What are neutron stars? Their formation and characteristics

When a supermassive star explodes as a supernova, its nucleus collapses and is converted into an ultra-dense celestial body. It is thought that in the Milky Way exist approximately a billion neutron stars. This type of star represents the final stage in the life of a supermassive star.

The stars that give birth to neutron stars are the biggest ones that exist in our universe, like the red or blue giants. These types of stars are a minority in our universe.

Their formation

Throughout the life of a star, gravity tries to collapse it, while the energy that is produced in its interior, via nuclear fusion, provokes the opposite effect. The result is that the star expands, in the case of the biggest stars, until having a diameter the size of Jupiter’s orbit.

In the case of red and blue giants, they reach the point of iron’s fusion. Arrived at this point, stars can no longer extract energy, and they consume. Gravity wins the battle, and, in a matter of seconds (one or two), the stars explode in the form of a supernova, ejecting to space most of its layers, and leaving behind what to name a neutron star.

Neutron stars have normally a mass between 1.3 and 2.5 solar masses, but in a sphere only 20 kilometres in diameter. As we can observe, these objects are enormously dense. A sugar-cube-sized of neutron star material would have a mass of a billion tonnes (a trillion kilograms) on the Earth. This is approximately what Mount Everest weights.

Comparison of the size of a neutron star compared to a city on Earth

The composition of neutron stars

When most of the star has exploded in the form of a supernova, the nucleus remains at the centre. Gravity continues to compress it, to a point where the atoms are so compacted and close together that electrons are thrust against the protons, in such a way that they are combined to form neutrons.

Thus, the name of neutron stars comes from their composition. They are mainly formed by these particles. The gravity on these bodies is so strong that there can’t be any mountains. It is calculated that the parts that stand out from their surface can have, at most, a height of about 5 centimetres.


The supernova that gives birth to the neutron star transfers an enormous quantity of energy to this object. In consequence, neutron stars rotate onto their axis extremely quick: between 0,1 and 60 rotations per second, and some reach 700 rotations each second. They gradually slow down over time. The extraordinary magnetic fields that these bodies have produces high-energy light beams from their magnetic poles. In the case that one of the poles of the neutron star is aimed at the Earth, the light beams can pass through or next to the Earth, producing the effect of a lighthouse. This creates what we name pulsars.

These light beams are produced in extremely constant and precise time intervals. It is such their precision that astronomers are considering using them for spaceflight navigation.


Neutron stars have extraordinary magnetic fields. The magnetic field of the Earth is approximately 1 gauss, and the one of the Sun of some hundred gauss. But, the average neutron star has magnetic fields of trillions of gauss.

Nevertheless, there are neutron stars with a much more powerful magnetic field than the average. These types of neutron stars are named magnetars. The latter have 1,000 times more powerful magnetic fields than the rest.

This makes magnetars the bodies with the most powerful magnetic field in the universe.

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