What is a neutron star?

Besides black holes, neutron stars are among the most baffling objects in the Universe.

A neutron star is formed in the last moments of the life of a very large star (with more than about eight times the mass as our Sun), when the nuclear fuel in its core eventually runs out. In a sudden and violent end, the outer layers of the star are ejected with monstrous energy in a supernova explosion, leaving behind spectacular clouds of interstellar material rich in dust and heavy metals. At the centre of the cloud (nebula), the dense stellar core further contracts to form a neutron star. A black hole can also form when the remaining core’s mass is greater than about three solar masses.

A neutron star is extraordinarily dense, packing more mass than the entire Sun (1.5 to 2.5 solar masses) in a globe with a diameter of 10-15 km (about the diameter of a city like Paris). Its density is so high that a sugar cube sized object of neutron star material would weigh as much as all the people on Earth.

Due to extreme pressure, the electrons and protons present in normal matter fuse together with the result that these exotic stars are composed almost entirely of neutrons. This ‘neutron compound’ generates enough force to support the inward pressure of gravity.

Newly formed neutron stars have extremely strong magnetic fields; thousands to billions of times more intense than any magnetic field we can generate in our labs. Often neutron stars are also rotating extremely fast (completing up to hundreds of revolutions per second) and beam light at radio wavelength from their magnetic poles. This beam can be detected by radio telescopes only when it is pointing toward Earth, similar to the way a lighthouse can be seen only when the light is pointed in the direction of an observer. Because of this, the radio signal appears to be pulsating and these neutron stars are called pulsars.

When a neutron star and a normal star are orbiting each other at a close distance, the neutron star can suck material away from its companion. This material falls with a high speed onto the collapsed object, becoming extremely hot and releasing energy in the form of X-rays. The star’s powerful magnetic field interact with the blazing gas and can form jets. Systems that consist of a neutron star ‘feeding’ on a normal star are known as X-ray binaries.

[Image description: At the centre of the image, there is a very bright white-blueish ball, representing the neutron star, with white/blue filaments are streaming out from its polar regions, representing magnetic field lines. Some filaments loop around the centre ball, connecting the magnetic north pole to the south. Two blueish beams stream out the two opposite poles towards space. The deep blue background depicting deep space is dotted with small bright-white spots symbolising stars.]