Which of the following is a characteristic of a black dwarf?

A black dwarf is theorized to be a cold stellar remnant that results when a white dwarf has cooled sufficiently that it no longer emits significant heat or light. Before reaching this state, a white dwarf is composed largely of electron-degenerate matter, and it radiates energy as it cools over billions of years. When it has cooled to the point where it no longer shines, it becomes a black dwarf.

This concept illustrates that a black dwarf is a white dwarf that has exhausted its thermal energy and is effectively "dead" in terms of stellar activity, merging the characteristics of fading luminosity with an eventual state of minimal temperature. Black dwarfs are not yet observed in the universe since the time taken for a white dwarf to reach this stage exceeds the current age of the universe.

Other options do not correctly define a black dwarf. The final stage of massive stars refers to supernova remnants or neutron stars rather than a black dwarf. The term "nebulae" pertains to regions where star formation occurs, which is distinct from the life cycle stage represented by a black dwarf. Therefore, the characteristic of a black dwarf being a white dwarf that has run out of light encapsulates its definition accurately.