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# 6 Neutron stars

The mass range of progenitor stars that collapse to form neutron stars is uncertain, but it is possible that all single progenitor stars with main-sequence masses in the range will do so. However, in some binary systems, neutron stars can also form from stars that were initially even more massive than , where mass transfer between the two stars influences their evolution significantly. Whatever their origin, a newly formed neutron star will have a temperature of 1011–1012 K, but it will cool quickly to around 109 K on a timescale of a day, and to around 108 K within a hundred years.

Neutron stars are commonly observed as rapidly spinning radio pulsars. The youngest of these, such as the Crab pulsar which formed only ~ 1000 years ago, are seen to still reside in the centre of an expanding supernova remnant. Some so-called millisecond radio pulsars are observed in binary systems with a white dwarf or non-pulsar neutron star companion. Other neutron stars are seen in binary systems with a main-sequence or supergiant companion star from which they accrete large amounts of material. The accreting matter gives rise to strong X-ray emission, and such objects are known as X-ray binary stars.

The neutron stars in radio pulsars and X-ray binaries are measured to have masses in the range , contained within a sphere of radius ; hence they have a density of . These extreme temperatures and densities give rise to some extraordinary consequences for the neutron star’s properties.

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