Neutron Stars

	INTRODUCTION
	PROPERTIES
	MAGNETARS

Introduction

A white dwarf star more massive than 1.4 solar masses cannot remain stable. It will collapse further to a new stable state, called a neutron star. In this lecture we shall explore some of the properties of these fantastically dense objects.

In 1934 Walter Baade (at Mount Wilson Observatory) and Fritz Zwicky (at Caltech) suggested that a very massive star would end its life by having its core crushed by gravity. This would trigger a gigantic explosion called a Type II supernova. They speculated that the collapsed core could be dense enough to fuse together electrons and protons to form neutrons. Such a collapsed star is called a neutron star.

Properties

The collapsing core at the center of supernovae can have a mass that exceeds this limit. Therefore we expect that in some supernovae the cores will collapse to neutron stars with the following properties:

Radius about 10 km.

Density ranges from 10⁴ g cm^-3  at the surface to about 5 x 10¹⁵ g cm^-3 at the center. This is about a billion times denser than a white dwarf.

Structure--We expect the star to be basically a very smooth rigid metallic shell filled with a neutron superfluid. A superfluid is a fluid that moves without resistance.

Conservation of Angular Momentum

Because of the law of conservation of angular momentum a collapsing star spins faster and faster the smaller it becomes, rather like a spinning ice skater who gradually draws in her arms. Since neutron stars are so much smaller than the cores from which they collapse neutron stars would be expected to spin extremely rapidly: hundreds to thousands of times a second! 

Any normal object spinning that fast would fly apart in an instance. However, the crust of a neutron star is so strong that it can keep the star from breaking up.

Pulsars

Stars have very strong magnetic fields. Therefore, if a star collapses to a neutron star we would expect the magnetic field to be immensely concentrated. Neutrons stars, indeed, have extremely intense magnetic fields. 

Moreover, the rapidly spinning magnetic field generates a powerful electric field that can strip electrons from the neutron star's surface. These electrons are then accelerated by the electric field, away from the star's surface, to almost the speed of light. 

The accelerating electrons emit electromagnetic radiation, which comes off in two oppositely directed beams. If one of the beams crosses our line of sight, as the neutron star spins, we will detect periodic pulses, much like the beam from a lighthouse. Such a neutron star is called a pulsar. 

The best known pulsar lies in the Crab Nebula. It is the remnant of a star whose explosion was witnessed by Chinese astronomers in 1054 AD.

MAGNETARS

Recently a new class of star has been identified called Magnetars. These stars are thought to be neutron stars with considerably stronger magnetic fields. The magnetic field on these stars is so enormous that it can stress the star's super hard metallic crust to breaking point, triggering violent star quakes. These violent star quakes can generate powerful bursts of gamma rays, which can be detected over thousands of light years.

Last updated October 11, 1999 Harrison B. Prosper