An Etymological Dictionary of Astronomy and Astrophysics

A mechanism, made possible by certain chance coincidences between → spectral lines of He II, O III and N III in some → planetary nebulae ,
that explains the presence with a high intensity of a selected group of O III and N III lines
while all other lines of these elements are missing.

See also: After I. S. Bowen who first discovered this mechanism in 1935; → fluorescence; → mechanism.