It is currently believed that the universe is not old enough for any white dwarfs to have cooled to black dwarf status. However, saying this conclusively is difficult, since black dwarfs would be exceptionally hard to detect. They do not emit radiation, so to detect them you'd have to look for their gravitational affects.
Black dwarfs, like white dwarfs, would be exceptionally dense, being composed of electron-degenerate matter. In normal matter, atoms are mostly empty space between the nucleus and electrons. In electron-degenerate matter, the electrons are compressed into the nucleus.
Game and Story Use
- Since they are not supposed to exist yet, a black dwarf's discovery might overturn existing theories - unless it proves to be a Precursor artifact of some type.
- Particularly nasty Eldritch Abominations might leave black dwarfs behind after eating most of a star or absorbing its energy.
- In this case, the discovery of black dwarfs, once their real cause was realized, might lead to a mass evacuation of planets in the star-eaters' path. A campaign might center around a planet whose star is being eaten: the PCs might be sent to find a new planet to house the doomed world's population, or to protect the evacuation against external threats (either human ones like piracy or lesser Eldritch Abomination aliens).
- Alternately, such star-eaters could prove an appropriate enemy for campaigns set in very advanced civilizations. These would be suitable foes for Dyson Sphere builders!
- A RAMBOs cluster of black dwarf stars and their pocked and battered planetoid orbitals serves as home to a horrible race of aggressive aliens. Their initial attacks are devastating and come without warning, and no one had ever even noticed that bleak black dwarf star there before. Their technology is well beyond what the PCs are used to. How could this alien culture have developed in isolation in such a resource-poor cold and lifeless system? Well, it turns out they're actually from the future! In the distant future the universe itself is cooling off, and most stars can no longer support life. These new aggressive aliens are from such a bleak future, and have made a Tipler Cylinder or other massive Portal To The Past to escape. This burned out star they brought with them through the portal.
- A particularly potent stellar engine powering some massive interstellar megastructure might have a black dwarf at its center. Once a bright and vibrant main sequence star, it was burned out and drained off by the nature of the machine built around it. But what could possibly require so much power that it sucks stars of their fuel and energy? The GM is encouraged to think big.
- A campaign set in the remotest far future, trillions of years from now, could feature black dwarf stars all over the map. With that much time for evolution and technology to do their thing, you can expect sufficiently advanced technology to feature prominently. This could be a high-fantasy "galactic end times" kind of game, sort of Mad Max meets Spelljammer. There'd be a high contrast between the amazing technology and the bleak scarcity of resources.
- For ideas on how a black dwarf star might serve as a navigational hazard, see MACHOs.