By Suvir Rathore

Neutron Stars:

When colossal stars, four to eight times the mass of the sun, explode in a Type IIa Supernova, they leave behind a super dense star (3x1017kg/m3) of roughly 1.4 times the mass of the sun but generally only 20 km across.

Electrons have a half-integer spin (in the unit of the reduced Planck constant), and so they can be classed as fermions. This means they obey the Pauli Exclusion Principle - where no two electrons can occupy the same quantum state - so when a star collapses the electron degeneracy pressure prevents the energy from the gravitational collapse to combine the electrons and protons to form neutrons, thus forming a white dwarf, which slowly radiates its energy away to eventually form a brown dwarf or a degenerate star. The electrons fill the lowest energy shells and the added electrons require more energy and thus lie in higher energy shells when compressed by gravity.

Casey Reed - Penn State University

Casey Reed - Penn State University

This, however, is only applicable to formed white dwarfs that do not exceed the Chandrasekhar limit, this is to say that they are no larger than approximately 1.44 solar masses. The inward pull of gravity beyond this limit is greater than the electron degeneracy pressure, allowing the electrons and protons to combine to form charge-neutral (and slightly heavier) neutrons after photodisintegration has taken place.

The densely packed nucleus, full of neutrons, also has its own pressure - neutron degeneracy pressure – which is a result of the same principle. This is because neutrons are made up of two down quarks and one up quark, where each quark is in itself a fermion, therefore the subatomic particle is also a fermion, thus also obeying the Pauli Exclusion Principle up till approximately 3 solar masses (where they then form black holes). The force of gravity is so high (1011 greater than Earth’s) that spaghettification would occur where the difference in the force on the bottom of the object is so much greater than at the top of the object that it stretches. It also allows for gravitational lensing as well as there being time dilation (relative to Earth).

Due to the conservation of angular momentum (after a red supergiant collapses), neutron stars tend to spin very fast, although the constant yet small spin down rate means they slow down over time unless the spin-up process takes place where they absorb matter from orbiting stars. The fastest known neutron star, PSR J1748-2446ad, has a surface speed of almost a quarter of the speed of light! There can also be glitches in the speed, for example, a known neutron star has sped up. The theory to explain it was that since the speed was decreasing over time, the shape became more spherical (from an oblate spheroid) which, after a defined process, reduced its size and increased its angular momentum.

Some neutron stars emit a lot of electromagnetic radiation from regions near their magnetic poles, which when the magnetic axis does not match with their rotational axis, can be detected and defined as pulsars. The magnetic fields of pulsars are much stronger than that of Earth’s magnetic field (some even trillions of times stronger) due to its high density.

Although not much is known about them, Magnetars are even stranger. Magnetars are neutron stars, but with a super strong magnetic field (as if normal pulsars weren’t enough). These spin slightly faster than normal neutron stars and have magnetic fields of roughly 1010 tesla equivalence, the strongest known in the universe. For comparison, the Earth’s magnetic field has the equivalence of roughly 30 micro-tesla, and the strongest man-made magnets would have the equivalence of no more than 50 tesla. These are very dangerous as seen in 2004 when gamma rays from SGR 1806-20 reached Earth and affected the Earth’s atmosphere despite the Magnetar being 50,000 light years away. The explanation for their strong magnetic fields is theorized to be a result of the magnetohydrodynamic dynamo process where the thermal and kinetic energy within the liquid layers of Magnetars produce and amplify the stronger magnetic fields.

The Eddington Limit is essentially the maximum balance between the radiation pressure and the gravitational force. If a star exceeds this limit, the luminosity would be very high, so high in fact that layers of the star can actually be blasted off. Such luminosities are associated only with (supermassive) black holes that form accretion disks however some ultra-luminous X-ray sources (ULXs) have such high luminosities that they in fact also break the Eddington limit. After a noticed dip in a particular ULX’s light spectrum, it was concluded that the sources are not black holes, for Cyclotron Resonance Scattering Features (CRSFs - where the movement of charged particles in a magnetic field caused the absorption line) are evidence of magnetised neutron stars, thus allowing them to shine so bright.

Finally, binary neutron star systems have been found and are more prevalent than predicted. The gravitational waves of a neutron star merger event which travelled 130 million light years to Earth were detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO - and over 70 other observatories) in August 2017 with a burst of gamma rays detected by Gamma-ray Burst Monitor on NASA’s Fermi space telescope. Kilonovas and like-events could be the explanation for the existence of elements heavier than iron, which may have been produced alongside supernova nucleosynthesis.