Atmosphere
Because of its density, the surface gravity of a white dwarf is very great. The force of this gravity causes the heavier atoms in its atmosphere to sink, while the lighter atoms remain at the surface. As a result, white dwarfs have atmospheres that are composed of hydrogen or helium, two of the lightest elements. This atmosphere forms a very thin layer above the surface of the star.
Core
The surface of the white dwarf itself is thought to be made up of a crust that is 50 kilometers thick. Underneath this crust is a crystalline lattice of carbon and oxygen atoms.
Unusual White Dwarfs
A very few white dwarfs, formed from larger stars, have atmospheres that are rich in oxygen. These white dwarfs have burned the carbon they once contained and the oxygen in their atmosphere is what remains of their cores.