In the core of a neutron star, the immense gravitational forces exerted by the star's mass cause profound effects on the neutrons and protons present. Neutron stars are incredibly dense, with masses several times that of our Sun compressed into a sphere only a few kilometers in diameter.
Under such extreme conditions, the gravitational force overwhelms the electromagnetic repulsion between protons, allowing them to come into close proximity. In this environment, protons can undergo a process called inverse beta decay or neutron capture. The gravitational pressure squeezes protons together, and one of the protons converts into a neutron by capturing an electron from the surroundings. This process is facilitated by the presence of a sea of free electrons in the core of a neutron star.
The conversion of protons to neutrons via neutron capture is crucial because neutrons do not experience electromagnetic repulsion and can exist in close proximity to each other. The neutrons, being electrically neutral, can coalesce and form a densely packed arrangement known as neutronium.
Neutronium is an exotic state of matter where neutrons are densely packed together, analogous to how atoms form a solid lattice in ordinary matter. Due to the incredibly strong gravity, the neutronium in the core of a neutron star is compressed to extreme densities, creating a state of matter not found in everyday life.
The process of fusion, as seen in the cores of stars like our Sun, does not typically occur in neutron stars. Fusion involves the combination of lighter atomic nuclei to form heavier ones, releasing energy in the process. However, in the core of a neutron star, the extreme densities and pressures inhibit the fusion of atomic nuclei.
Instead, the core of a neutron star is primarily composed of neutrons, with some protons and electrons interspersed. The gravitational force is responsible for maintaining the structural integrity of the neutron star, balancing the intense pressure created by the mass of the star against the outward push generated by neutron degeneracy pressure, a quantum mechanical effect that resists further compression of the neutrons.
It's important to note that the exact details of the behavior of matter in the core of neutron stars are still not fully understood, and there is ongoing research and theoretical work aimed at unraveling the intricacies of these extreme astrophysical objects.