What characterizes a black dwarf star?

A black dwarf star is characterized as a cooled white dwarf that emits no detectable light. After a star like our Sun exhausts its nuclear fuel, it goes through a red giant phase and sheds its outer layers, leaving behind a white dwarf. This white dwarf initially radiates heat and light due to the residual thermal energy from when it was still undergoing nuclear fusion. However, over billions of years, it will continue to cool and lose energy. Eventually, it will reach a temperature where it no longer emits light that can be detected, thus becoming a black dwarf.

To clarify the context around the other options, a star with significant luminosity (the first choice) does not describe a black dwarf, as black dwarfs are no longer luminous. The second choice indicates a white dwarf that emits detectable light, which describes a stage before a star becomes a black dwarf. Finally, the fourth option describes a massive star in the main sequence, which is unrelated to the lifecycle of white dwarfs and black dwarfs. In summary, the defining characteristic of a black dwarf is its complete inability to emit detectable light, resulting from the cooling process of a white dwarf.