metallicity

History

In the first half of the 20th century, while studying stellar spectra, astronomers noticed that some stars had stronger absorption lines of metals than others. In 1925, Cecilia Payne-Gaposchkin convincingly showed in her dissertation that hydrogen is the main element of stars, and differences in line intensities are explained by conditions in atmospheres. Later, Walter Baade introduced the concepts of stellar populations I and II, linking them to the abundance of heavy elements.

How it works

Heavy elements are born inside massive stars through thermonuclear reactions and are scattered into interstellar space by supernova explosions or stellar winds. New stars condense from this enriched gas, so each subsequent generation gets a little more metals. Thus, the oldest Population II stars are very metal-poor ([Fe/H] < –1), while young ones like our Sun (Population I) are relatively rich. It’s like a family heirloom: each generation adds new 'jewels' (elements) to the common treasury.

History

In the first half of the 20th century, while studying stellar spectra, astronomers noticed that some stars had stronger absorption lines of metals than others. In 1925, Cecilia Payne-Gaposchkin convincingly showed in her dissertation that hydrogen is the main element of stars, and differences in line intensities are explained by conditions in atmospheres. Later, Walter Baade introduced the concepts of stellar populations I and II, linking them to the abundance of heavy elements.

How it works

Heavy elements are born inside massive stars through thermonuclear reactions and are scattered into interstellar space by supernova explosions or stellar winds. New stars condense from this enriched gas, so each subsequent generation gets a little more metals. Thus, the oldest Population II stars are very metal-poor ([Fe/H] < –1), while young ones like our Sun (Population I) are relatively rich. It’s like a family heirloom: each generation adds new 'jewels' (elements) to the common treasury.

History

At the beginning of the 20th century, spectroscopists noticed that stars differ in the intensity of metal lines. The key breakthrough was made by Cecilia Payne-Gaposchkin in 1925: applying Saha’s ionization theory, she proved that stars consist mainly of hydrogen, and differences in spectra are explained by temperature, not composition. In the 1940s, Walter Baade, studying galaxies resolved into individual stars, identified Population I (metal-rich, disk) and Population II (metal-poor, halo). With the advent of high-sensitivity spectroscopy and large-scale surveys (SDSS, Gaia), the metallicities of millions of stars have been measured, allowing the reconstruction of the chemical evolution of the Milky Way.

How it works

Elements heavier than helium, except for a small amount of lithium and beryllium, are synthesized inside stars. On the main sequence, hydrogen is converted into helium, then in later stages helium is converted into carbon, oxygen, and so on up to iron (the peak of the binding energy curve). Heavy nuclei (copper, gold, uranium) are created by neutron capture processes: the slow s-process in the shells of asymptotic giant branch stars and the rapid r-process during supernova explosions or neutron star mergers. The products of nucleosynthesis are ejected into the interstellar medium through winds, planetary nebulae, and explosions. Subsequent generations of stars form from this enriched material, increasing the average metallicity with time. The observed distribution of [Fe/H] in different components of the Galaxy (thin disk, thick disk, halo) reflects their age and migration. Thus, massive elliptical galaxies retain metals in a deep potential well and exhibit high Z. In stellar models, metallicity enters into the equation of state, opacity (calculated using OPAL or OP grids), and nuclear reaction rates. For spectroscopic determination of [Fe/H], the measured equivalent widths of Fe I and Fe II lines are compared with synthetic spectra, solving the transfer equations assuming local thermodynamic equilibrium (LTE) or taking into account non-LTE deviations.

Open problems & fun facts

What is the true fraction of metals locked in dust grains in different phases of the interstellar medium?; How do inhomogeneous mixing and radial migration of stars affect the metallicity gradient of the Galaxy?; What are the properties of the very first stars (Population III) with zero metallicity, and how did they enrich the early intergalactic medium?; How does metallicity influence the formation of supermassive black holes and the initial mass function?; Does the predicted extremely metal-poor gas exist in the circumgalactic medium, and how can it be detected?

💡 The Sun belongs to the ~10% most metal-rich stars of its age in the solar neighborhood. This 'super-metallicity' likely aided the formation of rocky terrestrial planets and, perhaps, the emergence of life.