ay45c5-page37 - Actually this estimate is rather...

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Unformatted text preview: Actually, this estimate is rather inaccurate: (1) because at this density nuclear forces (strong force) between neutrons are appreciable, and this aids gravity, thus decreasing the limiting mass; and (2) because the Newtonian gravitational potential surface=c2  1, so Newtonian gravity is a poor approximation, and we should use General Relativity. The physics overall is rather uncertain, but the limiting mass of a neutron star is probably  3 M . Until we know the equation of state of a neutron uid, this mass will remain uncertain. What is the radius of a neutron star? Neutron stars are denser than white dwarf stars. In the limit, the central density should correspond to the neutrons almost touching. The neutron{neutron separation is then  1 fm  10 13 cm, so the neutron number density  1039 cm 3, and   2  1015 g cm 3. Hence, if M  1 M , R  10 km. We know that neutron stars exist because of evidence of pulsars. (We know that white dwarfs exist, too, because we can see them optically; e.g., Sirius B.) Pulsars are distinguished by the regular arrival of radio pulses separated by intervals of a few ms (0.5 ms = shortest) to a few seconds ( 5 sec = longest). 0 2 4 Time (s) 6 8 Chart record of individual pulses from one of the rst pulsars discovered, PSF 0329 + 54. They were recorded at a frequency of 410 MHz and with an instrumental time constant of 20 ms. The pulses occur at regular intervals of about 0.714 s. 149 ...
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