The physics of compact stars is in itself extremely interesting.
The features of these objects (forming during and following a supernova
explosion) strongly dependent on the equation-of-state (EOS) adopted
to describe the core. Its exact determination remains a formidable
theoretical problem. There is still no general agreement on the exact
composition of dense matter, and on its EOS, especially for densities
in excess several times nuclear matter density. Although EOSs could
be provided by high energy Nucleon-Nucleon scattering experiments, one
has to extrapolate the results to extreme conditions of high density
and high neutron-proton asymmetry, i.e. in a regime where the EOS is
poorly constrained by nuclear data and experiments.
Neutrinos play an important role in the collapse of massive stars
and in the evolution of neutron stars, which cool to temperatures in
the MeV range in a time of order minutes. Thus a detailed knowledge
of neutrino generation and scattering processes in dense matter with
temperatures up to those at which neutron stars are formed, 20-50 MeV,
is crucial for understanding the supernova phenomenon and how neutron
stars cool. The problems are challenging ones since they require insights
from nuclear, particle, and condensed matter physics, as well as from
astrophysics. C. Pethick
has made important contributions to this subject,
most recently by demonstrating that creation of
neutrino pairs by bremsstrahlung from electrons scattered by nuclei in
the crust of a neutron star is strongly affected by the band structure
of electrons due to the lattice of nuclei. Analysing observations of
cooling neutron stars in the light of knowledge of neutrino processes
holds the promise of putting constraints on the composition of matter
in the interiors of neutron stars.
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