A white dwarf is what remains after a star like the sun exhausts its nuclear fuel and sheds its outer layers. What is left behind is the hot, compact core, roughly the size of Earth but carrying a mass close to that of the entire original star.
A white dwarf no longer generates energy through fusion. Instead it is held up against gravity by electron degeneracy pressure, a quantum effect that keeps electrons from being squeezed any closer together, and it simply cools and dims over billions of years rather than collapsing further.
If a white dwarf sits in a binary system, it can pull in gas from its companion star and grow heavier over time. Should it approach the Chandrasekhar limit, that slow accumulation can end in a sudden thermonuclear explosion instead of a quiet fade. More detail is available from NASA’s Imagine the Universe.
The first white dwarf ever identified was Sirius B, the faint companion to the night sky’s brightest star, whose unusually high density puzzled astronomers when it was properly measured in the early twentieth century, since nothing in classical physics explained how so much mass could be packed into so small a volume. Left alone, a white dwarf simply radiates its leftover heat away into space over an extraordinarily long timescale, eventually cooling into a cold, dark object sometimes called a black dwarf. No black dwarfs are thought to actually exist yet anywhere in the universe, since the cooling process takes far longer than the roughly 13.8 billion years the universe has existed so far, meaning even the very first white dwarfs ever formed have not had nearly enough time to finish fading out completely.
A single teaspoon of white dwarf material would weigh close to a ton, roughly the weight of a small car packed into a space you could hold in your hand. That density comes directly from packing a star’s worth of mass into a sphere only about the size of the Earth, held up not by fusion but purely by the quantum pressure of tightly packed electrons.