The Lyman-alpha line is a specific spectral line in the ultraviolet part of the electromagnetic spectrum. It corresponds to a transition of an electron in a hydrogen atom from the second energy level (n=2) to the first energy level (n=1). This transition emits a photon with a wavelength of approximately 121.6 nanometers (nm).
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The Lyman-alpha line is the hydrogen electric-dipole transition, with photon energy about and vacuum wavelength about angstroms. Its emission cools collisionally excited gas, while absorption traces neutral hydrogen along a background-source sightline.