heliumdominated

Helium-dominated is an observational and descriptive term used to refer to a system in which helium constitutes the majority of atoms by abundance. It is most commonly used in astronomy to describe stellar atmospheres, envelopes, or remnants where helium outnumbers hydrogen and other elements, either by number or by mass, in the observable layers.

In the context of stars and white dwarfs, helium-dominated atmospheres occur in various evolutionary stages. Helium-rich or helium-dominated white dwarfs, such as the DB and DO types, have photospheres where helium lines dominate the spectrum and hydrogen features are weak or absent. Other helium-dominated stellar objects include certain hot subdwarfs and peculiar stars whose outer layers have been stripped or where diffusion has led to helium enrichment at the surface. The classification often depends on spectral analysis and atmospheric modeling to determine the relative abundances of helium and other elements, accounting for processes like gravitational settling and radiative diffusion.

Beyond stellar contexts, helium-dominated plasmas can arise in laboratory or fusion-plasma environments when helium becomes the principal ion species, for example as a byproduct of fusion reactions. In planetary science, the term is unlikely to describe typical planetary atmospheres, which are usually hydrogen-helium mixtures, but it may appear in theoretical scenarios or exotic compositions.

Overall, a helium-dominated designation indicates a surface or environment where helium is the leading constituent, influencing spectra, opacity, and the inferred physical properties. See also helium-rich star, DB white dwarf, and DO white dwarf.