Early Days of Planetary Nebular Theory

The spectra of gaseous nebulae differed strikingly from those of stars which were well understood since 1922 thanks to the work of Saha. Gaseous nebulae exhibited exotic bright line spectra characterized by strange emissions of unknown origin as well as familiar lines of hydrogen and helium. The strongest lines in most nebulae fell at 4959A and 5007A. They were originally attributed to an unknown element. First came the interpretation of the lines of H.