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A marked intensity drop at a wavelength of about 365 nanometres in the continuous spectrum of a star (or other astronomical object) in which absorption by hydrogen occurs.
The individual Balmer lines of atomic hydrogen become closer with decreasing wavelength until they merge.
The wavelength of 365 nanometres corresponds to the energy required to ionize a hydrogen atom when the electron is originally in the second energy level of the atom. At shorter wavelengths (higher energies) the hydrogen effectively causes continuous absorption.
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