If the collapsing core in a supernova explosion is less than about three solar masses, it can achieve a stable state with neutron pressure in balance against gravity. The result is a very compact object, a neutron star, with a radius of about 10 km and an extreme density of around 5 × 10 14 g/cm 3—at the surface, a 1 mm grain of sand would weigh 200,000 tons. During the collapse, angular momentum conservation results in rapid rotation (see Chapter 4), many times per second initially, and conservation of magnetic field lines produces a magnetic field billions of times stronger than a normal star. The interior temperature is on the order of a billion degrees, and the neutrons act as a fluid there. A much cooler, thin, solid crust overlies this interior. Its very small surface area, however, results in an extremely low luminosity. In fact, astronomers have not yet detected the thermal radiation coming directly from the surface of a neutron star, but these objects are observable in another fashion.
Pulsars, stars observed to emit radiation in precisely separated pulses, were discovered in 1967. The first to be identified is coincident in position with the central stellar remnant in the Crab Nebula. Pulsars were quickly matched with the hypothetical neutron stars predicted in the 1930s. The pulses of radiation are due to a lighthouse beaming effect. The rapid rotation (the Crab pulsar rotates 30 times per second) carries the star's magnetic field around it, but at a radius not far from the star, the magnetic field would be rotating at the speed of light in violation of the theory of special relativity. To avoid this difficulty, the magnetic field (which generally is tilted with respect to the star's rotational axis) is converted to electromagnetic radiation in the form of two lighthouse beams directed radially outward along the magnetic field. An observer can detect a pulse of radiation every time a light beam passes by. Ultimately, therefore, it is the rotation of the star that is the energy source for the pulses and for the radiation that keeps the surrounding supernova nebula excited. For the Crab pulsar, this is about 100,000 times the solar luminosity. As rotational energy is lost, the star slows.
Unlike normal stars, neutron stars have a solid surface, with the neutrons locked into a crystalline lattice. As these stars radiate away energy, the crust slows its rotation. Observationally, the pulses are seen to be slowing down at a rate in agreement with the measured energy emission. But the fluid interior does not slow down. At some point, the disparity between their rotations results in an abrupt speedup of the crust, with an instantaneous decrease (a glitch) in the period of the pulses that are produced by the lighthouse beaming. In August 1998, a readjustment of this phenomenon in a distant neutron star apparently split open its outer crust, revealing the billion degree interior. This produced a significant flux of X‐radiation, which momentarily bathed Earth, but fortunately for life on the planet's surface, was absorbed by the atmosphere.
The behavior of neutron stars in binary systems is analogous to binaries containing a white dwarf companion. Mass transfer can occur and form an accretion disk around the neutron star. Heated by the neutron star, this disk is hot enough to emit X rays. A number of X‐ray binaries are known. When hydrogen from the accretion disk accumulates on the surface of the neutron star, rapid conversion to helium may be initiated, producing a brief emission of X rays. X‐ray bursters may repeat this process every few hours to days.
In exceptional cases, mass infall onto an old neutron star (a dormant pulsar) with transfer of angular momentum may result in a significant spin‐up of the star. A renewed rapid rotation will reinitiate the beaming mechanism and produce an extremely short period millisecond pulsar. Under other circumstances, the intense X‐ray flux from a pulsar can actually heat the outer layers of a companion to the extent that this material escapes. Ultimately, the companion star may be completely vaporized.