A Balmer line is a hydrogen spectral line. The Balmer lines arise from transitions involving the second energy level and a higher level. The energies involved cause the Balmer lines to be in the optical spectrum. Balmer lines appear in a great variety of object, hydrogen being the most common chemical element in the universe. Balmer lines can appear in absorption or emission. In stars they appear in absorption, and they are "strongest" when radiating from a star with a temperature of 10,000K.
In nebulae such as H II regions, planetary nebulae,
and AGNs they appear in emission.
In the 1920s, Payne and Saha explained the relationship between the hydrogen spectrum and stellar temperature.