A neutron star is the collapsed core of a
giant star which before collapse had a total
of between 10 and 29 solar masses. Neutron
stars are the smallest and densest stars,
not counting hypothetical quark stars and
strange stars. Neutron stars have a radius
on the order of 10 kilometres (6.2 mi) and
a mass between 1.4 and 2.16 solar masses.
They result from the supernova explosion of
a massive star, combined with gravitational
collapse, that compresses the core past white
dwarf star density to that of atomic nuclei.
Once formed, they no longer actively generate
heat, and cool over time; however, they may
still evolve further through collision or
accretion. Most of the basic models for these
objects imply that neutron stars are composed
almost entirely of neutrons (subatomic particles
with no net electrical charge and with slightly
larger mass than protons); the electrons and
protons present in normal matter combine to
produce neutrons at the conditions in a neutron
star. Neutron stars are supported against
further collapse by neutron degeneracy pressure,
a phenomenon described by the Pauli exclusion
principle, just as white dwarfs are supported
against collapse by electron degeneracy pressure.
If the remnant star has a mass greater than
about 3 solar masses, it continues collapsing
to form a black hole.
Neutron stars that can be observed are very
hot and typically have a surface temperature
of around 600000 K. They are so dense that
a normal-sized matchbox containing neutron-star
material would have a weight of approximately
3 billion tonnes, the same weight as a 0.5
cubic kilometre chunk of the Earth (a cube
with edges of about 800 metres). Their magnetic
fields are between 108 and 1015 (100 million
to 1 quadrillion) times as strong as that
of the Earth. The gravitational field at the
neutron star's surface is about 2×1011 (200
billion) times that of the Earth.
As the star's core collapses, its rotation
rate increases as a result of conservation
of angular momentum, hence newly formed neutron
stars rotate at up to several hundred times
per second. Some neutron stars emit beams
of electromagnetic radiation that make them
detectable as pulsars. Indeed, the discovery
of pulsars by Jocelyn Bell Burnell in 1967
was the first observational suggestion that
neutron stars exist. The radiation from pulsars
is thought to be primarily emitted from regions
near their magnetic poles. If the magnetic
poles do not coincide with the rotational
axis of the neutron star, the emission beam
will sweep the sky, and when seen from a distance,
if the observer is somewhere in the path of
the beam, it will appear as pulses of radiation
coming from a fixed point in space (the so-called
"lighthouse effect"). The fastest-spinning
neutron star known is PSR J1748-2446ad, rotating
at a rate of 716 times a second or 43,000
revolutions per minute, giving a linear speed
at the surface on the order of 0.24 c (i.e.
nearly a quarter the speed of light).
There are thought to be around 100 million
neutron stars in the Milky Way, a figure obtained
by estimating the number of stars that have
undergone supernova explosions. However, most
are old and cold, and neutron stars can only
be easily detected in certain instances, such
as if they are a pulsar or part of a binary
system. Slow-rotating and non-accreting neutron
stars are almost undetectable; however, since
the Hubble Space Telescope detection of RX
J185635-3754, a few nearby neutron stars that
appear to emit only thermal radiation have
been detected. Soft gamma repeaters are conjectured
to be a type of neutron star with very strong
magnetic fields, known as magnetars, or alternatively,
neutron stars with fossil disks around them.Neutron
stars in binary systems can undergo accretion
which typically makes the system bright in
X-rays while the material falling onto the
neutron star can form hotspots that rotate
in and out of view in identified X-ray pulsar
systems. Additionally, such accretion can
"recycle" old pulsars and potentially cause
them to gain mass and spin-up to very fast
rotation rates, forming the so-called millisecond
pulsars. These binary systems will continue
to evolve, and eventually the companions can
become compact objects such as white dwarfs
or neutron stars themselves, though other
possibilities include a complete destruction
of the companion through ablation or merger.
The merger of binary neutron stars may be
the source of short-duration gamma-ray bursts
and are likely strong sources of gravitational
waves. In 2017, a direct detection (GW170817)
of the gravitational waves from such an event
was made, and gravitational waves have also
been indirectly detected in a system where
two neutron stars orbit each other.
In October 2018, astronomers reported that
GRB 150101B, a gamma-ray burst event detected
in 2015, may be directly related to the historic
GW170817 and associated with the merger of
two neutron stars. The similarities between
the two events, in terms of gamma ray, optical
and x-ray emissions, as well as to the nature
of the associated host galaxies, are "striking",
suggesting the two separate events may both
be the result of the merger of neutron stars,
and both may be a kilonova, which may be more
common in the universe than previously understood,
according to the researchers.
== Formation ==
Any main-sequence star with an initial mass
of above 8 times the mass of the sun (8 M☉)
has the potential to produce a neutron star.
As the star evolves away from the main sequence,
subsequent nuclear burning produces an iron-rich
core. When all nuclear fuel in the core has
been exhausted, the core must be supported
by degeneracy pressure alone. Further deposits
of mass from shell burning cause the core
to exceed the Chandrasekhar limit. Electron-degeneracy
pressure is overcome and the core collapses
further, sending temperatures soaring to over
5×109 K. At these temperatures, photodisintegration
(the breaking up of iron nuclei into alpha
particles by high-energy gamma rays) occurs.
As the temperature climbs even higher, electrons
and protons combine to form neutrons via electron
capture, releasing a flood of neutrinos. When
densities reach nuclear density of 4×1017
kg/m3, neutron degeneracy pressure halts the
contraction. The infalling outer envelope
of the star is halted and flung outwards by
a flux of neutrinos produced in the creation
of the neutrons, becoming a supernova. The
remnant left is a neutron star. If the remnant
has a mass greater than about 3 M☉, it collapses
further to become a black hole.As the core
of a massive star is compressed during a Type
II supernova, Type Ib or Type Ic supernova,
and collapses into a neutron star, it retains
most of its angular momentum. But, because
it has only a tiny fraction of its parent's
radius (and therefore its moment of inertia
is sharply reduced), a neutron star is formed
with very high rotation speed, and then over
a very long period it slows. Neutron stars
are known that have rotation periods from
about 1.4 ms to 30 s. The neutron star's density
also gives it very high surface gravity, with
typical values ranging from 1012 to 1013 m/s2
(more than 1011 times that of Earth). One
measure of such immense gravity is the fact
that neutron stars have an escape velocity
ranging from 100,000 km/s to 150,000 km/s,
that is, from a third to half the speed of
light. The neutron star's gravity accelerates
infalling matter to tremendous speed. The
force of its impact would likely destroy the
object's component atoms, rendering all the
matter identical, in most respects, to the
rest of the neutron star.
== Properties ==
=== 
Mass and temperature ===
A neutron star has a mass of at least 1.1
and perhaps up to 3 solar masses (M☉). The
maximum observed mass of neutron stars is
about 2.01 M☉. But in general, compact stars
of less than 1.39 M☉ (the Chandrasekhar
limit) are white dwarfs, whereas compact stars
with a mass between 1.4 M☉ and 3 M☉ (the
Tolman–Oppenheimer–Volkoff limit) should
be neutron stars (though there is an interval
of a few tenths of a solar mass where the
masses of low-mass neutron stars and high-mass
white dwarfs can overlap). Between 3 M☉
and 5 M☉, hypothetical intermediate-mass
stars such as quark stars and electroweak
stars have been proposed, but none have been
shown to exist. Beyond 10 M☉ the stellar
remnant will overcome the neutron degeneracy
pressure and gravitational collapse will usually
occur to produce a black hole, though the
smallest observed mass of a stellar black
hole is about 5 M☉.The temperature inside
a newly formed neutron star is from around
1011 to 1012 kelvin. However, the huge number
of neutrinos it emits carry away so much energy
that the temperature of an isolated neutron
star falls within a few years to around 106
kelvin. At this lower temperature, most of
the light generated by a neutron star is in
X-rays.
=== Density and pressure ===
Neutron stars have overall densities of 3.7×1017
to 5.9×1017 kg/m3 (2.6×1014 to 4.1×1014
times the density of the Sun), which is comparable
to the approximate density of an atomic nucleus
of 3×1017 kg/m3. The neutron star's density
varies from about 1×109 kg/m3 in the crust—increasing
with depth—to about 6×1017 or 8×1017 kg/m3
(denser than an atomic nucleus) deeper inside.
A neutron star is so dense that one teaspoon
(5 milliliters) of its material would have
a mass over 5.5×1012 kg, about 900 times
the mass of the Great Pyramid of Giza. In
the enormous gravitational field of a neutron
star, its weight would be 1.1×1025 N, which
is about 15 times the weight of the Moon.
The pressure increases from 3.2×1031 to 1.6×1034
Pa from the inner crust to the center.The
equation of state of matter at such high densities
is not precisely known because of the theoretical
difficulties associated with extrapolating
the likely behavior of quantum chromodynamics,
superconductivity, and superfluidity of matter
in such states along with the empirical difficulties
of observing the characteristics of neutron
stars that are at least hundreds of parsecs
away.
A neutron star has some of the properties
of an atomic nucleus, including density (within
an order of magnitude) and being composed
of nucleons. In popular scientific writing,
neutron stars are therefore sometimes described
as "giant nuclei". However, in other respects,
neutron stars and atomic nuclei are quite
different. A nucleus is held together by the
strong interaction, whereas a neutron star
is held together by gravity. The density of
a nucleus is uniform, while neutron stars
are predicted to consist of multiple layers
with varying compositions and densities.
=== Magnetic field ===
The magnetic field strength on the surface
of neutron stars ranges from c. 104 to 1011
tesla. These are orders of magnitude higher
than in any other object: for comparison,
a continuous 16 T field has been achieved
in the laboratory and is sufficient to levitate
a living frog due to diamagnetic levitation.
Variations in magnetic field strengths are
most likely the main factor that allows different
types of neutron stars to be distinguished
by their spectra, and explains the periodicity
of pulsars.The neutron stars known as magnetars
have the strongest magnetic fields, in the
range of 108 to 1011 tesla, and have become
the widely accepted hypothesis for neutron
star types soft gamma repeaters (SGRs) and
anomalous X-ray pulsars (AXPs). The magnetic
energy density of a 108 T field is extreme,
exceeding the mass-energy density of ordinary
matter. Fields of this strength are able to
polarize the vacuum to the point that the
vacuum becomes birefringent. Photons can merge
or split in two, and virtual particle-antiparticle
pairs are produced. The field changes electron
energy levels and atoms are forced into thin
cylinders. Unlike in an ordinary pulsar, magnetar
spin-down can be directly powered by its magnetic
field, and the magnetic field is strong enough
to stress the crust to the point of fracture.
Fractures of the crust cause starquakes, observed
as extremely luminous millisecond hard gamma
ray bursts. The fireball is trapped by the
magnetic field, and comes in and out of view
when the star rotates, which is observed as
a periodic soft gamma repeater (SGR) emission
with a period of 5–8 seconds and which lasts
for a few minutes.The origins of the strong
magnetic field are as yet unclear. One hypothesis
is that of "flux freezing", or conservation
of the original magnetic flux during the formation
of the neutron star. If an object has a certain
magnetic flux over its surface area, and that
area shrinks to a smaller area, but the magnetic
flux is conserved, then the magnetic field
would correspondingly increase. Likewise,
a collapsing star begins with a much larger
surface area than the resulting neutron star,
and conservation of magnetic flux would result
in a far stronger magnetic field. However,
this simple explanation does not fully explain
magnetic field strengths of neutron stars.
=== Gravity and equation of state ===
The gravitational field at a neutron star's
surface is about 2×1011 times stronger than
on Earth, at around 2.0×1012 m/s2. Such a
strong gravitational field acts as a gravitational
lens and bends the radiation emitted by the
neutron star such that parts of the normally
invisible rear surface become visible.
If the radius of the neutron star is 3GM/c2
or less, then the photons may be trapped in
an orbit, thus making the whole surface of
that neutron star visible from a single vantage
point, along with destabilizing photon orbits
at or below the 1 radius distance of the star.
A fraction of the mass of a star that collapses
to form a neutron star is released in the
supernova explosion from which it forms (from
the law of mass–energy equivalence, E = mc2).
The energy comes from the gravitational binding
energy of a neutron star.
Hence, the gravitational force of a typical
neutron star is huge. If an object were to
fall from a height of one meter on a neutron
star 12 kilometers in radius, it would reach
the ground at around 1400 kilometers per second.
However, even before impact, the tidal force
would cause spaghettification, breaking any
sort of an ordinary object into a stream of
material.
Because of the enormous gravity, time dilation
between a neutron star and Earth is significant.
For example, eight years could pass on the
surface of a neutron star, yet ten years would
have passed on Earth, not including the time-dilation
effect of its very rapid rotation.Neutron
star relativistic equations of state describe
the relation of radius vs. mass for various
models. The most likely radii for a given
neutron star mass are bracketed by models
AP4 (smallest radius) and MS2 (largest radius).
BE is the ratio of gravitational binding energy
mass equivalent to the observed neutron star
gravitational mass of "M" kilograms with radius
"R" meters,
B
E
=
0.60
β
1
−
β
2
{\displaystyle BE={\frac {0.60\,\beta }{1-{\frac
{\beta }{2}}}}}
β
=
G
M
/
R
c
2
{\displaystyle \beta \ =G\,M/R\,{c}^{2}}
Given current values
G
=
6.67408
×
10
−
11
m
3
kg
−
1
s
−
2
{\displaystyle G=6.67408\times 10^{-11}\,{\text{m}}^{3}{\text{kg}}^{-1}{\text{s}}^{-2}}
c
=
2.99792458
×
10
8
m
/
s
{\displaystyle c=2.99792458\times 10^{8}\,{\text{m}}/{\text{s}}}
M
⊙
=
1.98855
×
10
30
kg
{\displaystyle M_{\odot }=1.98855\times 10^{30}\,{\text{kg}}}
and star masses "M" commonly reported as multiples
of one solar mass,
M
x
=
M
M
⊙
{\displaystyle M_{x}={\frac {M}{M_{\odot }}}}
then the relativistic fractional binding energy
of a neutron star is
B
E
=
886.0
M
x
R
[
in meters
]
−
738.3
M
x
{\displaystyle BE={\frac {886.0\,M_{x}}{R_{\left[{\text{in
meters}}\right]}-738.3\,M_{x}}}}
A 2 M☉ neutron star would not be more compact
than 10,970 meters radius (AP4 model). Its
mass fraction gravitational binding energy
would then be 0.187, −18.7% (exothermic).
This is not near 0.6/2 = 0.3, −30%.
The equation of state for a neutron star is
not yet known. It is assumed that it differs
significantly from that of a white dwarf,
whose equation of state is that of a degenerate
gas that can be described in close agreement
with special relativity. However, with a neutron
star the increased effects of general relativity
can no longer be ignored. Several equations
of state have been proposed (FPS, UU, APR,
L, SLy, and others) and current research is
still attempting to constrain the theories
to make predictions of neutron star matter.
This means that the relation between density
and mass is not fully known, and this causes
uncertainties in radius estimates. For example,
a 1.5 M☉ neutron star could have a radius
of 10.7, 11.1, 12.1 or 15.1 kilometers (for
EOS FPS, UU, APR or L respectively).
== Structure ==
Current understanding of the structure of
neutron stars is defined by existing mathematical
models, but it might be possible to infer
some details through studies of neutron-star
oscillations. Asteroseismology, a study applied
to ordinary stars, can reveal the inner structure
of neutron stars by analyzing observed spectra
of stellar oscillations.Current models indicate
that matter at the surface of a neutron star
is composed of ordinary atomic nuclei crushed
into a solid lattice with a sea of electrons
flowing through the gaps between them. It
is possible that the nuclei at the surface
are iron, due to iron's high binding energy
per nucleon. It is also possible that heavy
elements, such as iron, simply sink beneath
the surface, leaving only light nuclei like
helium and hydrogen. If the surface temperature
exceeds 106 kelvin (as in the case of a young
pulsar), the surface should be fluid instead
of the solid phase that might exist in cooler
neutron stars (temperature <106 kelvin).The
"atmosphere" of a neutron star is hypothesized
to be at most several micrometers thick, and
its dynamics are fully controlled by the neutron
star's magnetic field. Below the atmosphere
one encounters a solid "crust". This crust
is extremely hard and very smooth (with maximum
surface irregularities of ~5 mm), due to the
extreme gravitational field.Proceeding inward,
one encounters nuclei with ever-increasing
numbers of neutrons; such nuclei would decay
quickly on Earth, but are kept stable by tremendous
pressures. As this process continues at increasing
depths, the neutron drip becomes overwhelming,
and the concentration of free neutrons increases
rapidly. In that region, there are nuclei,
free electrons, and free neutrons. The nuclei
become increasingly small (gravity and pressure
overwhelming the strong force) until the core
is reached, by definition the point where
mostly neutrons exist. The expected hierarchy
of phases of nuclear matter in the inner crust
has been characterized as "nuclear pasta",
with fewer voids and larger structures towards
higher pressures.
The composition of the superdense matter in
the core remains uncertain. One model describes
the core as superfluid neutron-degenerate
matter (mostly neutrons, with some protons
and electrons). More exotic forms of matter
are possible, including degenerate strange
matter (containing strange quarks in addition
to up and down quarks), matter containing
high-energy pions and kaons in addition to
neutrons, or ultra-dense quark-degenerate
matter.
== Radiation ==
=== 
Pulsars ===
Neutron stars are detected from their electromagnetic
radiation. Neutron stars are usually observed
to pulse radio waves and other electromagnetic
radiation, and neutron stars observed with
pulses are called pulsars.
Pulsars' radiation is thought to be caused
by particle acceleration near their magnetic
poles, which need not be aligned with the
rotational axis of the neutron star. It is
thought that a large electrostatic field builds
up near the magnetic poles, leading to electron
emission. These electrons are magnetically
accelerated along the field lines, leading
to curvature radiation, with the radiation
being strongly polarized towards the plane
of curvature. In addition, high energy photons
can interact with lower energy photons and
the magnetic field for electron-positron pair
production, which through electron–positron
annihilation leads to further high energy
photons.The radiation emanating from the magnetic
poles of neutron stars can be described as
magnetospheric radiation, in reference to
the magnetosphere of the neutron star. It
is not to be confused with magnetic dipole
radiation, which is emitted because the magnetic
axis is not aligned with the rotational axis,
with a radiation frequency the same as the
neutron star's rotational frequency.If the
axis of rotation of the neutron star is different
to the magnetic axis, external viewers will
only see these beams of radiation whenever
the magnetic axis point towards them during
the neutron star rotation. Therefore, periodic
pulses are observed, at the same rate as the
rotation of the neutron star.
=== Non-pulsating neutron stars ===
In addition to pulsars, neutron stars have
also been identified with no apparent periodicity
of their radiation. This seems to be a characteristic
of the X-ray sources known as Central Compact
Objects in Supernova remnants (CCOs in SNRs),
which are thought to be young, radio-quiet
isolated neutron stars.
=== Spectra ===
In addition to radio emissions, neutron stars
have also been identified in other parts of
the electromagnetic spectrum. This includes
visible light, near infrared, ultraviolet,
X-rays and gamma rays. Pulsars observed in
X-rays are known as X-ray pulsars if accretion-powered;
while those identified in visible light as
optical pulsars. The majority of neutron stars
detected, including those identified in optical,
X-ray and gamma rays, also emit radio waves;
the Crab Pulsar produces electromagnetic emissions
across the spectrum. However, there exist
neutron stars called radio-quiet neutron stars,
with no radio emissions detected.
== Rotation ==
Neutron stars rotate extremely rapidly after
their formation due to the conservation of
angular momentum; like spinning ice skaters
pulling in their arms, the slow rotation of
the original star's core speeds up as it shrinks.
A newborn neutron star can rotate many times
a second.
=== Spin down ===
Over time, neutron stars slow, as their rotating
magnetic fields in effect radiate energy associated
with the rotation; older neutron stars may
take several seconds for each revolution.
This is called spin down. The rate at which
a neutron star slows its rotation is usually
constant and very small.
The periodic time (P) is the rotational period,
the time for one rotation of a neutron star.
The spin-down rate, the rate of slowing of
rotation, is then given the symbol
P
˙
{\displaystyle {\dot {P}}}
(P-dot), the negative derivative of P with
respect to time. It is defined as periodic
time decrease per unit time; it is a dimensionless
quantity, but can be given the units of s⋅s−1
(seconds per second).The spin-down rate (P-dot)
of neutron stars usually falls within the
range of 10−22 to 10−9 s⋅s−1, with
the shorter period (or faster rotating) observable
neutron stars usually having smaller P-dot.
However, as a neutron star ages, the neutron
star slows (P increases) and the rate of slowing
decreases (P-dot decreases). Eventually, the
rate of rotation becomes too slow to power
the radio-emission mechanism, and the neutron
star can no longer be detected.P and P-dot
allow minimum magnetic fields of neutron stars
to be estimated. P and P-dot can be also used
to calculate the characteristic age of a pulsar,
but gives an estimate which is somewhat larger
than the true age when it is applied to young
pulsars.P and P-dot can also be combined with
neutron star's moment of inertia to estimate
a quantity called spin-down luminosity, which
is given the symbol
E
˙
{\displaystyle {\dot {E}}}
(E-dot). It is not the measured luminosity,
but rather the calculated loss rate of rotational
energy that would manifest itself as radiation.
For neutron stars where the spin-down luminosity
is comparable to the actual luminosity, the
neutron stars are said to be "rotation powered".
The observed luminosity of the Crab Pulsar
is comparable to the spin-down luminosity,
supporting the model that rotational kinetic
energy powers the radiation from it. With
neutron stars such as magnetars, where the
actual luminosity exceeds the spin-down luminosity
by about a factor of one hundred, it is assumed
that the luminosity is powered by magnetic
dissipation, rather than being rotation powered.P
and P-dot can also be plotted for neutron
stars to create a P–P-dot diagram. It encodes
a tremendous amount of information about the
pulsar population and its properties, and
has been likened to the Hertzsprung–Russell
diagram in its importance for neutron stars.
=== Spin up ===
Neutron star rotational speeds can increase,
a process known as spin up. Sometimes neutron
stars absorb orbiting matter from companion
stars, increasing the rotation rate and reshaping
the neutron star into an oblate spheroid.
This causes an increase in the rate of rotation
of the neutron star of over a hundred times
per second in the case of millisecond pulsars.
The most rapidly rotating neutron star currently
known, PSR J1748-2446ad, rotates at 716 revolutions
per second. A 2007 paper reported the detection
of an X-ray burst oscillation, which provides
an indirect measure of spin, of 1122 Hz from
the neutron star XTE J1739-285, suggesting
1122 rotations a second. However, at present,
this signal has only been seen once, and should
be regarded as tentative until confirmed in
another burst from that star.
=== Glitches and starquakes ===
Sometimes a neutron star will undergo a glitch,
a sudden small increase of its rotational
speed or spin up. Glitches are thought to
be the effect of a starquake—as the rotation
of the neutron star slows, its shape becomes
more spherical. Due to the stiffness of the
"neutron" crust, this happens as discrete
events when the crust ruptures, creating a
starquake similar to earthquakes. After the
starquake, the star will have a smaller equatorial
radius, and because angular momentum is conserved,
its rotational speed has increased.
Starquakes occurring in magnetars, with a
resulting glitch, is the leading hypothesis
for the gamma-ray sources known as soft gamma
repeaters.Recent work, however, suggests that
a starquake would not release sufficient energy
for a neutron star glitch; it has been suggested
that glitches may instead be caused by transitions
of vortices in the theoretical superfluid
core of the neutron star from one metastable
energy state to a lower one, thereby releasing
energy that appears as an increase in the
rotation rate.
=== "Anti-glitches" ===
An "anti-glitch", a sudden small decrease
in rotational speed, or spin down, of a neutron
star has also been reported. It occurred in
a magnetar, that in one case produced an X-ray
luminosity increase of a factor of 20, and
a significant spin-down rate change. Current
neutron star models do not predict this behavior.
If the cause was internal, it suggests differential
rotation of solid outer crust and the superfluid
component of the magnetar's inner structure.
== Population and distances ==
At present, there are about 2000 known neutron
stars in the Milky Way and the Magellanic
Clouds, the majority of which have been detected
as radio pulsars. Neutron stars are mostly
concentrated along the disk of the Milky Way
although the spread perpendicular to the disk
is large because the supernova explosion process
can impart high translational speeds (400
km/s) to the newly formed neutron star.
Some of the closest known neutron stars are
RX J1856.5-3754, which is about 400 light
years from Earth, and PSR J0108-1431 at about
424 light years. RX J1856.5-3754 is a member
of a close group of neutron stars called The
Magnificent Seven. Another nearby neutron
star that was detected transiting the backdrop
of the constellation Ursa Minor has been nicknamed
Calvera by its Canadian and American discoverers,
after the villain in the 1960 film The Magnificent
Seven. This rapidly moving object was discovered
using the ROSAT/Bright Source Catalog.
== Binary neutron star systems ==
About 5% of all known neutron stars are members
of a binary system. The formation and evolution
of binary neutron stars can be a complex process.
Neutron stars have been observed in binaries
with ordinary main-sequence stars, red giants,
white dwarfs or other neutron stars. According
to modern theories of binary evolution it
is expected that neutron stars also exist
in binary systems with black hole companions.
The merger of binaries containing two neutron
stars, or a neutron star and a black hole,
are expected to be prime sources for the emission
of detectable gravitational waves.
=== X-ray binaries ===
Binary systems containing neutron stars often
emit X-rays, which are emitted by hot gas
as it falls towards the surface of the neutron
star. The source of the gas is the companion
star, the outer layers of which can be stripped
off by the gravitational force of the neutron
star if the two stars are sufficiently close.
As the neutron star accretes this gas its
mass can increase; if enough mass is accreted
the neutron star may collapse into a black
hole.
=== Neutron star binary mergers and nucleosynthesis
===
Binaries containing two neutron stars are
observed to shrink as gravitational waves
are emitted. Ultimately the neutron stars
will come into contact and coalesce.
The coalescence of binary neutron stars is
one of the leading models for the origin of
short gamma-ray bursts. Strong evidence for
this model came from the observation of a
kilonova associated with the short-duration
gamma-ray burst GRB 130603B, and finally confirmed
by detection of gravitational wave GW170817
and short GRB 170817A by LIGO, Virgo and 70
observatories covering the electromagnetic
spectrum observed the event. The light emitted
in the kilonova is believed to come from the
radioactive decay of material ejected in the
merger of the two neutron stars. This material
may be responsible for the production of many
of the chemical elements beyond iron, as opposed
to the supernova nucleosynthesis theory.
== Planets ==
Neutron stars can host exoplanets. These can
be original, circumbinary, captured, or the
result of a second round of planet formation.
Pulsars can also strip the atmosphere off
from a star, leaving a planetary-mass remnant,
which may be understood as a chthonian planet
or a stellar object depending on interpretation.
For pulsars, such pulsar planets can be detected
with the pulsar timing method, which allows
for high precision and detection of much smaller
planets than with other methods. Two systems
have been definitively confirmed. The first
exoplanets ever to be detected were the three
planets Draugr, Poltergeist and Phobetor around
PSR B1257+12, discovered in 1992–1994. Of
these, Draugr is the smallest exoplanet ever
detected, at a mass of twice that of the Moon.
Another system is PSR B1620-26, where a circumbinary
planet orbits a neutron star-white dwarf binary
system. Also, there are several unconfirmed
candidates. Pulsar planets receive little
visible light, but massive amounts of ionizing
radiation and high-energy stellar wind, which
makes them rather hostile environments.
== History of discoveries ==
At the meeting of the American Physical Society
in December 1933 (the proceedings were published
in January 1934), Walter Baade and Fritz Zwicky
proposed the existence of neutron stars, less
than two years after the discovery of the
neutron by Sir James Chadwick. In seeking
an explanation for the origin of a supernova,
they tentatively proposed that in supernova
explosions ordinary stars are turned into
stars that consist of extremely closely packed
neutrons that they called neutron stars. Baade
and Zwicky correctly proposed at that time
that the release of the gravitational binding
energy of the neutron stars powers the supernova:
"In the supernova process, mass in bulk is
annihilated". Neutron stars were thought to
be too faint to be detectable and little work
was done on them until November 1967, when
Franco Pacini pointed out that if the neutron
stars were spinning and had large magnetic
fields, then electromagnetic waves would be
emitted. Unbeknown to him, radio astronomer
Antony Hewish and his research assistant Jocelyn
Bell at Cambridge were shortly to detect radio
pulses from stars that are now believed to
be highly magnetized, rapidly spinning neutron
stars, known as pulsars.
In 1965, Antony Hewish and Samuel Okoye discovered
"an unusual source of high radio brightness
temperature in the Crab Nebula". This source
turned out to be the Crab Pulsar that resulted
from the great supernova of 1054.
In 1967, Iosif Shklovsky examined the X-ray
and optical observations of Scorpius X-1 and
correctly concluded that the radiation comes
from a neutron star at the stage of accretion.In
1967, Jocelyn Bell Burnell and Antony Hewish
discovered regular radio pulses from PSR B1919+21.
This pulsar was later interpreted as an isolated,
rotating neutron star. The energy source of
the pulsar is the rotational energy of the
neutron star. The majority of known neutron
stars (about 2000, as of 2010) have been discovered
as pulsars, emitting regular radio pulses.
In 1971, Riccardo Giacconi, Herbert Gursky,
Ed Kellogg, R. Levinson, E. Schreier, and
H. Tananbaum discovered 4.8 second pulsations
in an X-ray source in the constellation Centaurus,
Cen X-3. They interpreted this as resulting
from a rotating hot neutron star. The energy
source is gravitational and results from a
rain of gas falling onto the surface of the
neutron star from a companion star or the
interstellar medium.
In 1974, Antony Hewish was awarded the Nobel
Prize in Physics "for his decisive role in
the discovery of pulsars" without Jocelyn
Bell who shared in the discovery.In 1974,
Joseph Taylor and Russell Hulse discovered
the first binary pulsar, PSR B1913+16, which
consists of two neutron stars (one seen as
a pulsar) orbiting around their center of
mass. Einstein's general theory of relativity
predicts that massive objects in short binary
orbits should emit gravitational waves, and
thus that their orbit should decay with time.
This was indeed observed, precisely as general
relativity predicts, and in 1993, Taylor and
Hulse were awarded the Nobel Prize in Physics
for this discovery.In 1982, Don Backer and
colleagues discovered the first millisecond
pulsar, PSR B1937+21. This object spins 642
times per second, a value that placed fundamental
constraints on the mass and radius of neutron
stars. Many millisecond pulsars were later
discovered, but PSR B1937+21 remained the
fastest-spinning known pulsar for 24 years,
until PSR J1748-2446ad (which spins more than
700 times a second) was discovered.
In 2003, Marta Burgay and colleagues discovered
the first double neutron star system where
both components are detectable as pulsars,
PSR J0737-3039. The discovery of this system
allows a total of 5 different tests of general
relativity, some of these with unprecedented
precision.
In 2010, Paul Demorest and colleagues measured
the mass of the millisecond pulsar PSR J1614–2230
to be 1.97±0.04 M☉, using Shapiro delay.
This was substantially higher than any previously
measured neutron star mass (1.67 M☉, see
PSR J1903+0327), and places strong constraints
on the interior composition of neutron stars.
In 2013, John Antoniadis and colleagues measured
the mass of PSR J0348+0432 to be 2.01±0.04
M☉, using white dwarf spectroscopy. This
confirmed the existence of such massive stars
using a different method. Furthermore, this
allowed, for the first time, a test of general
relativity using such a massive neutron star.
In August 2017, LIGO and Virgo made first
detection of gravitational waves produced
by colliding neutron stars.In October 2018,
astronomers reported that GRB 150101B, a gamma-ray
burst event detected in 2015, may be directly
related to the historic GW170817 and associated
with the merger of two neutron stars. The
similarities between the two events, in terms
of gamma ray, optical and x-ray emissions,
as well as to the nature of the associated
host galaxies, are "striking", suggesting
the two separate events may both be the result
of the merger of neutron stars, and both may
be a kilonova, which may be more common in
the universe than previously understood, according
to the researchers.
== Subtypes table ==
Neutron star
Isolated neutron star (INS): not in a binary
system.
Rotation-powered pulsar (RPP or "radio pulsar"):
neutron stars that emit directed pulses of
radiation towards us at regular intervals
(due to their strong magnetic fields).
Rotating radio transient (RRATs): are thought
to be pulsars which emit more sporadically
and/or with higher pulse-to-pulse variability
than the bulk of the known pulsars.
Magnetar: a neutron star with an extremely
strong magnetic field (1000 times more than
a regular neutron star), and long rotation
periods (5 to 12 seconds).
Soft gamma repeater (SGR).
Anomalous X-ray pulsar (AXP).
Radio-quiet neutron stars.
X-ray dim isolated neutron stars.
Central compact objects in supernova remnants
(CCOs in SNRs): young, radio-quiet non-pulsating
X-ray sources, thought to be Isolated Neutron
Stars surrounded by supernova remnants.
X-ray pulsars or "accretion-powered pulsars":
a class of X-ray binaries.
Low-mass X-ray binary pulsars: a class of
low-mass X-ray binaries (LMXB), a pulsar with
a main sequence star, white dwarf or red giant.
Millisecond pulsar (MSP) ("recycled pulsar").
Sub-millisecond pulsar.
X-ray burster: a neutron star with a low mass
binary companion from which matter is accreted
resulting in irregular bursts of energy from
the surface of the neutron star.
Intermediate-mass X-ray binary pulsars: a
class of intermediate-mass X-ray binaries
(IMXB), a pulsar with an intermediate mass
star.
High-mass X-ray binary pulsars: a class of
high-mass X-ray binaries (HMXB), a pulsar
with a massive star.
Binary pulsars: a pulsar with a binary companion,
often a white dwarf or neutron star.
Theorized compact stars with similar properties.
Protoneutron star (PNS), theorized.
Exotic star
Quark star: currently a hypothetical type
of neutron star composed of quark matter,
or strange matter. As of 2018, there are three
candidates.
Electroweak star: currently a hypothetical
type of extremely heavy neutron star, in which
the quarks are converted to leptons through
the electroweak force, but the gravitational
collapse of the neutron star is prevented
by radiation pressure. As of 2018, there is
no evidence for their existence.
Preon star: currently a hypothetical type
of neutron star composed of preon matter.
As of 2018, there is no evidence for the existence
of preons.
== Examples of neutron stars ==
RX J0806.4-4123 – neutron star source of
infrared radiation.
PSR J0108-1431 – closest neutron star
LGM-1 – the first recognized radio-pulsar
PSR B1257+12 – the first neutron star discovered
with planets (a millisecond pulsar)
SWIFT J1756.9-2508 – a millisecond pulsar
with a stellar-type companion with planetary
range mass (below brown dwarf)
PSR B1509-58 source of the "Hand of God" photo
shot by the Chandra X-ray Observatory.
PSR J0348+0432 – the most massive neutron
star with a well-constrained mass, 2.01 ± 0.04
M☉.
== Gallery ==
=== Video – animation ===
== 
See also ==
Dragon's Egg
Neutronium
Preon-degenerate matter
Rotating radio transient
== Notes
