1973: On 6 February 1973, Ira Sprague Bowen died.
Bowen was an American astrophysicist. His investigation of the ultraviolet spectra of highly ionised atoms led to his explanation of the unidentified strong green spectral lines of gaseous nebulae as 'forbidden lines' of ionised oxygen and nitrogen.
This emission, appearing to match no known element, had formerly been suggested to be due to a hypothetical element, 'nebulium.' Bowen was able to show, that in reality, the emission lines exactly matched those calculated to be the 'forbidden lines' of ionised oxygen and nitrogen under extremely low pressure. This made a major advance in the knowledge of celestial composition.