A white dwarf is a:
low-mass star at the end of its life.
Explanation:
A white dwarf is the remnant of a low-mass star that has exhausted the nuclear fuel in its core. After such a star has gone through its life cycle, including the red giant phase, it sheds its outer layers, leaving behind a dense core that no longer undergoes fusion reactions. This core is what we call a white dwarf. It is extremely dense and hot initially, but it gradually cools and fades over time.
- Hot, main-sequence star: This does not describe a white dwarf, as main-sequence stars are still in the active phase of nuclear fusion.
- Type of protostar: This is incorrect; a protostar is an early stage in the formation of a star, not a final stage like a white dwarf.
- Object like Jupiter that was not massive enough to become a star: This describes a brown dwarf, not a white dwarf.
Thus, the correct choice is that a white dwarf is a low-mass star at the end of its life.