Complex sentence: They are characterized by the ionized lines of nitrogen, carbon or oxygen and typically show strong broad emission lines in their spectra, mostly with helium, nitrogen, carbon, silicon, and oxygen, but with hydrogen lines usually weak or absent.
Split into simpler sentences: They are characterized by the ionized lines of nitrogen, carbon or oxygen. They typically show strong broad emission lines in their spectra, mostly with helium, nitrogen, carbon, silicon, and oxygen, but with hydrogen lines usually weak or absent.