Primordial abundances require extrapolation to material minimally processed by stars. Helium is inferred from hydrogen and helium emission in low-metallicity ionized regions, with ionization and temperature corrections. Deuterium is inferred from isotope-shifted absorption in low-metallicity quasar systems. Lithium is measured in old metal-poor stellar atmospheres and requires a correction, or a bound, for stellar depletion. These observables probe different astrophysical environments and systematic errors.
The Spite plateau is the approximately constant lithium abundance in warm, metal-poor halo stars. It provides an important estimate of primordial lithium only after accounting for stellar atmosphere physics and possible transport and nuclear destruction of lithium within the stars.
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