What Is A Neutron Star?

Core Interior Crust Pulsars


Arm Yourself With Truth

A neutron star has a typical density of about 10^14 g/cm^3, while the average density of our sun is about one and a half g/cm^3. The radius of a neutron star is typically around ten kilometers; the radius of our sun is 696,000 km. The mass of a neutron star can range from 1.4 to about 3 solar masses. One of the more exotic bodies encountered in the exploration of space, neutron star represents a stage of stellar evolution between main-sequence evolution and total gravitational collapse.


Core

Little is known about the core of the neutron star. It is impossible to penetrate through the interior with normal EM radiation, and working theoretical models have yet to come forth. Speculations about the core have suggested neutrons decomposing into pions and protons, but pions are exotic particles and therefore difficult to model.

Interior

The interior of a neutron star is a degenerate gas of neutrons which typically has about the same density as an atomic nucleus, and at this density it becomes a fluid. A degenerate gas of neutrons forms when all of the available momentum and position (or "quantum") states for neutrons are occupied. The Pauli Exclusions principle states that no two particles (fermions) can have the same quantum state. Compression of the gas attempts to force two neutrons into the same state, therefore there is resistance to compression and it is this "degeneracy pressure" which supports the star against further collapse. If the gravitational forces are greater than the resistance of the neutron degeneracy pressure, the star will collapse into a black hole. It has been suggested that the upper mass limit for neutron stars occurs around three solar masses. The measured masses of more than 400 neutron stars are all less than this upper limit. Neutron degeneracy pressure only becomes significant at very high densities, and this explains why neutron stars have a radius one thousandth of that of the Moon while remaining more massive than our sun.

The interior also consists of an electron dengenerate gas and a superconducting fluid of protons. Neutrons in the interior are inhibited from decay into protons and electrons because of electron degeneracy pressure. As the density of the core approaches the density of an atomic nucleus, the ratio of neutrons to protons to electrons approaches 8:1:1. This ratio represents the equilibrium between neutron decay and formation.


Crust

Near the surface of the neutron star, density drops to the point that separate atoms can form. Atoms in this outer crust are typically iron or heavier nuclei. These nuclei are so neutron-heavy that they would normally undergo beta decay, converting a neutron into a proton, an electron and an anti-neutrino. Electron pressure prevents beta decay from taking place, however, because there are no levels for the ejected electrons to fall into.


Pulsars

A pulsar is a point in the night sky that produces electro-magnetic radiation with a period that is incredibly constant. Typical pulsars might slow down by something like 10^(-12) seconds/year. This type of accuracy is an improvement on our best atomic clocks, and pulsars exist with even smaller slow-down rates.

Pulsars are rapidly spinning neutron stars. Neutrons stars are extremely small, but during their formation angular momentum and magnetic flux are conserved. This results in a very small star with a very large magnetic field and a rapid rotation period. Large magnetic fields sweep charged particles (mostly electrons) from the crust of the neutron star and accelerate them into space. These particles travel along the magnetic field lines, and therefore they travel the furthest distance along the poles of the magnetic field. If the poles of the magnetic field are not located at the rotational poles of the star (those of the Earth are not, for example), particles shot from the star will sweep out through space once each rotation. If the Earth is in the path of one of these sweeps, we will observe radiation pulses.

Keeping in mind that a neutron star is about the size of San Diego, before the discovery of pulsars astronomers were pessimistic about their chances of ever observing a neutron star. Pulsars have swept out inconsistant theories and brought neutron stars into the light.

"Burster" pulsar...