DA – a hydrogen-rich atmosphere or outer layer, indicated by strong Balmer hydrogen spectral lines. DB – a helium-rich atmosphere, indicated by neutral helium, He I, spectral lines. DO – a helium-rich atmosphere, indicated by ionized helium, He II, spectral lines. DQ – a carbon-rich atmosphere, indicated by atomic or molecular carbon lines. DZ – a metal-rich atmosphere, indicated by metal spectral lines (a merger of the obsolete white dwarf spectral types, DG, DK, and DM). DC – no strong spectral lines indicating one of the above categories. DX – spectral lines are insufficiently clear to classify into one of the above categories. The type is followed by a number giving the white dwarf's surface temperature. This number is a rounded form of 50400/Teff, where Teff is the effective surface temperature, measured in kelvins. Originally, this number was rounded to one of the digits 1 through 9, but more recently fractional values have started to be used, as well as values below 1 and above 9.(For example DA1.5 for IK Pegasi B) Two or more of the type letters may be used to indicate a white dwarf that displays more than one of the spectral features above. Extended white dwarf spectral types DAB – a hydrogen- and helium-rich white dwarf displaying neutral helium lines DAO – a hydrogen- and helium-rich white dwarf displaying ionized helium lines DAZ – a hydrogen-rich metallic white dwarf DBZ – a helium-rich metallic white dwarf A different set of spectral peculiarity symbols are used for white dwarfs than for other types of stars: Non-stellar spectral types: Classes P and Q Finally, the classes P and Q are left over from the system developed by Cannon for the Henry Draper Catalogue. They are occasionally used for certain non-stellar objects: Type P objects are stars within planetary nebulae (typically young white dwarfs or hydrogen-poor M giants); type Q objects are novae. Stellar remnants Stellar remnants are objects associated with the death of stars. Included in the category are white dwarfs, and as can be seen from the radically different classification scheme for class D, non-stellar objects are difficult to fit into the MK system. The Hertzsprung–Russell diagram, which the MK system is based on, is observational in nature so these remnants cannot easily be plotted on the diagram, or cannot be placed at all. Old neutron stars are relatively small and cold, and would fall on the