Subatomic Physics

19.4. Stellar Collapse and Neutron Stars 595
occur, as in the formation process of the neutron star in the pre-supernova. At
the increased pressure, more neutron rich nuclei are formed; the electron capture
processes continue and at about 4 × 10 11 g/cm 3 , nuclei with 82 neutrons, such
as 118 Kr, are most stable. (35,38) Ordinary krypton on Earth has A = 84. The
most stable nuclides at high pressure thus are very neutron-rich. Under ordinary
circumstances, such nuclides would decay by electron emission. However, at the
pressure under discussion here, all available energy levels are already occupied by
electrons and the Pauli principle prevents simple beta decay.
The last neutron in 118 Kr is barely bound. As the density increases beyond
4 × 10 11 g/cm 3 , the neutrons begin to leak out of the nuclei and form a degenerate
liquid. As the pressure increases further, the nuclei in this neutron drip regime
become more neutron rich and grow in size. At a density of about 2.5 ×10 14 g/cm 3 ,
they essentially touch, merge together, and form a continuous fluid of neutrons,
protons, and electrons. Neutrons predominate and protons and electrons constitute
only about 5% of the matter. Neutrons cannot decay to protons by simple beta
decay because the decay electron would have an energy below the electron Fermi
energy; the decay is thus forbidden by the Pauli principle.
Figure 19.8: Composition of neutron-star matter as a function
of the density. At higher densities, muons and strange
particles appear. [Courtesy M. Ruderman.]
At still higher densities, it becomes
energetically feasible to
create more massive elementary
particles through electron
captures such as
e − n −→ νΣ − ;
these particles can again be
stable because of the exclusion
principle. (36) The number of
constituents of matter as a
function of density is shown in
Fig. 19.8. (37)
We now turn again to the internal pressure in a neutron star. We have seen above
that the degenerate electron gas provides pressure that prevents collapse at lower
pressures. At higher pressure (or densities), complete collapse is prevented by a combination
of two features, the repulsive core in the nucleon–nucleon force (Fig. 14.15),
and the degeneracy energy of the neutrons. Fig. 19.8 indicates that neutrons pre-
35 G. Baym, C. Pethick, and P. Sutherland, Astrophys. J. 170, 299 (1971).
36 V.R. Pandharipande, Nucl. Phys. A178, 123 (1971).
37 For an updated version of the composition at densities larger than nuclear see T.Takatsuka
and R. Tamagaki, Prog. Theor. Phys. 112, 37 (2004).