A neutron star has a mass of about 1.4 times the mass of the sun, but is not much bigger than a small city, about 15 km in radius.
A teaspoon of neutron star material would weigh about 10 million tons. The gravitational field is intense; the escape velocity is about 0.4 times the speed of light.
The collapsed star is so dense that electrons and protons do not exist separately, but are fused to form neutrons. The outer layers form a rigid crust surrounded by an atmosphere of a highly energetic electrons and excited atoms.
The neutron star acts like an enormous magnet, with the magnetic poles tipped at an angle to the axis of rotation. Like the Earth, the pulsar is surrounded by a magnetosphere, a region in which electrons and other particles are accelerated by the magnetic field. However, the magnetic field of the neutron star is much stronger than the Earth's and the electrons move at velocities close to the speed of light, emitting synchrotron radiation in a narrow beam along the direction of the magnetic poles.
Since nuclear fusion is no longer possible, the neutron star has no new source of internal energy generation. With time, its rotation should slow and its magnetic field should decrease. Unless the neutron is "spun-up", it will eventually become "invisible".
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