In 1967, legendary astrophysicist Jocelyn Bell, then a research student at the University
of Cambridge, was studying variations in radio signals using a detector constructed by her
advisor and Nobel laureate Antony Hewish.
When she noticed sharp, rapid, and regular radio pulses coming from a source in the constellation
Vulpecula, several astronomers thought the waves were of extra-terrestrial origin.
This radio source was called "little green men," or LGM for short.
Later, they realized that they had in fact stumbled upon a unique type of star—a neutron
star.
This was the first ever observation of neutron stars.
Over the decades, many other kinds of neutron stars were found.
But what exactly are neutron stars and why were they given that name?
In 1934, German astronomer Walter Baade and Swiss astronomer Fritz Zwicky predicted the
existence of neutron stars.
They proposed that when a dying star explodes as a supernova, the leftover iron core collapses
and becomes…
something new.
For this to happen, the mass of the star's core should exceed the Chandrasekhar limit,
which has a value of 1.4 times the mass of the Sun, or around 2.765*1030 kgs.
Once the core collapses, the energy released by the collapse causes the matter around the
core to be ejected.
Once all of this surrounding matter gets blown away, the only remnant is the core, which
goes on to become a neutron star.
We have discovered more than 2,500 neutron stars so far.
Some well-known examples include the Crab Pulsar, located in the Crab Nebula, and the
Vela Pulsar, found in the Vela Supernova Remnant.
Neutron stars have their name because they are composed almost entirely of neutrons.
There are also trace amounts of an equal number of electrons and protons, given the whole
object a net neutral charge.
Neutron stars generally have a radius of only around 12 km.
With masses exceeding 1.4 times that of the Sun, these are some of the densest objects
in the Universe.
A tablespoonful of neutron star material could weigh as much as Mount Everest.
Neutron stars also create one of the most powerful magnetic fields in the universe.
Their magnetic fields can be as much as one million billion (or one quadrillion), times
stronger than that of Earth.
In fact, in 2004, a neutron star, located around 50,000 light-years away, underwent
an outburst.
Its magnetic field was so strong that it affected the Earth's ionosphere, even from that monumental
distance!
In such extreme environments, the matter inside neutron stars likely exhibits properties that
are very different from what we usually see.
This state of matter is called exotic quark matter, and it could display properties like
superconductivity and superfluidity.
To understand how neutron stars became like this, we need to know more about their formation,
at the time just as stars are about to die.
Neutron stars are classified either as isolated or as members of a binary system.
Isolated neutron stars are neutron stars that freely float in space.
These neutron stars emit some form of radiation, like visible, infrared, X-ray, or gamma rays.
Isolated neutron stars have several sub-classes.
One such subclass is the compact central objects (COO) found at the center of supernova remnants.
Another subclass consists of dim isolated neutron stars (DINS), which are relatively
older and emit X-rays.
The most common subclass, however, are pulsars.
Pulsars are rapidly rotating neutron stars that give a periodic pulse or signal.
This pulse usually appears as radio emissions, but could in fact be X-rays, optical, and
gamma.
While most pulsars spin at roughly once per second, the fastest one ever observed spins
around 650 times every second!
The pulses we see are emanations of radiation that emerge from a small area on the neutron
star's surface.
As the neutron star spins, it sweeps this radiation out across space.
Whenever the beam gets pointed in the direction of Earth, we see a pulse.
This is similar to how a lighthouse functions, and has often been called 'The Lighthouse
Model.'
Pulsars are one of the most common types of neutron stars that we see.
As mentioned, the first neutron star we observed, the one seen by Jocelyn Bell and Antony Hewish,
was a pulsar.
Neutron stars do not have any nuclear fusion process happening at their cores.
Since there is no energy production, they eventually cool off and reduce the amount
of radiation they emit.
They also lose their spin and their magnetic field over time.
Binary neutron stars with a sufficiently close companion star will start accreting matter
from the star itself.
This would either result in the companion star becoming a white dwarf or a neutron star
itself.
However, it could also lead to the destruction of the companion star.
There is still a lot left to learn about neutron stars.
We have some idea about their properties, which we have learned from pulsars and the
gravitational waves emitted by their mergers.
However, we still need to get observational evidence on properties like the maximum mass
they could have, their radii, and their internal composition.
Neutron stars, alongside black holes, are some of the most extreme objects in the Universe.
Studying such objects has helped us unveil new aspects of physics, not only in astronomy,
but also in high-energy physics.
It has helped us discover several things, including novel nuclear processes, new states
of matter, and the eventual fate of the stars.