Neutron Star Formation

Stars are a ubiquitous feature of galaxies, where they live out a never-ending cycle of birth, life and death. This lifecycle is by no means meaningless though, and in fact contributes to the creation and dispersion of elements intricately linked to the planetary systems that make up the universe.

One remnant left behind following the death of the more massive stars is a neutron star. The greater size of stars generally contributes to a shorter lifespan, but this can still span billions of years for all but the largest stars. At the end of this lifespan, the main sequence stars that weigh over eight solar masses erupt into an enormous explosion called a supernova.

It is these supernovae that play the crucial role in the formation of neutron stars. On average, a supernova explosion is experienced by each galaxy once every hundred years. Unlike a nova, where only the surface of the star explodes, a supernova is fuelled by the core of the star collapsing and exploding. The tremendous force of this collapse causes energy to be thrust outwards from the centre, causing an explosion where the external layers of the star are violently thrown outwards.

For a neutron star to be created, the core of the exploding star must contain between 1.4 and 3 solar masses. Here, the collapse of the core continues, and fuels the combination of electrons and protons, which are crushed together to form neutrons.

The neutrons often bring a halt to the collapse, leaving behind a neutron star. These neutron stars, made up almost entirely of neutrons, are the densest objects known. This incredible density of neutron stars – not much different from the density of an atomic nucleus – lends them an extremely strong gravitational pull at the surface.

The neutron star also emerges from the explosion with a vastly reduced radius, which gives it a very high rotation speed, with periods of approximately 1.4 ms to 30s.

If the right conditions exist, the neutrons can pair up to form superfluid, a substance that cannot be experimentally created on Earth. The first direct evidence of this phenomenon recently came to light when the core of a star that collapsed close to Earth was found to be cooling rapidly.

The fall in temperature here came from the neutrons attaining superfluidity. The joining up of neutrons caused the release of neutrinos, which took away a significant amount of energy as they passed through the star. This resulted in the cooling effect, which was detected here in the case of this star.