A possible site where large amounts of strangeness-bearing matter
may be found is in the interior of a neutron star. Relative to matter involving
only nucleons, properties of neutron stars such as their masses and radii
are affected by the presence of strangeness (hyperons, or a K- condensate,
or strange quarks [22,23]).
Furthermore, the thermal and structural evolution of a neutron star depends
sensitively on the strangeness baryon content of its interior. Some examples
of phenomena that occur due to the presence of strangeness are listed below.
Typically, neutron stars with kaon condensates or strange quark
matter in the star's core have substantially smaller radii, and maximum
masses, than those without these exotic components. However, the addition
of strangeness through hyperons, although lowering the maximum mass, does
not greatly reduce the stellar radii .
There are no direct measurements of the radius or mass of any neutron star
outside a binary system. The most promising possibility for a firm radius
measurement is the isolated neutron star RX J185635-3754 .
Metastable neutron stars are possible with strangeness-bearing matter
and allow the formation of a low-mass black hole during the deleptonization
stage of a protoneutron star [23,26,27].
This may be confirmed by the abrupt cessation of neutrino emission during
the first 10-20 seconds of the evolution of a protoneutron star.
The long-term, up to 106 years, thermal evolution of a neutron star
is also affected by strangeness-bearing components [28,29].
Multi-wavelength photon observations of neutron stars (ROSAT, HST, AXAF,
and XMM) have been planned to elucidate the thermal histories of neutron
stars. Laboratory studies such as phase-shift analyses of nucleon-hyperon,
nucleon-meson and hyperon-hyperon interactions are crucial both for many-body
calculations of dense matter and for studies of superfluidity and superconductivity.
NASA is actively supporting both theoretical and observational efforts
under it's Astrophysical Theory and Long-Term-Space-Astrophysics Programs
in order to unravel the properties of matter under extreme conditions of
density and temperature through observations of neutron stars. The NSF
and DOE are also actively supporting theoretical studies of the role of
strangeness in the equation of state in nuclear astrophysics and in relativistic
heavy ion collisions. Strong support of laboratory studies aimed at the
determination of hyperon and kaon interactions both in free space and in
nuclei is therefore extremely important.