<P> The class D (for Degenerate) is the modern classification used for white dwarfs--low - mass stars that are no longer undergoing nuclear fusion and have shrunk to planetary size, slowly cooling down . Class D is further divided into spectral types DA, DB, DC, DO, DQ, DX, and DZ . The letters are not related to the letters used in the classification of other stars, but instead indicate the composition of the white dwarf's visible outer layer or atmosphere . </P> <P> The white dwarf types are as follows: </P> <Ul> <Li> DA: a hydrogen - rich atmosphere or outer layer, indicated by strong Balmer hydrogen spectral lines . </Li> <Li> DB: a helium - rich atmosphere, indicated by neutral helium, He I, spectral lines . </Li> <Li> DO: a helium - rich atmosphere, indicated by ionized helium, He II, spectral lines . </Li> <Li> DQ: a carbon - rich atmosphere, indicated by atomic or molecular carbon lines . </Li> <Li> DZ: a metal - rich atmosphere, indicated by metal spectral lines (a merger of the obsolete white dwarf spectral types, DG, DK and DM). </Li> <Li> DC: no strong spectral lines indicating one of the above categories . </Li> <Li> DX: spectral lines are insufficiently clear to classify into one of the above categories . </Li> </Ul> <Li> DA: a hydrogen - rich atmosphere or outer layer, indicated by strong Balmer hydrogen spectral lines . </Li>

Which of the following luminosity classes refers to stars on the main sequence