Neutron Star Facts

Due to extreme compactification, a spoonful of a neutron star may weigh billions of tons, which is equivalent by mass, to a few mountains on Earth.

Stars are gigantic balls of gas, mostly made up of hydrogen and helium, powered by nuclear fusion at their core. Their life is a constant tussle between the crushing force of gravity and the fusion energy combating it. More massive a star, hotter is its core, and faster is the rate at which is burns up fuel.

When some massive stars run out of their fuel, they implode in a cataclysmic explosion known as a supernova, leaving a rapidly spinning, dense, extremely compact core, with one of the most powerful gravitational and magnetic fields, in their wake. This stellar remnant, forged out of a dying massive stellar object, is known as a neutron star.

What happens when you take the mass of a star that weighs more than 1.4 times the mass of the Sun and compress it into a sphere extending 20 Km in diameter?

You get a stellar object that has a density of 1017 Kg/m3 (which is about 1014 times Earth's density), with the most mind-boggling set of inherent properties. A neutron star whose magnetic beam axis is directed towards the Earth, is known as a pulsar (pulsating star).

Every aspect of this stellar remnant tests the limits and extremes of the laws of physics. One by one, this article will unravel the enigma of these tiniest, yet most massive stellar remnants. As you will be convinced further, truth is stranger than fiction.

Neutron stars are created in violent explosions called supernovas, triggered by gravitational collapse of massive stars, which have run out of fuel. The core of the massive star is compressed by intense gravitational pressure, which results into a highly dense stellar remnant.

Any star that has a mass greater than 10 ten times the mass of our Sun (i.e. greater than 1.9891 × 10³¹ Kg), will most likely end up as a neutron star.

In a high mass neutron star, the crunching force of gravity is halted by the heat generated through fusion of progressively heavier elements, until the core becomes iron-rich, which is an element that cannot be fused further. At this point, the core starts collapsing due to the ceasing of fusion and the density goes on increasing, with the temperature rising to about 5 billion Kelvin.

During the formation of a neutron star, when the temperature rises, a process called photodisintegration is triggered, which splits iron nuclei into alpha particles (helium nuclei). As the temperature goes on rising further, protons and electrons start fusing to form neutrons, which releases a dense flux of neutrinos.

When neutron stars are formed, midway through the process, the climbing density reaches the mark of 4 x 10¹⁷ kg/cubic meter, and the degeneracy pressure (arising due to the 'Pauli Exclusion principle') exerted by neutrons, halts the collapse.

The degeneracy pressure arising during neutron star formation doesn't allow fermions (neutrons) to occupy the same quantum mechanical state, leading to an implosion, which ejects the entire outer envelope of the star, triggering a Type II or Type 1b supernova, with the core remaining at the center, forming a neutron star.

Just as a ballerina rotates faster as she wraps her hands around her body, the neutron star rotates very fast as its radius is reduced by gravitational compression. This happens due to conservation of angular momentum.

Considering the large decrease in radius during the collapse, the neutron star spins very fast, at a rate greater than 43,000 per minute, with periods ranging from 1.4 ms to 30 seconds. In a binary system, they can rotate at a rate greater than 600 per second due to mass accretion around them.

Neutrons stars form only when the mass of the stellar core is greater than 1.44 solar masses, which is known as the Chandrasekhar limit. But, if the core mass is greater than 3 solar masses (known as the 'Tolman-Oppenheimer-Volkoff limit'), it will most probably end up as a black hole, as even neutron degeneracy pressure won't be able to halt the gravitational collapse to a point of singularity.

A star with mass in the range of 15 to 20 solar masses, will most certainly end up as a black hole.

Neutron stars are some of the most compact objects in the universe and due to compression, their surface gravity is phenomenally high, that is about 10¹¹ times that of Earth. An object falling towards the star surface will be accelerated at about 10¹²m/s² which will shred and rip it out into constituent atoms.

To escape from this star's surface gravity, one would have to travel at a velocity of 100,000 km/s, which is about a third of the speed of light.

The stars are so dense that a spoonful of neutron star will weigh more than a mountain. A teaspoon of the star could weigh as much as 5 x 10¹² kg.

A neutron star extends about 20 km in diameter. As depicted in the graphic presented below, such a star will be comparable in size to the island of Manhattan, but will weigh several times more than the Sun.

These stars have very high magnetic fields. The measured fields are in the order of 1012 Gauss, which is about 1013 times our Earth's magnetic field. They are the strongest magnets in the world.

The rapid spinning of the Earth's magnetic field causes the creation of an electric charge and emission of electromagnetic radiation along the magnetic axis. This axis may not be necessarily pointed towards the Earth, as it is mostly non-aligned with the rotation axis. When it is pointed towards the Earth, we observe it as a pulsar.

• The atmosphere is 1 meter thick and completely dominated by the intense magnetic field of the order of 10¹² Gauss.
• As you go deeper, density of the star increases, and the core is populated by nuclei of high neutron numbers. The central core is supposed to be a superconducting fluid of protons and electrons.

Neutron stars were first hypothesized right after discovery of the neutron, in 1933, by Walter Baade and Fritz Zwicky, to explain the triggering of a supernova.

In 1965, Antony Hewish and Samuel Okoye first discovered an object emitting radiation in radio waves, in the Crab Nebula, located in the Orion constellation. After intense research, it was confirmed to be a neutron star and later a rotating neutron star, called a pulsar.

It is known as the Crab Pulsar (PSR B0531+21), which is one of the youngest known pulsars, created through a supernova, whose occurrence was recorded by Chinese astronomers on July 4, 1054.

Among the known neutrons stars, almost 5% are part of binary stellar systems (i.e., two stars revolving around each other).

The closest known pulsar (PSR J0108-1431) has been detected at a distance of 770 light years, in the Cetus constellation.

PSR B1919+21 was the first radio-pulsar to be detected and was nicknamed as LGM-1 (denoting 'Little Green Men'), as its periodic pulse was prima facie suspected to be an intelligent signal from alien life forms.

PSR B1257+12 is a pulsar, located about 1000 light years away, that was the first to be detected with a planetary system. It has two planets revolving around it.

Magnetars are a type of neutron stars, known to possess extremely strong magnetic fields, emitting X-ray and gamma radiation.

A millisecond pulsar is a neutron star radiating X-rays and gamma rays, while spinning at a very high rate. Among the 200 such objects discovered so far, the first was PSR B1937+21, which was known to spin about 641 times per second. PSR J1748-2446ad is the fastest of the lot, spinning at a phenomenal rate of 716 times per second.

Neutron stars in the form of pulsars are a subject of intense research today. The various phenomena associated with their magnetic fields and emission spectra are particularly interesting. Hope this rapid fire overview of this unique astrophysical object has inspired you to explore stellar phenomena at a deeper level.