Recent observations reveal that the ratio of deuterium to hydrogen atoms in the dense, cold cores that form stars can reach at least 10 %. This level of enrichment is measured through the abundance of deuterated molecules in gas colder than 10 K and denser than about 10⁵ cm⁻³.

To map the structure of these cores, astronomers use the high‑J rotational transitions of N₂H⁺ and N₂D⁺ at around 460 GHz. These lines have critical densities of about 10⁷ cm⁻³, so they trace the deepest, densest parts of the core. They are observed together with the ground‑state lines of ortho‑H₂D⁺ (372 GHz) and para‑D₂H⁺ (692 GHz). The combined map gives a spatially resolved view of the density, velocity, and ionisation state.

A dust‑emission map of the core L1544, overlaid with the ortho‑H₂D⁺ spectrum taken at its centre, illustrates this high deuteration. The spectrum shows deep absorption features that can only arise when D/H ≥ 0.1.