Final answer:
The density of a collapsing high-mass star's core before a supernova explosion is similar to that of an atomic nucleus, which is extremely high. A neutron star formed from such a collapse is so dense that human analogies like compressing all humans into a sugar cube are used to convey its compactness. These events occur when the core collapses due to gravitational forces exceeding any known counteracting pressures.
Step-by-step explanation:
Understanding the Density of a Neutron Star
In the moments before a supernova explosion, the core of a high-mass star undergoes a dramatic collapse. As the core contracts, its density surges to extremities greater than any typical matter we encounter. When this collapse is halted by a quantum mechanical pressure from degenerate neutrons, the core reaches densities comparable to that of an atomic nucleus, regarded as the densest form of known matter. A neutron star emerges from this process, embodying a state of matter so compact that likening its density to human comprehension requires fantastical analogies - for instance, compressing the entire human population into the volume of a raindrop.
The densities reached during this collapse are indeed immense. Just before the ultimate explosion - the supernova - the core's density can approximate a million times the density of water, or about 200,000 times the average density of Earth. A typical neutron star, which is a possible outcome of this collapse, is so dense that to match its density we would have to condense all humans on Earth into the space of a single sugar cube.
This extraordinary phenomenon occurs due to gravitational collapse when the fuel for nuclear fusion is depleted in a high-mass star. Electrons are absorbed into nuclei, the material of the star falls inward at substantial fractions of the speed of light, and the core ultimately reaches the size of mere kilometers across with masses up to around 3 solar masses (Msun). If the core's mass exceeds this, it may collapse further, potentially forming a black hole.