At the last stage of stellar evolution, a heavy star can collapse into an extremely dense object made mostly of neutrons

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At the last stage of stellar evolution, a heavy star can collapse into an extremely dense object made mostly of neutrons. The star is called a neutron star. Suppose we represent the star as a uniform, solid, rigid sphere, both before and after the collapse. The star’s initial radius was the solar radius 8.1x105 km; its final radius is 7.8 km. If the original star rotated with the solar rotation period 17 days, find the rotation period of the collapsed neutron star in the unit of millisecond.