The helium 1083 nm line (metastable helium line, sometimes referred to as the helium 1080 nm line) is an absorption line of interest in the study of extra-solar planet atmospheres. It is a near infrared line but within visible-light atmospheric window, and little affected by the interstellar medium, thus useful for ground observation. Its detection through transmission spectroscopy of the planet's atmosphere (during a transit) is an indication of atmospheric escape of helium.
The line is a triplet (i.e., three fine structure lines) of helium in a metastable state, the line produced by absorption of passing EMR by helium that is in a particular metastable excited state which generally lasts only hours. Thus the line's presence indicates something is putting some helium in this state, which can happen during an atom's recombination after ionization. Atmospheric escape can be modeled using plausible values of temperature, pressure and constituents, that calculate how much helium would be ionized and how much would be in this metastable state.
The metastable state consists of electrons in both the first and second electron shells that have parallel spin, making the excited electron's straight-forward transition to the ground state forbidden, i.e., sufficiently improbable to remain in the state for hours before transitioning. The eventual transition results in emission of a photon with a wavelength of 62.6 nm (if the atom is not otherwise disturbed before that). The 1083 nm absorption line is not forbidden: it corresponds to photon energy that further excites the electron to another allowed energy level. A helium atom in the ground state (no excitation) is not in the position to absorb a photon of that energy, thus the absorption line reveals the presence of helium in the metastable state.