A neutron star is a type of stellar remnant
that can result from the gravitational collapse
of a massive star during a Type II, Type Ib
or Type Ic supernova event. Neutron stars
are the densest and tiniest stars known to
exist in the universe; although having only
the radius of about 10 km, they may have
a mass of a few times that of the Sun. Neutron
stars probably appear white to the naked eye.
Neutron stars are the end points of stars
whose inert core's mass after nuclear burning
is greater than the Chandrasekhar limit for
white dwarfs, but whose mass is not great
enough to overcome the neutron degeneracy
pressure to become black holes. Such stars
are composed almost entirely of neutrons,
which are subatomic particles without net
electrical charge and with slightly larger
mass than protons. Neutron stars are very
hot and are supported against further collapse
by quantum degeneracy pressure due to the
phenomenon described by the Pauli exclusion
principle. This principle states that no two
neutrons can occupy the same place and quantum
state simultaneously.
The discovery of pulsars in 1967 suggested
that neutron stars exist. Born in supernova
explosions, these bodies are only ~12-13 kilometers
by radius. In contrast, the Sun's radius is
about 60,000 times that. They spin around
as rapidly as 716 times a second, or approximately
43,000 revolutions per minute. A typical neutron
star has a mass between ~1.4 and about 2 solar
masses with a surface temperature of ~6 x
105 Kelvin . Neutron stars have overall densities
of 3.7×1017 to 5.9×1017 kg/m3, which is
comparable to the approximate density of an
atomic nucleus of 3×1017 kg/m3. The neutron
star's density varies from below 1×109 kg/m3
in the crust - increasing with depth - to
above 6×1017 or 8×1017 kg/m3 deeper inside.
This density is approximately equivalent to
the mass of a Boeing 747 compressed to the
size of a small grain of sand. A normal-sized
matchbox containing neutron star material
would have a mass of approximately 5 billion
tonnes or ~1 km³ of Earth rock.
In general, compact stars of less than 1.44 solar
masses – the Chandrasekhar limit – are
white dwarfs and a compact star weighing between
that and 3 solar masses should be a neutron
star. The maximum observed mass of neutron
stars is about 2 solar masses. Gravitational
collapse will usually occur on any compact
star between 10 and 25 solar masses and produce
a black hole. The smallest observed mass of
a black hole is about 3.8 solar masses. Between
these, hypothetical intermediate-mass stars
such as quark stars and electroweak stars
have been proposed, but none have been shown
to exist. The equations of state of matter
at such high densities are not precisely known
because of the theoretical and empirical difficulties.
Some neutron stars rotate very rapidly and
emit beams of electromagnetic radiation as
pulsars. Gamma-ray bursts may be produced
from rapidly rotating, high-mass stars that
collapse to form a neutron star, or from the
merger of binary neutron stars. There are
thought to be on the order of 108 neutron
stars in the galaxy, but they can only be
easily detected in certain instances, such
as if they are a pulsar or part of a binary
system. Non-rotating and non-accreting neutron
stars are virtually undetectable; however,
the Hubble Space Telescope has observed one
thermally radiating neutron star, called RX
J185635-3754.
Formation
Any star with an initial main-sequence mass
of around 10 solar masses or above has the
potential to become 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 material 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 billion Kelvin. At these temperatures,
photodisintegration occurs. As the temperature
climbs even higher, electrons and protons
combine to form neutrons, releasing a flood
of neutrinos. When densities reach nuclear
density of 4 x 1017 kilograms per cubic meter,
neutron degeneracy pressure halts the contraction.
The infalling outer atmosphere of the star
is flung outwards, becoming a Type II or Type
Ib supernova. The remnant left is a neutron
star. If it has a mass greater than about
5 solar masses, it collapses further to become
a black hole. Other neutron stars are formed
within close binaries.
As the core of a massive star is compressed
during a supernova, and collapses into a neutron
star, it retains most of its angular momentum.
Since it has only a tiny fraction of its parent's
radius, a neutron star is formed with very
high rotation speed, and then gradually slows
down. Neutron stars are known that have rotation
periods from about 1.4 ms to 30 seconds. The
neutron star's density also gives it very
high surface gravity, up to ~ 1013 m/s2 with
typical values of a few ×1012 m/s2. One measure
of such immense gravity is the fact that neutron
stars have an escape velocity of around 100,000
km/s, about a third of the speed of light.
Matter falling onto the surface of a neutron
star would be accelerated to tremendous speed
by the star's gravity. The force of impact
would likely destroy the object's component
atoms, rendering all its matter identical,
in most respects, to the rest of the star.
Properties
The surface of a neutron star is made of iron.
In the presence of a strong magnetic field
the atoms of iron polymerize. The polymers
pack to form a lattice with density about
ten thousand times that of terrestrial iron
and strength a million times that of steel.
It has excellent electrical conductivity along
the direction of the magnetic field, but is
a good insulator perpendicular to this direction.
Immediately beneath this surface the neutron
star is still solid, but its composition is
changing. Larger nuclei, particularly rich
in neutrons, are formed, and materials that
on Earth would be radioactive are stable in
this environment. With increasing depth, the
density rises. When its density reaches 400
billion times that of water, the nuclei can
get no larger and neutrons start ‘dripping’
out. As the density increases further, the
nuclei dissolve in a sea of neutrons. The
neutron fluid is a superfluid – it has no
viscosity and no resistance to flow or movement.
Within a few kilometres of the surface the
density has reached the density of the atomic
nucleus. Up to this point the properties of
matter are reasonably well understood, but
beyond it understanding becomes increasingly
difficult. The composition of the core of
the star is particularly uncertain: it may
be liquid or solid; it may consist of other
nuclear particles; and there may be another
phase change, where quarks start ‘dripping’
out of the neutrons, forming another liquid.
A neutron star has a mass comparable to that
of the Sun, but as it is only about 10 km
in radius, it has an average density 1 quadrillion
times that of water. Such a large mass in
such a small volume produces an intense gravitational
force: objects weigh 100 billion times more
on the surface of a neutron star than on the
surface of the Earth. The intense gravitational
field affects light and other electromagnetic
radiation emitted by the star, producing significant
redshift. The strong gravitational attraction
allows neutron stars to spin rapidly without
disintegrating. Such spin rates are expected
if the core of the original star collapses
without loss of angular momentum - if the
original star has a magnetic field, then this
too may be conserved and concentrated in the
collapse to a neutron star. Pulsars, gamma-ray
burst sources, and the neutron stars in some
X-ray binaries are believed to have magnetic
fields with a strength of about 100 million
teslas.
The gravitational field at the star's surface
is about 2×1011 times stronger than on Earth.
Such a strong gravitational field acts as
a gravitational lens and bends the radiation
emitted by the star such that parts of the
normally invisible rear surface become visible.
If the radius of the neutron star is or less,
then the photons may be trapped in an orbit,
thus making the whole surface of that neutron
star visible, along with destabilizing orbits
at that and less than that of the radius.
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. The
energy comes from the gravitational binding
energy of a neutron star.
Neutron star relativistic equations of state
provided by Jim Lattimer include a graph of
radius vs. mass for various models. The most
likely radii for a given neutron star mass
are bracketed by models AP4 and MS2. BE is
the ratio of gravitational binding energy
mass equivalent to observed neutron star gravitational
mass of "M" kilograms with radius "R" meters,
      
Given current values
and star masses "M" commonly reported as multiples
of one solar mass,
then the relativistic fractional binding energy
of a neutron star is
A two-solar-mass neutron star would not be
more compact than 10,970 meters radius. Its
mass fraction gravitational binding energy
would then be 0.187, −18.7%. This is not
near 0.6/2 = 0.3, −30%.
A neutron star is so dense that one teaspoon
of its material would have a mass over 5.5×1012 kg,
about 900 times the mass of the Great Pyramid
of Giza. Hence, the gravitational force of
a typical neutron star is such that if an
object were to fall from a height of one meter,
it would only take one microsecond to hit
the surface of the neutron star, and would
do so at around 2000 kilometers per second,
or 7.2 million kilometers per hour.
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 falls
within a few years to around 106 kelvin. Even
at 1 million kelvin, most of the light generated
by a neutron star is in X-rays. In visible
light, neutron stars probably radiate approximately
the same energy in all parts of visible spectrum,
and therefore appear white.
The pressure increases from 3×1033 to 1.6×1035
Pa from the inner crust to the center.
The equation of state for a neutron star is
still not known. It is assumed that it differs
significantly from that of a white dwarf,
whose EOS is that of a degenerate gas which
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 EOS have been
proposed 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 solar mass neutron
star could have a radius of 10.7, 11.1, 12.1
or 15.1 kilometres.
Structure
Current understanding of the structure of
neutron stars is defined by existing mathematical
models, but it might be possible to infer
through studies of neutron-star oscillations.
Similar to asteroseismology for ordinary stars,
the inner structure might be derived by analyzing
observed frequency spectra of stellar oscillations.
On the basis of current models, the 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 element cores,
such as iron, simply sink beneath the surface,
leaving only light nuclei like helium and
hydrogen cores. If the surface temperature
exceeds 106 kelvin, the surface should be
fluid instead of the solid phase observed
in cooler neutron stars.
The "atmosphere" of the star is hypothesized
to be at most several micrometers thick, and
its dynamic is fully controlled by the star's
magnetic field. Below the atmosphere one encounters
a solid "crust". This crust is extremely hard
and very smooth, because of 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, neutron
drip becomes overwhelming, and the concentration
of free neutrons increases rapidly. In this
region, there are nuclei, free electrons,
and free neutrons. The nuclei become increasingly
small until the core is reached, by definition
the point where they disappear altogether.
The composition of the superdense matter in
the core remains uncertain. One model describes
the core as superfluid neutron-degenerate
matter. More exotic forms of matter are possible,
including degenerate strange matter, matter
containing high-energy pions and kaons in
addition to neutrons, or ultra-dense quark-degenerate
matter.
History of discoveries
In 1934, Walter Baade and Fritz Zwicky proposed
the existence of the neutron star, only a
year 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 Nebula neutron star
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 and Antony Hewish discovered
regular radio pulses from CP 1919. 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 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 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 objects 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+12 remained the fastest-spinning
known pulsar for 24 years, until PSR J1748-2446ad
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 solar masses, using Shapiro
delay. This was substantially higher than
any previously measured neutron star mass,
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
solar masses, 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.
Rotation
Neutron stars rotate extremely rapidly after
their creation 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 several
times a second; sometimes, the neutron star
absorbs orbiting matter from a companion star,
increasing the rotation to several hundred
times per second, reshaping the neutron star
into an oblate spheroid.
Over time, neutron stars slow down because
their rotating magnetic fields radiate energy;
older neutron stars may take several seconds
for each revolution.
The rate at which a neutron star slows its
rotation is usually constant and very small:
the observed rates of decline are between
10−10 and 10−21 seconds for each rotation.
Therefore, for a typical slow down rate of
10−15 seconds per rotation, a neutron star
now rotating in 1 second will rotate in 1.000003
seconds after a century, or 1.03 seconds after
1 million years.
Sometimes a neutron star will spin up or undergo
a glitch, a sudden small increase of its rotation
speed. Glitches are thought to be the effect
of a starquake — as the rotation of the
star slows down, the shape becomes more spherical.
Due to the stiffness of the "neutron" crust,
this happens as discrete events when the crust
ruptures, similar to tectonic earthquakes.
After the starquake, the star will have a
smaller equatorial radius, and since angular
momentum is conserved, rotational speed increases.
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 superfluid core of the
star from one metastable energy state to a
lower one.
Neutron stars have been observed to "pulse"
radio and x-ray emissions believed to be caused
by particle acceleration near the magnetic
poles, which need not be aligned with the
rotation axis of the star. Through mechanisms
not yet entirely understood, these particles
produce coherent beams of radio emission.
External viewers see these beams as pulses
of radiation whenever the magnetic pole sweeps
past the line of sight. The pulses come at
the same rate as the rotation of the neutron
star, and thus, appear periodic. Neutron stars
which emit such pulses are called pulsars.
The most rapidly rotating neutron star currently
known, PSR J1748-2446ad, rotates at 716 rotations
per second. A recent paper reported the detection
of an X-ray burst oscillation at 1122 Hz
from the neutron star XTE J1739-285. However,
at present, this signal has only been seen
once, and should be regarded as tentative
until confirmed in another burst from this
star.
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 most 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 speeds to the newly created neutron
star.
Some of the closest neutron stars are RX J1856.5-3754
about 400 light years away and PSR J0108-1431
at about 424 light years. RX J1856.5-3754
is members 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 stars
About 5% of all known neutron stars are members
of a binary system. The formation and evolution
scenario of binary neutron stars is a rather
exotic and complicated process. The companion
stars may be either ordinary stars, 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.
Such binaries are expected to be prime sources
for emitting gravitational waves. Neutron
stars in binary systems often emit X-rays
which is caused by the heating of material
accreted from the companion star. Material
from the outer layers of a companion star
is sucked towards the neutron star as a result
of its very strong gravitational field. As
a result of this process binary neutron stars
may also coalesce into black holes if the
accretion of mass takes place under extreme
conditions. It has been proposed that coalescence
of binaries consisting of two neutron stars
may be responsible for producing short gamma-ray
bursts. Such events may also be responsible
for creating all chemical elements beyond
iron, as opposed to the supernova nucleosynthesis
theory.
Subtypes
Neutron star
Protoneutron star, theorized.
Radio-quiet neutron stars
Radio loud neutron star
Single pulsars–general term for neutron
stars that emit directed pulses of radiation
towards us at regular intervals.
Rotation-powered pulsar
Magnetar–a neutron star with an extremely
strong magnetic field, and long rotation periods.
Soft gamma repeater
Anomalous X-ray pulsar
Binary pulsars
Low-mass X-ray binaries
Intermediate-mass X-ray binaries
High-mass X-ray binaries
Accretion-powered 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.
Millisecond pulsar
Sub-millisecond pulsar
Exotic star
Quark star–currently a hypothetical type
of neutron star composed of quark matter,
or strange matter. As of 2008, 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 star is prevented by radiation
pressure. As of 2010, there is no evidence
for their existence.
Preon star–currently a hypothetical type
of neutron star composed of preon matter.
As of 2008, there is no evidence for the existence
of preons.
Giant nucleus
A neutron star has some of the properties
of an atomic nucleus, including density 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. In particular, a nucleus
is held together by the strong interaction,
whereas a neutron star is held together by
gravity, and thus the density and structure
of neutron stars is more variable. It is generally
more useful to consider such objects as stars.
Examples of neutron stars
PSR J0108-1431 – closest neutron star
LGM-1 – the first recognized radio-pulsar
PSR B1257+12 – the first neutron star discovered
with planets
SWIFT J1756.9-2508 – a millisecond pulsar
with a stellar-type companion with planetary
range mass
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, .
See also
Notes
References
External links
Introduction to neutron stars
Neutron Stars for Undergraduates and its Errata
NASA on pulsars
"NASA Sees Hidden Structure Of Neutron Star
In Starquake". SpaceDaily.com. April 26, 2006
"Mysterious X-ray sources may be lone neutron
stars". New Scientist.
"Massive neutron star rules out exotic matter".
New Scientist. According to a new analysis,
exotic states of matter such as free quarks
or BECs do not arise inside neutron stars.
"Neutron star clocked at mind-boggling velocity".
New Scientist. A neutron star has been clocked
traveling at more than 1500 kilometers per
second.
