What is a white dwarf?
A red giant starts shrinking when its stock of hydrogen is nearing exhaustion. At a certain stage, the star becomes so dense that ever its gravity cannot matter in which shrink it any more. White dwarfs consist of degenerate matter in which gravity has packed the protons and electrons together as tightly as is physically possible so that a spoonful of it, weighs several tonnes. At this stage, the star becomes stable again though it is much smaller in size than it was at the red giant stage. The star is now called a white dwarf.
How does a white dwarf shine?
A white dwarf shines mainly by radiating away its stored internal energy, besides the fusion of any residual helium into other heavier elements. It slowly cools and darkness over millions of years.
What is a black dwarf?
A white dwarf shines by radiating its internal energy. When the internal energy is completely exhausted the star ceases to shine and becomes a cold, dark body called a black dwarf.