H-alpha

H-alpha (Hα) is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28 nm; it occurs when a hydrogen electron falls from its third to second lowest energy level. H-alpha light is important to astronomers as it is emitted by many emission nebulae and can be used to observe features in the Sun's atmosphere, including solar prominences.

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:

For the Lyman series the naming convention is:

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 as it does to ionize the hydrogen atom, the probability of the electron being excited to n = 3 without being removed from the atom is very small. Instead, after being ionized, the electron and proton recombine to form a new hydrogen atom. In the new atom, the electron may begin in any energy level, and subsequently cascades to the ground state (n = 1), emitting photons with each transition. Approximately half the time, this cascade will include the n = 3 to n = 2 transition and the atom will emit H-alpha light. Therefore, the H-alpha line occurs where hydrogen is being ionized.

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