H-alpha is a specific wavelength of light that is emitted by hydrogen atoms when their electrons transition from the third energy level to the second energy level. This transition produces light at a wavelength of 656.28 nanometers, which falls within the red part of the visible spectrum.
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The hydrogen Balmer emission line from the transition between principal quantum numbers three and two. Recombination-line emission from gas photoionized by short-lived massive stars makes it a star-formation tracer, subject to dust attenuation, escape of ionizing photons and contamination by other excitation sources.