Neutron Stars
Neutron stars are a class of very compact astrophysical objects which are remnants of massive stars that collapse after exhausting their capacity for thermonuclear burning in their interiors. The typical mass of a neutron star is about one and a half times that of the Sun (which is about 4.4x1030 lb, or 2x1030 kg), while its radius is only about 6 mi (10 km)--one hundred thousand times smaller than the solar radius. Such a combination exists through compression of matter in the interior of the star to a density greater than 2.7 x 1016 lb per cubic inch (1015 g per cubic cm) 2 cm, several times the density in atomic nuclei. Unlike most naturally occurring systems in equilibrium, where matter exists in the form of atoms surrounded by electrons, matter in the cores of neutron stars is believed to be a uniform mixture of nucleons, electrons and possibly other species of elementary particles.
The concept of a star composed of neutrons was first raised by the prominent Russian physicist, L. D. Landau, shortly after the discovery of the neutron in 1932. Landau pointed out that since the neutron has no electric charge, a macroscopic number of neutrons could be bound together by gravity. Two years later, astronomers F. Badde and W. Zwicky suggested that the unexplained phenomena of Supernovae arise from the collapse of normal star into a neutron star. Both suggestions remained theoretical speculation for several decades, but were revived shortly after the discovery of pulsars in 1967. The very accurate pulsation of pulsars is attributed to rotation, and only very compact stars can rotate at the observed rates--with periods as low as millisceonds. Neutron stars are the only viable candidate for such objects, and it is well accepted that pulsars offer conclusive evidence that neutron stars exist. The identification of pulsars in the remnants of known supernovae, such as the Crab and Vela, confirmed Badde and Zwicky's hypothesis about the supernova-neutron star connection as well.
Theoretical models of stellar evolution suggest that stars whose initial mass is in the range 10-25 solar masses will end their lives in a supernovae explosion that will leave behind a neutron star. If this is correct, there should be over one hundred million neutron stars in our galaxy, but the vast majority of them are undetectable by any astronomical method. A small minority of the younger neutron stars have sufficiently high surface magnetic fields, typically 1012 Gauss (over one million times stronger than the highest manmade magnetic field), and are spinning rapidly enough to emit beamed electromagnetic emission that enables their identification. As of early 2000, over 700 pulsars had been discovered. Several tens of other neutron stars have been identified by other means, mostly as sources of x-ray emission, or by their gravitational effect on an observable binary companion pulsars.
Neutron stars are the only known system in nature where matter at extremely high densities can exist in a stable manner. In this sense, neutron stars are cosmological laboratories for nuclear and hadronic physics, which are dominated by the strong interaction. Moreover, the microscopic physics of matter at supernuclear densities determine the star's main observable macroscopic quantities, such as mass and radii, so that astronomical observation of neutron stars can, in fact, serve as a primary research tool for the study of microscopic physics. A different but closely related topic is matter at densities just below that of atomic nuclei, which includes a mixture of nuclei and free nucleons. Such matter occupies the "crust" of the neutron star (the outer 1/2 mile), which is believed to be responsible for violent quakes that have been detected in several young neutron stars.
Neutron stars also offer laboratories for very strong magnetic field and gravitational field physics. In particular, the gravitational filed in a neutron star and its vicinity is large enough that general relativity leads to effects of the order of 10-20% with respect to Newtonian gravity. Observations related to gravitational influence of neutron stars in their close vicinity can indeed serve as probes of general relativity. This applies to surface emission from the star, to inter-stellar material falling on to the star, and to the motion of a close binary companion. The "textbook" manifestation of general relativity is the double-neutron star system PSR 1913+16, which undergoes a shifting of its pulsation period. This shift in excellent agreement with predictions general relativity, indicating that gravitational waves are emitted from the system (the discovery of this system earned Taylor and Hulse the Nobel prize in 1994).
Many neutron stars are also the sites of strong x-ray sources in the range of a few to a few tens of keV. These sources are neutron stars which accrete material ejected by a binary companion star. The material is compressed as it accretes on to the neutron star, and the very strong gravitational field near the surface causes temperature of the accreting material to rise to millions of degrees, hence emitting in the x-ray range. The detailed features of x-ray emission vary considerably between different sources. Some sources are very periodic, probably due to strong emissions forming a hot-spot that rotates along with the neutron star; the "classic" case of her X-1, with a period of 1.24 seconds in its x-ray emission, is such an x-ray pulsar. In other cases, the period of oscillations in the x-ray emission seems to vary, implying that more complex phenomena determines the rate of accretion and the emission efficiency. Some sources show strong, transient bursts of x rays, which are probably due to thermonuclear ignition of accreted hydrogen on the surface of the star. Since the x-ray emission originates from near or at the surface of the neutron star, it can provide important clues concerning the properties of neutron stars and the physics of strong gravitational fields. Improved space-based detectors of cosmological x-ray sources (and NASA's Chandra x-ray satellite in particular) have great potential in improving our knowledge of these exciting fields in the near future.
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