Neutron stars are created as the result of a supernova. A supernova signals the death of a large star, particularly supergiants. During the supernova the outer parts of the star are blasted off into space. What happens is that as these outer layers are released, the force of the explosion implodes the core. The core is referred to as being in a state of collapse. The force is so great that it is crushed into a superdense object that can be as large as London, but has the mass of our Sun.

Composition of a Neutron Star

Surprisingly they are not composed of gas (plasma state) but the neutrons form the core of the star, hence the name. Surrounding the core is a layer of (tightly-packed) iron nuclei. It is as if the neutrons were a liquid surrounded by solid iron. During the collapse of the star a large percentage of the atoms are compressed. This forces the protons and electrons (the subatomic particles) of these atoms to merge, resulting in the creation of neutrons. The neutrons form the liquid core of the star, hence the name. Surrounding the core is a layer of solid iron.

Neutrons are very small since they are subatomic particles and in a neutron star they are arranged tightly together which is why they are so dense. So dense, in fact, that a teaspoon of neutron star material would weigh a few thousand tons. If you were on a spacecraft you would have to have a thrust equal to 0.5c (or half the speed of light) to escape if you are on the surface. After a neutron star reaches a density more than three solar masses (three suns) it will collapse further due to the excessive gravity and will result in the creation of a black hole. There are other types of neutron stars, called Pulsars.

Pulsars

These are neutron stars that have strong magnetic fields and spin on their axis. They send out regular spurts of radio waves. They can spin several hundred times per second, or a slow as a few times per minute. Some pulsars emit bursts of light, around 15 times a second, although this varies between each star. These bursts are so precise that they are more accurate than any clock found on Earth. They work like this because the radio beams are emitted from the magnetic poles of the star and we can detect them here on Earth as they pass by us. Eventually they slow down as they lose energy, then will eventually stop producing radio waves and die.

Types of Pulsar

The three main categories are:

• Binary - A Pulsar can orbit another star and this companion may be a different class of star altogether, not necessarily a neutron star. They attract each other due to gravity as they orbit and will eventually merge. The resulting mass of the two may be too much and they will collapse into a black hole.

• X-Ray - Pulsars don't necessarily produce radio waves, they can produce X-rays. This occurs because in binary systems the gas from its companion is attracted to the pulsar and forms a stream as it approaches. As the gas collides with the surface of the pulsar it forms a region where the material is heated up to millions of degrees which produces the X-rays. When this area rotates into our direction as the star spins we see this as a pulse of X-rays. X-ray pulsars may also have been resurrected from dead pulsars because as they extract gas from a companion, they speed up.

• Magnetars - These are pulsars with amazingly strong magnetic fields than those described above. It is thought that this property of magnetars is related to gamma-ray bursts in space.

Where Neutron Stars Can Be Found

The best place to start is in the heart of nebulas, such as the Crab Nebula which contains an optical pulsar and it is thought that the energy released from this star is what heats the surrounding nebula to make it visible. Of course, they can probably be found all over the galaxy but they are being detected mostly in globular clusters where they are thought to occur in large numbers.

Suggested Reading

Science Fiction

• Neutron Star written by Larry Niven. Part of the short story collection, Neutron Star
• Dragon's Egg written by Professor Robert L Forward.

Factual

• The Universe Explained written by Colin Ronan.