As the star's envelope expands, we see in to a hotter part of the star, and it moves to the left in the HR diagram. The hot core lights up the outer envelope, turning it into a spectacular, glowing planetary nebula . We are left with the former red giant's hot, dense core, mainly carbon and oxygen, perhaps with some helium and hydrogen near the surface. This white dwarf is supported by degenerate electron pressure, so it doesn't contract even though it radiates heat. The surface cools to a few thousand degrees over about 10 billion years.
White dwarfs are extremely dense.
A strange feature of degenerate electron pressure is that a more massive white dwarf is smaller than a less massive white dwarf. Its electrons must therefore move faster to provide the necessary pressure support. 1.4 M sun cannot exist. The most massive white dwarf known is about 1 M sun , and most are about 0.7 M sun , in agreement with Chandrasekhar's theoretical deduction. A very massive star, which would produce a carbon core more massive than 1.4 M sun , must have a different fate.