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 trasitions 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 6563Å (6.563×10−7m; see angstrom) 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.