE LARGE MAGELLANIC CLOUD. Astrophysical Journal 2009, 707 (2), 1269–1295.
↑Segura, A.; Walkowicz, L. M.; Meadows, V.; Kasting, J.; Hawley, S., The Effect of a Strong Stellar Flare on the Atmospheric Chemistry of an Earth-like Planet Orbiting an M Dwarf. Astrobiology 2010, 10 (7), 751–771.
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