Re: What causes the Large magnetic fields of Neutron Stars

The Earth's magnetic field probably results from the motion of liquid iron in the Earth's core. Iron is a conductor, so it can produce an electric field. A basic result of electricity and magnetism is that a changing electric field, like that produced by a spinning mass of iron, can produce a magnetic field.

Neutron stars probably don't have iron in their cores. How do they produce their magnetic fields? In order to answer this question, we have to look at how NSs are produced.

An NS is produced at the end of the lifetime of a massive star, say, one ten times more massive than the Sun. Stars, including the Sun, have magnetic fields. In the case of stars, the magnetic field is produced most likely from the motions of gas rather than liquid iron. A star is hot enough that electrons are stripped from atoms producing free electrons and ions (mostly hydrogen ions, which are just free protons). Motions of this ionized gas or plasma can then produce an electric field which in turn produces a magnetic field.

At the end of a massive star's life, it encounters a crisis. Stars are intricate balancing acts. The mass of a star produces gravity, which tries to crush the star. The force of gravity is balanced by pressure at the center of the star, pressure that is produced by the heat from the nuclear reactions occuring in the star's core. When a star exhausts its fuel to support the nuclear reactions, the reactions cease, and gravity wins. The star collapses.

A basic result in electricity and magnetism is that magnetic fields cannot simply disappear. Consider how the magnetic field of the star looks just before it collapses. The magnetic field extends out from the surface of the star. As it collapses, the surface area of the star becomes much smaller (by a huge factor). Now consider how the magnetic field looks just after the star has collapsed. Once again, the magnetic field extends out from the surface of the star, but now this surface is much smaller. From basic electricity and magnetism it's possible to demonstrate that this means the strength of the magnetic field must have become much stronger.

This is what causes an NS' strong magnetic field: The magnetic field is a highly compressed relic of the original star's magnetic field. For reference, a typical NS magnetic field might be 10¹² Gauss; by comparison, the Earth's magnetic field has a strength of about 0.05 Gauss. An NS' magnetic field is more than a trillion times stronger than the Earth's.

There's still some debate on what happens to this magnetic field over time. Some think that various processes (not all of which are well understood) could cause the strength of the magnetic field to weaken over time. Other astronomers disagree. Even if the strength of an NS' magnetic field does decay over time, though, the rate at which it decays is quite slow.