Hydrogen-alpha

Hydrogen-alpha, typically shortened to H-alpha or Hα, is a deep-red visible spectral line of the hydrogen atom with a wavelength of 656.28 nm in air and 656.46 nm in vacuum. It is the first spectral line in the Balmer series and is emitted when an electron falls from a hydrogen atom's third- to second-lowest energy level. H-alpha has applications in astronomy where its emission can be observed from emission nebulae and from features in the Sun's atmosphere, including solar prominences and the chromosphere.

Balmer series

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 principal quantum number n = 1, 2, 3, ... . Electrons may only exist in these states, and may only transit between these states.

The set of transitions from n ≥ 3 to n = 2 is called the Balmer series and its members are named sequentially by Greek letters:

n = 3 to n = 2 is called Balmer-alpha or H-alpha,

n = 4 to n = 2 is called Balmer-beta or H-beta,

n = 5 to n = 2 is called Balmer-gamma or H-gamma, etc.

For the Lyman series the naming convention is:

n = 2 to n = 1 is called Lyman-alpha,

n = 3 to n = 1 is called Lyman-beta, etc.

H-alpha has a wavelength of 656.281 nm, is visible in the red part of the electromagnetic spectrum, and is the easiest way for astronomers to trace the ionized hydrogen content of gas clouds. Since it takes nearly as much energy to excite the hydrogen atom's electron from n = 1 to n = 3 (12.1 eV, via the Rydberg formula) as it does to ionize the hydrogen atom (13.6 eV), ionization is far more probable than excitation to the n = 3 level.