NEUTRON STAR

any of a class of extremely dense, compact stars thought to be composed primarily of neutrons. Neutron stars are typically about 20 km (12 miles) in diameter but have a mass roughly the same as the Sun's. Thus, their mean densities are extremely highabout 1014 times that of water. This approximates the density inside the atomic nucleus, and in some ways a neutron star can be conceived of as a gigantic nucleus held together by its own gravity. In the centre of the star, where the pressure is greatest, the material is thought to consist mainly of hyperons and mesons. The intermediate layers are mostly neutrons and are probably in a superfluid state. The outer 1 km (0.6 mile) is solid, in spite of the high temperatures, which might be as high as 1,000,000 K. The surface of this solid layer, where the pressure is lowest, is composed of an extremely dense form of iron. Another important characteristic of neutron stars is the presence of very strong magnetic fields, upwards of 1012 gauss, which causes the surface iron to be polymerized in the form of long chains of iron atoms. The individual atoms become compressed and elongated in the direction of the magnetic field and can bind together end-to-end, forming whiskers, which probably extend out from the surface. Below the surface, the pressure becomes much too high for individual atoms to exist. The discovery of pulsars in 1967 provided the first evidence of the existence of neutron stars. Pulsars are generally believed to be rotating neutron stars. The very short periods of the Crab (NP 0532) and Vela pulsars (33 and 83 milliseconds, respectively) rule out the possibility that they might be white dwarfs. The pulses appear to result from electrodynamic phenomena generated by their rotation and their strong magnetic fields, as in a dynamo. There is also evidence that certain binary X-ray sources, such as Hercules X-1, contain neutron stars. Cosmic objects of this kind appear to emit X rays by compression of material from companion stars accreted onto their surfaces. Most investigators believe that neutron stars are formed by supernova explosions in which the collapse of the central core of the supernova is halted by rising neutron pressure as the core density increases to about 1015 grams per cubic cm. If the collapsing core is more massive than about two solar masses, however, a neutron star cannot be formed, and the core would presumably become a black hole.