What is a Black Dwarf?
A black dwarf is what a white dwarf becomes once it has radiated away the last of its heat: a cold, dark, planet-sized lump of degenerate matter emitting essentially nothing. No black dwarf exists anywhere in the universe, and none can for a very long time — the cooling takes something like 10^15 years, about a hundred thousand times the current age of the cosmos.
A white dwarf has no energy source. It cannot fuse, it cannot contract; all it can do is leak the heat it was born with into space. That process starts fast, at over 100,000 kelvin, and then slows to a crawl as the surface cools and the star radiates less. The coldest white dwarfs found so far, some of the oldest objects in the galaxy, are still around 3,000 K — nowhere near finished.
The endpoint is also a moving target. A black dwarf is defined as being in thermal equilibrium with its surroundings, but the surroundings are the cosmic microwave background, and the CMB is itself cooling as the universe expands. A cinder chasing a falling floor never quite lands. In practice the label means an object too cold to detect by anything but its gravity.
This is one of the few pages in the section describing something that has never been observed and never could be, yet. It is here because it is where the overwhelming majority of stars end up: over 97% of the stars in the Milky Way, the Sun included, are headed for a white dwarf and then, in deep time, for this. Speculative work has even suggested the most massive black dwarfs might eventually detonate, on a timescale with more than a thousand digits.