The question was raised, are neutron stars made up mostly of neutrons,
or do hyperons (like lambda's and sigma's) also appear, since
(conceivably) they may be energetically favored?
The issue of the equation of state of neutron star matter is discussed
at some length in Harrison, Thorne, Wakano, and Wheeler's U. of Chicago
Press book "Gravitation Theory and Gravitational Collapse" back in
1965. Roughly, the mix of hadrons (strongly interacting particles,
i.e., nucleons and hyperons) is about one proton to eight neutrons,
with TINY admixtures of hyperons from the exponential tail of Fermi
distributions. The governing principle is that the Fermi energy of the
protons plus the Fermi energy of the electrons must equal the Fermi
energy of the neutrons (i.e., there is a "chemical" equilibrium between
protons combining with electrons to make neutrons {and spitting out
e-neutrinos} VERSUS neutrons decaying into protons, electrons, and
anti-e-neutrinos). The energy difference between neutrons and protons
versus lambda's and sigma's is a few hundred electron masses --
supplying that would require a VERY energetic (high temperature)
gamma-radiation field in equilibrium with the neutron star matter. (Xi
hyperons are yet a few hundred electron masses still higher.)
[Gee, I haven't thought about the baryon octet for about 25 years! I
feel young again!]
Peter Vajk
St. Joseph Notre Dame High School
Alameda, CA 94501