What is the final stage of a white dwarf star believed to be?

The final stage of a white dwarf star is understood to be a black dwarf. A white dwarf is the remnant core of a star that has exhausted its nuclear fuel and shed its outer layers, resulting in a hot, dense core primarily composed of carbon and oxygen. Over time, as it radiates its heat into space and cools down, it will eventually no longer emit detectable light or heat, transitioning into what is termed a black dwarf.

This stage represents a theoretical end point for white dwarfs, as the universe is not old enough for any black dwarfs to yet exist, since it would take longer than the current age of the universe for a white dwarf to cool sufficiently to become a black dwarf.

In contrast, other choices represent different stages of stellar evolution or different types of celestial objects. A black star is not a recognized classification in the lifecycle of stars, while red giants are phases in the evolution of stars before they become white dwarfs. A neutron star is a remnant left after a supernova explosion of a massive star, quite distinct from the pathway white dwarfs take.