The helium absorption lines seen in the spectra of DB white dwarfs are formed by excited He I atoms with one electron in the lowest (n=1) orbital and the other in an n=2 orbital. White dwarfs of spectral type DB are not observed with temperatures below about 11,000 K. Using what you know about spectral line formation, give a qualitative explanation why the helium lines would not be seen at lower temperatures. As a DB white dwarf cools below 12,000 K, into what spectral type does it change?