Astronomy Practice Exam

What is the term for a white dwarf that has ceased to emit light?

1. Black dwarf

2. Dim star

3. Red giant

4. Neutron star

The correct answer is: Black dwarf

The correct term for a white dwarf that has ceased to emit light is known as a black dwarf. After a white dwarf forms from the remnants of a star that has exhausted its nuclear fuel, it initially emits light due to residual heat. However, over an extremely long period, it continues to cool down and eventually becomes colder and dimmer. Once it has cooled sufficiently that it no longer emits detectable light or significant heat, it is classified as a black dwarf. This phenomenon is theoretical because the universe is not old enough for any white dwarfs to have completely cooled into black dwarfs yet; they would take longer than the current age of the universe to reach this state. Other terms listed do not align with the context of a white dwarf. The term "dim star" does not specifically refer to a type of stellar remnant, while "red giant" describes a different stage of stellar evolution characterized by the expansion and cooling of a star that has exhausted the hydrogen in its core. A neutron star, on the other hand, represents an entirely different end state of a more massive star, resulting from a supernova explosion, and is not related to the evolution of white dwarfs.