Neutron star
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A neutron star is a type of remnant
that can result from the gravitational
collapse of a massive star
during a Type
II, Type
Ib or Type Ic supernova
event. Such stars are composed almost entirely of neutrons,
which are subatomic particles without electrical
charge and roughly the same mass as protons.
Neutron stars are very hot and are supported against further collapse
because of the Pauli
exclusion principle. This principle states that no two
neutrons (or any other fermionic
particle) can occupy the same place and quantum
state simultaneously.
A typical neutron star has a mass
between 1.35 and about 2.1 solar
masses, with a corresponding radius
of about 12 km
if the Akmal-Pandharipande-Ravenhall (APR) Equation
of state (EOS) is used.[1][2]
In contrast, the Sun's
radius is about 60,000 times that. Neutron stars have overall
densities predicted by the APR EOS of 3.7×1017 to
5.9×1017 kg/m3 (2.6×1014 to
4.1×1014 times the density of the sun),[3]
which compares with the approximate density of an atomic
nucleus of 3×1017 kg/m3.[4]
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.[5]
This density is approximately equivalent to the mass of the entire
human population compressed into the size of a sugar cube.
In general, compact stars of less than 1.44 solar masses, the
Chandrasekhar
limit, are white
dwarfs; above 2 to 3 solar masses (the
Tolman-Oppenheimer-Volkoff
limit), a quark
star might be created, however this is uncertain.
Gravitational
collapse will always occur on any compact
star over 5 solar masses, inevitably producing a black
hole.[citation
needed]
Formation
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 (and therefore its moment
of inertia is sharply reduced), a neutron star is
formed with very high rotation speed, and then gradually slows down.
Neutron stars are known to have rotation periods between about 1.4 ms
to 30 seconds. The neutron star's compactness also gives it very high
surface
gravity, up to 7 × 1012 m/s²
with typical values of a few × 1012 m/s²
(that is more than 1011 times of that of Earth).
One measure of such immense gravity is the fact that neutron stars
have an escape
velocity of around 100,000
km/s, about 33% 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
Gravitational light deflection at a neutron star. Due to relativistic
light deflection more than half of the surface is visible (each
chequered patch here represents 30 degrees by 30 degrees).[6]
The mass of the star depicted here is 1 and its radius 4, in natural
units[6]
from a Geometrized
unit system such that it has double its Schwarzschild
radius of 2.
The gravitational field at the star's surface is about 2 × 1011
times stronger than on Earth. The escape velocity is about
100,000 km/s, which is about one third the speed of light. 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.[6]
The gravitational
binding energy of a neutron star with two solar masses
is equivalent to the total conversion of one solar mass to energy
(from the law of mass-energy equivalence, E = mc2).
That energy was released during the supernova explosion.
A neutron star is so dense that one teaspoon (5 milliliters)
of its material would have a mass over 5×1012 kg.[7]
The resulting force of gravity is so strong that if an object were to
fall from just one meter high 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.[8]
The temperature inside a newly formed neutron star is from around
1011
to 1012
kelvins.[5]
However, the huge number of neutrinos
it emits carries away so much energy that the temperature falls
within a few years to around 1 million kelvins.[5]
Even at 1 million kelvins, 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 equation
of state (EOS) 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 (FPS, UU, APR, L, SLy, and
others) and current research is still attempting to constrain the
theories to make predictions of neutron star matter.[1][9]
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 (for EOS FPS, UU, APR or L
respectively).[9]
All EOS show that neutronium compresses with pressure.
Structure
A model of a neutron star's internal structure
Cross-section of neutron star. Densities are in terms of ρ0
the saturation nuclear matter density, where nucleons begin to touch.
Patterned after Haensel et al.[1],
page 12
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.[1]
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.[10]
It is also possible that heavy element cores, such as iron, simply
drown beneath the surface, leaving only light nuclei like helium
and hydrogen
cores[10].
If the surface temperature exceeds 106 kelvins (as in the
case of a young pulsar),
the surface should be fluid instead of the solid phase observed in
cooler neutron stars (temperature <10 sup="">6
kelvins)[10].
The "atmosphere" of the star is roughly one meter 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 (with maximum surface
irregularities of ~5 mm), because of the extreme gravitational
field.[11]
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.
Proceeding deeper, one comes to a point called neutron
drip where free neutrons leak out of nuclei. In this
region, there are nuclei, free electrons, and free neutrons. The
nuclei become smaller and smaller until the core is reached, by
definition the point where they disappear altogether. The exact
nature of the superdense matter in the core is still not well
understood. While this theoretical substance is referred to as
neutronium
in science
fiction and popular literature, the term "neutronium"
is rarely used in scientific publications, due to ambiguity over its
meaning. The term neutron-degenerate
matter is sometimes used, though not universally as
the term incorporates assumptions about the nature of neutron star
core material.
Neutron star core material could be a superfluid
mixture of neutrons with a few protons
and electrons, or it could incorporate high-energy particles like
pions
and kaons
in addition to neutrons, or it could be composed of strange
matter incorporating quarks
heavier than up
and down
quarks, or it could be quark
matter not bound into hadrons.
(A compact star composed entirely of strange matter would be called a
strange
star.) However, so far, observations have neither
indicated nor ruled out such exotic states of matter.
History of discoveries
The first direct observation of a neutron star in visible light. The
neutron star is RX
J185635-3754.
The neutron subatomic particle was discovered in 1932 by Sir James
Chadwick.[12]
By bombarding the hydrogen
atoms in paraffin
with emissions from beryllium
that was itself being bombarded with alpha
particles, he demonstrated that these emissions
contained a neutral particle that had about the same mass as a
proton. In 1935 he was awarded the Nobel Prize in Physics for this
discovery.[13]
In 1934, Walter
Baade and Fritz
Zwicky proposed the existence of the neutron
star,[14][15]
only a year after Chadwick's discovery of the neutron.[16]
In seeking an explanation for the origin of a supernova,
they proposed that the neutron star is formed in a supernova.
Supernovae are suddenly appearing dying stars in the sky, whose
luminosity in the optical night outshine an entire galaxy
for days to weeks. 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". If the central part of a massive star
before its collapse contains (for example) 3 solar masses, then a
neutron star of 2 solar masses can be formed. The binding energy E
of such a neutron star, when expressed in mass units via the
mass-energy
equivalence formula E = mc²,
is 1 solar mass. It is ultimately this energy that powers the
supernova.
In 1965, Antony
Hewish and Samuel
Okoye discovered "an unusual source of high radio
brightness temperature in the Crab
Nebula".[17]
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.[18]
In 1967, Jocelyn
Bell and Antony
Hewish discovered regular radio pulses from the
location of the Hewish and Okoye radio source. This pulsar was later
interpreted as originating from an isolated, rotating neutron star.
The energy source of the pulsar is the rotational energy of the
neutron star. The largest number of known neutron stars are of this
type (See Rotation-powered
pulsar).
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 (See Accretion-powered
pulsar).
In 1974, Antony
Hewish was awarded the Nobel
Prize in Physics "for his decisive role in the
discovery of pulsars" without Samuel
Okoye and Jocelyn
Bell who shared in the discovery.
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.
A "starquake", or "stellar quake"
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 as 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.[19]
Neutron stars have been observed to "pulse" radio and x-ray
emissions believed 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 revolutions per
second.[20]
A recent paper reported the detection of an X-ray burst oscillation
(an indirect measure of spin) at 1122 Hz from the neutron star XTE
J1739-285.[21]
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.
The population of neutron stars is concentrated along the disk of the
Milky Way although the spread perpendicular to the disk is fairly
large. The reason for this spread is that neutron stars are born with
high speeds (400 km/s) as a result of an imparted momentum-kick
from an asymmetry during the supernova
explosion process. The closest known neutron star is PSR
J0108-1431 at a distance of about 85 parsecs
(or 280 light
years)[22].
Another nearby neutron star that was detected transiting the backdrop
of the constellation Ursa Minor has been catalogued as 1RXS
J141256.0+792204. This rapidly moving object,
nicknamed by its Canadian and American discoverers "Calvera",
was discovered using the ROSAT/Bright
Source Catalog. Initial measurements placed its
distance from earth at 200 to 1,000 light years away, with later
claims greater—about 450 light-years.
Binary neutron stars
About 5% of all neutron stars are members of a binary
system. The formation and evolution scenario of binary
neutron stars is a rather exotic and complicated process.[23]
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 (gas) accreted from the
companion star. Material from the outer layers of a (bloated)
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.[24]
Subtypes
- Neutron star
- Protoneutron star (PNS), theorized.[25]
- Radio loud neutron star
- Single pulsars–general term for neutron stars that emit directed pulses of radiation towards us at regular intervals (due to their strong magnetic fields).
- Rotation-powered pulsar ("radio pulsar")
- 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)
-
- Low-mass X-ray binaries (LMXB)
- High-mass X-ray binaries (HMXB)
- Accretion-powered pulsar ("X-ray 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 (MSP) ("recycled pulsar")
- Sub-millisecond pulsar[26]
-
- Quark star–currently a hypothetical type of neutron star composed of quark matter, or strange matter. As of 2008, there are three candidates.
- 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.
- Q star–currently a hypothetical type of heavy neutron star with an exotic state of matter. As of 2008, there is no evidence for their existence.
Giant nuclei
A neutron star has some of the properties of an atomic
nucleus, including density, and being made 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
force, while a neutron star is held together by
gravity.
It is generally more useful to consider such objects as stars.
