Press Release

Neutron Stars Chaisson & McMillan, Ch. 22

Stars allow the study of the physics of materials on sub-nuclear scales. For example, there are exotic stars known as Neutron Stars .

Neutron stars are roughly as massive as the Sun, i.e., 2x1030 kilograms or around 1057 particles (neutrons, protons, and electrons) but have radii on the order of the size of Eugene (10-20 kilometers in depending on the nature of the strong [nuclear] force). The Sun has a radius of 0.7 million kilometers. Interestingly, unless the neutron star has an unusually high temperature, it will have a solid outer crust. If you could withstand, the intense gravity at the surface of a neutron star (because of its large mass and small size), you would be able to stand and walk about on a neutron star. The large mass and small size also means that neutron stars have extremely high densities. Roughly speaking, the density of a neutron star is so high that the particles are only 3 x 10-13 centimeters apart. This is roughly the separation between the protons and neutrons in the nucleus of an atom. Neutron stars, in a certain sense, are huge nuclei whose structures are determined by the Nuclear Force and the Gravitational Force. A consequence of this is that there is a maximum mass for neutron stars, as there is for white dwarfs. Depending on the assumed nature of the strong force, estimates for the maximum mass for a neutron star range from 2 to 3 MSun. The study of neutron stars may someday shed light on the nature of one of the four fundamental forces of the Universe.

Pulsars

The first clear-cut example of a neutron star in nature was driven by the discovery and subsequent anlysis of data from objects known as pulsars (pulsars were first discovered by Bell & Hewish in 1967). The case was firmly made by the Crab pulsar, the neutron star produced by the SN explosion that produced the Chinese guest star in 1054, the Crab supernova. The strongest evidence was that:

the Crab pulsar pulses with a period of 33 milliseconds (msec), that is, with a frequency of 30 Hz. This high spin rate leads to a large centrifugal acceleration at its equator, an acceleration that would tear an object like a Main Sequence star or even the more compact white dwarfs apart. This point has been driven home since, by the discovery of the class of millisecond pulsars, the fastest spinning of which has a period of 1.56 msec.

Magnetic Fields and Spin Rates

It is now generally accepted that Pulsars are rapidly rotating, strongly magnetic neutron stars.

The Earth has a magnetic field of strength B ~ 1 Gauss. The Sun has an average magnetic field of B ~ 10 Gauss (but can be as large as several thousand Gauss in sunspots). There are upper main sequence stars that have average magnetic fields of several thousand Gauss. The magnetic fields of pulsars are the most extreme, ranging from 100 million Gauss to 100 trillion Gauss!

The Sun spins with a period of around 25 days at its equator; even the fastest spinning Main Sequence stars spin with periods only on the order of a day. The fastest spinning neutron star spins with a period of 1.56 msec

How are pulsars produced from the collapse of massive stars?

If a magnetic, rotating star happens to collapse (through a Supernova outburst) to form a neutron star, its magnetic fields is trapped in the material and intensifies greatly during the collapse. Also, because of the law of the conservation of angular momentum, the spin rate of the core also greatly increases during the collapse. Both the magnetic field and the spin frequency are enhanced by a factor of (R_*/R_ns)² during collapse. Because of the small size of a neutron star compared to the size of the pre-collapse core of a massive star, very large magnetic fields and spin rates result and strongly magnetic and rapidly spinning neutron stars can be produced leading to the pulsar phenomenon.

Pulsars power themselves by tapping their supply of rotational energy. Given their masses and sizes, pulsars are essentially humongous flywheels. This coupled with their strong magnetic fields, makes them act like huge electrical generators. Pulsars produce huge electric fields, fields that can eject particles from the surface of the star and accelerate them to high energies. The high energy particles produce the electromagnetic radiation that allows us to see the pulsars. A popular version of this idea is the lighthouse model depicted below.

This type of scenario has been confirmed by the detection of a gradual slowing of the spin of the observed pulsar. The slowing of the spin suggests that the pulsar loses rotational energy. Given the assumption that we are dealing with a strongly magnetic, rotating neutron star, the observed luminosity of the pulsar matches the rotational energy loss rate of the pulsar.

The energy loss rate compared to the total spin energy of the pulsar allows us to estimate that typical pulsars live for tens of millions of years.