In certain neutron stars the shedding of angular momentum and energy over time causes the star to contract. As the star's core density increases, some neutrons can "melt," creating an environment in which normally unstable particles such as hyperons and strange quarks can survive indefinitely and can even come to predominate in more and more of the star (a, b, c). An estimated 1% of all neutron stars are undergoing this sort of transition. (Illustration by Malcolm Tarlton, American Institute of Physics.)
This topic is described by Norman K. Glendenning, S. Pei, and F. Weber in the 1 September 1997 issue of Physical Review Letters.
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