Final answer:
The remaining exposed core of a low-mass star after it has ejected its outer envelope as a red giant is called a white dwarf. This is a dense, hot stellar remnant that represents one of the possible end stages of stellar evolution, depending on the star's initial mass.
Step-by-step explanation:
When a low-mass star, such as our Sun, has exhausted the nuclear fuel in its core, it transitions into a red giant. As this red giant evolves, its outer layers are often expelled, creating a beautiful structure known as a planetary nebula. The core that remains after this ejection of the outer envelope is no longer capable of sustaining nuclear fusion and becomes an incredibly dense and hot remnant known as a white dwarf. This white dwarf will eventually cool and fade away over time as a black dwarf, but this takes many billions of years longer than the current age of the universe.
It is important to note that the final fate of a star depends heavily on its initial mass. Stars with core masses between 1.4 and about 3 solar masses (Msun) will evolve into neutron stars, while those with core masses greater than 3 Msun become black holes, marking the end of a star's life cycle through very different, much more violent processes than the formation of white dwarfs.