Nuclear matter is an idealized system of interacting
nucleons (protons and neutrons) that exists
in several phases that as yet are not fully
established.
It is not matter in a nucleus, but a hypothetical
substance consisting of a huge number of protons
and neutrons interacting by only nuclear forces
and no Coulomb forces.
Volume and the number of particles are infinite,
but the ratio is finite.
Infinite volume implies no surface effects
and translational invariance (only differences
in position matter, not absolute positions).
A common idealization is symmetric nuclear
matter, which consists of equal numbers of
protons and neutrons, with no electrons.
When nuclear matter is compressed to sufficiently
high density, it is expected, on the basis
of the asymptotic freedom of Quantum chromodynamics,
that it will become quark matter, which is
a degenerate Fermi gas of quarks.
Some authors use "nuclear matter" in a broader
sense, and refer to the model described above
as "infinite nuclear matter", and consider
it as a "toy model", a testing ground for
analytical techniques.
However, the composition of a neutron star,
which requires more than neutrons and protons,
is not necessarily locally charge neutral,
and does not exhibit translation invariance,
often is differently referred to, for example,
as neutron star matter or stellar matter and
is considered distinct from nuclear matter.
In a neutron star, pressure rises from zero
(at the surface) to an unknown large value
in the center.
Methods capable of treating finite regions
have been applied to stars and to atomic nuclei.
One such model for finite nuclei is the liquid
drop model, which includes surface effects
and Coulomb interactions.
== See also ==
QCD vacuum
Quark–gluon plasma
Degenerate matter
Neutron-degenerate matter
Strange matter
Nuclear structure
Neutronium
Nuclear physics
