The Lyman series is ultraviolet spectral lines (Lyman lines) of the hydrogen atom as an electron goes from state of excitation n = 2 or greater down to n = 1. It is named after Theodore Lyman. (As opposed, for example, to the Balmer series for n = 3 or greater down to n = 2.)
1/wavelength = RH ( 1 - 1 / n2 )
RH is the Rydberg constant for hydrogen, 1.09737316 × 107m-1 = 13.6eV / hc
A photon at or shorter than the Lyman limit can ionize any hydrogen. Photons from stars that are shorter than this limit are called Lyman continuum photons or LyC photons. Such photons are likely to ionize any neutral hydrogen in clouds they pass through, whether the hydrogen is excited or not. Cases where photons, e.g., from a galaxy, avoid such absorption (Lyman continuum escape) are of research interest.