Sizes and Densities
As we have seen,
neutron stars are extremely small and extremely dense. The adjacent image compares the size of
a neutron star with the size of a white dwarf, which is gigantic compared with the neutron star
but is itself more than a hundred times smaller
than a star like the Sun. The white dwarf on the left side is so much larger than the neutron
star that we can barely see the curvature of its surface on this scale.
Yet the white dwarf and the neutron star typically contain about a
solar mass of material. Thus, the densities of neutron stars are enormous
(see the right panel).
Densities of 1014-1015
g/cm3 are common in neutron stars.
This is a billion
times more dense than even white dwarf matter. As noted earlier, these incredibly high densities
result from the absence of electrical (Coulomb)
repulsion between neutrons. Normal matter is full of nuclei with
positive electrical charge. It is the electrical repulsion between protons that keeps the
density low and
normal
matter is mostly empty space. In a neutron star, the neutrons are free to pack closely together in
response to gravity, leading to matter having a density comparable to the densities in atomic nuclei.