The spectral classification of white dwarf stars. The original sequence used was DB, DA, DF, DG, and DK, in order of declining effective temperature from 100 000 to 4000 K (i.e. the second letter follows the normal sequence of spectral types, B to K). White dwarfs with a continuous (featureless) spectrum were classified as type DC. Subsequently, types DO (extremely hot, showing only ionized helium lines) and DQ (containing atomic or molecular carbon in addition to their mainly helium atmospheres) were added, and type DZ was introduced to combine the former types DF, DG, and DK. Also, a temperature index was added from 0 (hottest) to 9 (coolest). In the fully developed scheme, a DC9 white dwarf, for example, is very cool and contains no detectable absorption lines. The majority of white dwarfs are type DA (as is Sirius B), showing only broad absorption lines of hydrogen, their breadth resulting from the star’s enormous surface gravity. Most of the rest are type DB, which show only neutral helium lines. The metal lines in type DZ are thought to be caused by material accreted by the white dwarf from interstellar space. DQZ is a rare type with molecular carbon bands as well as metal lines; Procyon B is an example.