What is the temperature range of a black dwarf star?

The hypothetical result of a white dwarf losing all of its energy is a black dwarf, which has zero energy and a temperature of two to three degrees K, the same as space. Since no energy is being produced—white dwarfs no longer generate heat; instead, they gradually lose it into space—the range is also zero.

Given that the universe is only 14 billion years old and that it will take a star over a trillion years to cool completely, the existence of black dwarfs is entirely speculative. The oldest known white dwarfs are approximately 11 billion years old and remain approximately 3000 degrees.

Since the universe is too young for white dwarfs to evolve into black dwarfs, black dwarf stars cannot exist today. According to theoretical models, black dwarfs would have a temperature that is very close to absolute zero over a very long timescale.