Hydrogen-alpha

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.

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,[1] 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.

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.

This combination will pass only a narrow (<0.1 nm) range of wavelengths of light centred on the H-alpha emission line.

Commercially available H-alpha filters for amateur solar observing usually state bandwidths in Angstrom units and are typically 0.7Å (0.07 nm).

In the Bohr model of the hydrogen atom, the electron transition from energy level to results in the emission of an H-alpha photon.
The four visible hydrogen emission spectrum lines in the Balmer series. The red line at far-right is H-alpha
The Sun observed through an optical telescope with an H-alpha filter
A Milky Way view by Wisconsin H-Alpha Mapper survey
An amateur image of NGC 6888 , using an H-alpha (3 nm) filter