A white dwarf is the former core of a star like the Sun, squeezed down to the size of Earth. A teaspoon of its material would weigh several tons. A neutron star is an even denser remnant: a city the size of Manhattan, but with more mass than the Sun. Both objects seem different, but there's a bridge between them: when a white dwarf gains mass, its matter gradually turns into neutron matter.
Physicists have explored this bridge with a unified model. Inside a white dwarf, atomic nuclei (e.g., helium, carbon, or oxygen) sit at the nodes of a crystalline lattice, like oranges in a crate. In a neutron star, the lattice disappears — nuclei merge into a uniform superdense fluid. The model traces how one state transitions into the other and computes the limiting masses. For helium white dwarfs, the limit is about 1.4 solar masses; for carbon, about 1.0.
Such a unified description, from white dwarf to neutron star, is needed to calculate mergers of these objects. When a white dwarf and a neutron star collide, gravitational waves and flashes of light are born. The unified model provides precise equations of state for matter, without which such a cataclysm cannot be simulated.
A white dwarf the size of Earth can be denser than a neutron star the size of a city: it all depends on how much mass it has managed to accrete from a neighboring star.