What process creates the characteristic red light of the $ ext{H} ext{alpha}$ line at $656.3 ext{ nm}$?
Electron recombination with protons after hydrogen ionization.
The $ ext{H} ext{alpha}$ ($ ext{H} ext{ extalpha}$) optical emission line is generated by the intense output of nearby massive, hot stars. These stars emit copious amounts of high-energy photons in the ultraviolet spectrum. These energetic photons are powerful enough to ionize the surrounding hydrogen gas clouds—meaning they strip electrons away from the hydrogen atoms, creating free protons. When these liberated electrons eventually recombine with the protons, they transition through various energy levels, ultimately emitting light at a specific wavelength of $656.3 ext{ nm}$, which is observed as the characteristic red light associated with the $ ext{H} ext{ extalpha}$ spectral feature.
