A self-gravitating object which is supported by neutron-degeneracy Pressure is Known as a NEUTRON STAR; they were investigated Theoretically long before they were discovered in space. Their properties are analogous to those of white dwarfs except that their radii are smaller by a factor of about 1840 (the ratio of neutron mass to electron mass). Thus a one solar-mass neutron star has the tiny radius of only 10km. Similarly, we expect an upper limit to the mass of a neutron star, analogous to the Chandrasekhar limit for a white dwarf. However, when the density increases to such an extent (about 10l8kgm-3) that the neutrons are moving at relativistic velocities, collisions between the particles lead to the formation of other elementary particles. In addition, although neutron degeneracy pressure is important, the density is so high that nuclear interactions cannot be neglected, and the equation of state is more complicated than we have postulated. Therefore, although an upper limit to the mass of a neutron star is expected, it is not easy to calculate, but is estimated to be in the region of 1.5-3Mo.

The structure of a neutron star more closely resembles a planet than a star, but on a smaller and denser scale The atmosphere, if any, is unlikely to be more than a few centimetres thick. The outer layer of the star, known as the crust, is solid and crystalline and is probably a kilometre or so thick. The gravitational field at the surface is so large that the highest mountains are only about half a centimetre high. However, a would-be mountaineer requires 1011 joules of energy to climb such a mountain -more energy than a human being generates in his whole lifetime! Inside the crust is a region of neutron fluid containing a few protons and electrons to stabilize it. As we mentioned before, this fluid may have the properties of a superfluid. This fluid region may extend to the centre of the star. It may be, however, that in the more massive neutron stars a solid crystalline core forms at the centre

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