Which term describes a massive star that has ended nuclear fusion at its core?

The term that accurately describes a massive star that has ended nuclear fusion at its core is a black hole. When a massive star exhausts the fuel for nuclear fusion in its core, the lack of pressure from fusion causes the core to collapse under its own gravity. This collapse can lead to the formation of a black hole if the remaining mass is sufficient. Black holes are regions in space where the gravitational pull is so strong that not even light can escape.

In the lifecycle of a massive star, after exhausting its nuclear fuel, it may first become a red supergiant, and during its final stages, it could produce a supernova explosion. Depending on the mass of the core that remains after the explosion, it could either become a neutron star or collapse further into a black hole. Thus, while a neutron star is also a result of stellar evolution, it represents a different endpoint than a black hole, which specifically denotes a more extreme condition following the end of fusion in a very massive star.