H-alpha
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In physics and astronomy, H-alpha, also written Hα, is a particular emission line created by hydrogen.
According to the Bohr model of the atom, electrons exist in quantized energy levels surrounding the atom's nucleus. These energy levels are described by the angular momentum quantum number, n = 1, 2, 3, ... . Electrons may only exist in these states, and may only transition between these states. The set of transitions from n ≥ 3 to n = 2 are called the Balmer series and are named sequentially by Greek letter: n = 3 to n = 2 is called H-alpha, 4 to 2 is H-beta, 5 to 2 is H-gamma, etc. For the Lyman series the naming convention is 2 to 1 is Lyman-alpha, 3 to 1 is Lyman-beta, etc.
H-alpha has a wavelength of 656.3 nm, is visible in the red part of the electromagnetic spectrum, and is the easiest way for astronomers to trace the Hydrogen content of gas clouds.
See also: Bohr model, Rydberg formula, Balmer series, Lyman series, Paschen series, Bracket series