Atoms inside a neutron star are so violently compressed that they cease to be atoms. If you squeezed all 8 billion humans to that density, they would fit inside a single sugar cube.
Atoms are almost entirely empty space. If the nucleus of a hydrogen atom were the size of a marble sitting on the 50-yard line of a football stadium, the single electron would be a speck of dust buzzing around the highest seats in the stands. This vast, empty void between the nucleus and the electron cloud is what gives ordinary matter its physical size.
When a massive star goes supernova, its core collapses inward. At first, the atomic structure tries to survive. Electrons push back against the crushing gravity, creating a phenomenon called electron degeneracy pressure. But in a neutron star, gravity wins. The inward pressure becomes so catastrophic that the electrons have nowhere left to go. They are forced violently into the atomic nucleus, where they collide with protons and merge to form neutrons.
The entire electron cloud—that vast, stadium-sized void—vanishes. What remains is a solid, ultra-dense sphere composed almost entirely of neutrons packed shoulder-to-shoulder, exactly like the particles inside a single atomic nucleus. In effect, a neutron star is one giant, city-sized atomic nucleus.
Because all the empty space has been deleted from the matter, the density becomes incomprehensible. A standard neutron star packs roughly 1.5 times the mass of our entire Sun into a sphere just 12 miles (20 kilometers) across.
If you could scoop out just one teaspoon of this material, it would weigh about a billion tons—roughly the weight of Mount Everest.