Item of Terminology in Astronomy, and much used in sf. In an ordinary star, such as the Sun, the gravitational pressure tending to make it collapse is balanced by the outward pressure created by the continuous nuclear fusion within it. As a star's fuel burns out, Gravity takes over. A star of mass less than the Chandrasekhar limit – a value calculated by Indian physicist Subrahmanyan Chandrasekhar (1910-1995) to be about 1.4 times the mass of our Sun – would usually contract under the force of gravity into a very dense White Dwarf, with a radius of maybe only a few thousand kilometres; but a further, more extreme compression is possible, as under pressure the empty space within the atoms of the star's matter is annihilated, the electrons being crushed down to the atomic nucleus, there to fuse with the protons of the nucleus to form neutrons. The resulting degenerate matter – neutronium – is incredibly dense because of the loss of the intra-atomic emptiness: a neutron star having the same mass as our Sun would have a radius of about 10 kilometres (6 miles). Its surface gravity would be so strong that no "mountain" (in the sense of a prominent surface irregularity) could exist on it higher than about 5mm (0.2in); and, initially at least, it would rotate very rapidly owing to the conservation of angular momentum (i.e., for the same reason as ice skaters can increase their rate of spin by pulling in their limbs). A modestly sized chunk of neutronium and its inordinate gravitational effects are central to Larry Niven's Physics-puzzle story "There is a Tide" (July 1968 Galaxy).
Beginning in 1968, radio telescopes discovered many celestial sources which emitted regular bursts of microwave radiation with very short periods (from only a couple of seconds down to tiny fractions of a second) between pulses. These objects were named pulsars, and were soon shown almost certainly to be neutron stars. Their powerful electromagnetic fields channel the radiation associated with the pulsar into two continuous beams which, because of the object's rapid rotation, we see (assuming we are in a suitable line-of-sight) in the form of pulses, much as we might see the light from the rotating lamp of a lighthouse. The period of a pulsar's pulses (i.e., its rate of rotation) can be used as a measure of the pulsar's age – the rotation slows with time – and there is excellent correlation between such measures and the ages of pulsars whose dates of formation are known (notably the pulsar at the core of the Crab Nebula, the remnant of the supernova observed in 1054 CE).
The tidal forces created in proximity to such a star would be lethal, as imagined in Larry Niven's story "Neutron Star" (October 1966 If), in which a Spaceship pilot who has ventured too close is almost ripped apart because, in such an intense gravitational field, the length of his ship represents a significant distance in terms of field variation. Thus the force exerted by Gravity at the end of the vessel nearest to the neutron star is very considerably greater than that exerted on his head; it is this difference in pull that so nearly proves fatal to him. (He barely survives by remaining at the ship's centre of gravity, avoiding the worst of the tidal effects that would tend to crush him into the nose or tail. However, Niven subsequently learned that the spin imparted to the ship during its close approach to the rotating neutron star would be inevitably fatal.) In Gregory Benford's The Stars in Shroud (1978) a neutron star's gravity is exploited by spacecraft whipping round it to accelerate into new courses – a higher-energy version of the manoeuvre whereby space-probes in the solar system take advantage of the gravitational fields of the larger planets for slingshot manoeuvres. The most extreme neutron-star stories may be Robert L Forward's Dragon's Egg (1980) and its sequel Starquake! (1985), which have an Alien race – who live on a hugely accelerated timescale – evolving on the unfriendly surface of such a star, and ultimately making contact with human observers.
Stellar collapse for stars with a mass greater than the Chandrasekhar limit can, it is theorized, lead to a different and even more bizarre form, the Black Hole. Collisions between or involving neutron stars are potentially apocalyptic events, generating radiation bursts that could be lethal at interstellar distances. One such burst event destroys Earth's remaining non-uploaded "fleshers" early in Greg Egan's Diaspora (1997); Stephen Baxter's Space: Manifold 2 (2000; vt Manifold: Space 2000) imagines the construction of vast protective shields to preserve something from the inevitable sterilization events to come; the television series Threshold (2005-2006) begins six years before the expected extinction of life on Earth by the effects of a long-ago neutron-star collision. [PN/DRL]
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