What do we call a star that has collapsed under gravity to the point where electrons and protons merge to form neutrons?

A star that has collapsed under gravity to the extent where electrons and protons combine to form neutrons is referred to as a neutron star. This process occurs during the later stages of a massive star’s life cycle. When the core of the star runs out of nuclear fuel, it can no longer support itself against gravitational collapse. The extreme pressure and temperature lead to neutronization, effectively transforming the remnants of the stellar core into neutrons.

Neutron stars are incredibly dense, with a mass greater than that of the Sun concentrated in a small volume. This remarkable density arises from the forces at work in a supernova explosion, where the outer layers of the star are expelled, and the core compresses under its own gravity, resulting in the formation of a star primarily composed of neutrons.

The other options refer to different stellar phenomena: a white dwarf is the end stage of a medium-sized star, a red supergiant is a later stage in the life of a massive star before it goes supernova, and a black hole is the result of an even more extreme collapse, where the gravitational pull is so strong that not even light can escape. Each of these options represents unique characteristics of stellar life cycles, distinctly different from a neutron star.