Neutron Star : The most extreme and violent object in the Universe

 

When a massive supergiant star (around 10 to 30 solar masses or more) undergoes gravitational collapse due to its own mass it forms a highly compact core called the Neutron star. Except for black holes and some other hypothetical objects (white holes, strange stars), neutron stars are the densest and smallest stellar objects known to us. The average radius of a neutron star is around 10 km and its mass is around 1.4 solar masses.

Formation

When a star uses up all its thermonuclear fuel at the end of its lifetime it undergoes a gravitational collapse due to its own mass. If the star is not massive enough the collapse stops at an intermediate stage known as a white dwarf. In this stage, further collapse is hindered by the electron degeneracy pressure. Due to the extreme gravity of the compact object, it starts to accrete matter from the surrounding, until its mass becomes 1.4 solar masses. This is the Chandrasekhar limit, beyond which the electron degeneracy pressure is no longer able to restrict further collapse. So the core further collapses until its density is comparable to the atomic nuclei. This collapse is associated with a supernova explosion, which generates the enormous amount of energy required to compress the core beyond the white dwarf stage. This stage is called the neutron star and further collapse at this stage is hindered by the neutron degeneracy pressure and repulsive nuclear forces. The core of the neutron star is primarily made of iron which is formed as a result of nuclear fusion of the lighter elements. Since iron core cannot be further fused neutron star stops burning and cools down with time.

Properties

Neutron stars may or may not be a stable configuration depending on various conditions. They may further undergo collapse by accreting matter and go on to form singularities like black holes. They may undergo collision with other astronomical bodies to produce the most violent events in the universe. Once a neutron star forms, it no longer generates heat, and gradually cools down with time. However, the neutron stars that are observable from the earth have surface temperatures of around 600000 K. The magnetic fields possessed by the neutron stars are between 100 million to 1 quadrillion times that of the Earth's magnetic field. The most striking feature of a neutron star is its extreme density. For reference, a matchbox containing neutron star material will weigh around 3 billion tonnes. The gravitational field at the surface of a neutron star is about 200 billion times that of the Earth's gravitational field.

As a star collapses its rotation speed increases, which is necessary for the conservation of angular momentum. So the neutron stars rotate at an extreme speed ranging up to several hundred times per second. Some neutron stars emit electromagnetic radiation, as they rotate. These are called Pulsars. Indeed it was the discovery of pulsars by Jocelyn Bell Burnell in 1967, that indicated the existence of extreme compact objects like the neutron stars. Neutron stars with very strong magnetic fields are known as magnetars.

Most neutron stars are unknown to us since they are cold and non-radiating in nature. However, it is estimated that there are around 1 billion neutron stars in the Milky Way. This can be estimated from the number of supernova explosions that have occurred since the formation of a neutron star is always accompanied by an extreme event like the supernova explosion. That is why these are the most extreme objects in the universe!!

Two neutron stars can form a binary system and accrete mass which makes the system bright in X-rays. Sometimes the binary system can go for a merger, and such neutron star mergers result in gamma-ray bursts, acting as strong sources of gravitational waves. Such a direct detection was made in 2017. Finally, if a neutron star accumulates a huge amount of mass through accretion, then the repulsive nuclear forces are no longer able to restrict further collapse of the core. Under such circumstances, the iron core of a neutron star further collapses all the way to a singularity, which may be a black hole.

by

Prabir Rudra