Neutron stars

For stars between about 8 and 30 solar masses, the intense gravitational forces cause other fusion reactions to occur beyond helium burning, and in this way heavier elements such as oxygen, magnesium, silicon, and iron are produced. Eventually though no further such fusion reactions take place, and the star begins to collapse again. However, in this case, when the star reaches the point at which the electrons start to be squeezed together, the intense gravitational forces present forces the electrons to combine with protons to form neutrons. The star keeps on collapsing until a neutron degeneracy pressure sets in, due to the fact that neutrons also obey the Pauli exclusion principle. The star becomes a neutron star, an incredibly dense star about the size of a large city.

During the collapse it can happen that the outer envelope of the star collides with the inner neutron core, which sets up shock wave that results in the outer layer of the star exploding. This results in a supernova, a spectacular event seen occasionally here on Earth (the last one was in 1987). In such a collision, iron nuclei can absorb neutrons to form heavier elements such as Uranium. This means of producing and releasing heavy elements into the cosmos is the primary way that planets such as the Earth contain these elements - in a very real sense, the Earth is a product of some supernova explosions that happened billions of years ago.

The remaining neutron core left after a supernova explosion forms the neutron star. This core typically is rotating rapidly, and together with the strong magnetic fields that are present, can create what we call a pulsar. A pulsar is a neutron star that emits radio waves at regular intervals, which are the result of fast moving particles being accelerated by the intense magnetic fields present.