Final answer:
The process of core collapse at the end of the life of a massive star takes less than a second. The core shrinks to a diameter of less than 20 kilometers and reaches a density higher than that of an atomic nucleus. This leads to the formation of a neutron star.
Step-by-step explanation:
The process of core collapse at the end of the life of a massive star takes less than a second. During this collapse, the core of the star, which originally had a mass of about 1 Msun and a size similar to Earth, shrinks to a diameter of less than 20 kilometers. The material falls inward at a speed reaching one-fourth the speed of light. The collapse only stops when the density of the core exceeds the density of an atomic nucleus, which is the densest form of matter known.
Once the core reaches this extreme density, it can no longer collapse further, and a new and different way for matter to behave kicks in, leading to the formation of a neutron star. A neutron star is so compressed that to duplicate its density, all the people in the world would need to be squeezed into a single sugar cube.