Stellar chemistry

Stellar chemistry is the study of the chemical composition of stars and the physical and chemical processes that govern how elements are formed, transported, and observed in stellar environments. It is a subfield of astrophysics and is closely connected to astrochemistry, nuclear astrophysics, and galactic chemical evolution. Stellar chemistry encompasses both the internal nucleosynthesis reactions that create new elements and the observable atmospheric signatures—such as spectral lines and abundance patterns—that reveal a star’s origin, age, and evolutionary state.

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.

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]]].

Metallicity varies systematically among different stellar populations:

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.

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:

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.

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.