Neutron star

A neutron star is formed from the collapsed remnant of a massive star; i.e. a Type II, Type Ib, or Type Ic supernova. Models predict that neutron stars consist mostly of neutrons, hence the name. Such stars are very hot, as supported by the Pauli exclusion principle indicating repulsion between neutrons. A neutron star is one of the few possible conclusions of stellar evolution.

A typical neutron star has a mass between 1.35 and about 2.1 solar masses, with a corresponding radius between 20 and 10 km,[1] respectively — 30,000 to 70,000 times smaller than the Sun. Thus, neutron stars have overall densities of 8.4×10¹⁶ to 1×10¹⁸ kg/m³,[2] which compares with the approximate density of an atomic nucleus of 3×10¹⁷ kg/m³.[3] The neutron star's density varies from below 1×10⁹ kg/m³ in the crust increasing with depth to above 6 or 8×10¹⁷ kg/m³ deeper inside.[4]

In general, compact stars of less than 1.38 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 star over 5 solar masses, inevitably producing a black hole.

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 1.4ms to thirty seconds. The neutron star's compactness also gives it very high surface gravity, 2×1011 to 3×1012 times stronger than that of Earth. One measure of such immense gravity is the fact that neutron stars have an escape velocity of around 150,000 km/s, about 50% of the speed of light. Matter falling onto the surface of a neutron star would be super-accelerated by this gravity and 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.

Structure

A model of a neutron star's internal structure

Current understanding of the structure of neutron stars is defined by existing mathematical models. A neutron star is so dense that one teaspoon (5 millilitre) of its material would mass over 5×10¹² kg.[5] On the basis of current models, the matter at the surface of a neutron star is composed of ordinary atomic nuclei as well as electrons. The "atmosphere" of the star is roughly one meter thick, below which one encounters a solid "crust". Proceeding inward, one encounters nuclei with ever increasing numbers of neutrons; such nuclei would quickly decay 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

In 1932, Sir James Chadwick discovered the neutron as an elementary particle,[6] for which he was awarded the Nobel Prize in Physics in 1935.

In 1933, Walter Baade and Fritz Zwicky proposed the existence of the neutron star,[7] only a year after Chadwick's discovery of the neutron. 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 might 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".[8] This source turned out to be the Crab Nebula neutron star that resulted from the great supernova of 1054 CE.

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 an ice skater pulling in his or her 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, when they orbit a companion star and are able to accrete matter from it, they can increase this to several hundred times per second, distorting into an oblate spheroid shape despite their own immense gravity (an equatorial bulge).

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 down its rotation is usually constant and very small: the observed rates are between 10^-10 and 10^-21 second for each rotation. In other words, for a typical slow down rate of 10^-15 seconds per rotation, then 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 rapid and unexpected increase of its rotation speed (of the same, extremely small scale as the constant slowing down). 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.[9]

Neutron stars may "pulse" due to particle acceleration near the magnetic poles, which are not 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.[10] 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.[11] 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 far majority of neutron stars are 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)[12].

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[13]. 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.

Subtypes

Neutron star
- Radio-quiet neutron stars
- Radio emitting
  - 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
        
        Anomalous X-ray pulsar
  - Binary pulsars
    - 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 ("recycled pulsar")
  - Quark Star – a currently still hypothetical type of neutron star composed of quark matter, or strange matter. As of February 2007, there are three candidates.
  - Preon star – a currently still hypothetical type of neutron star composed of preon matter. As of 2007, there is no evidence for the existence of preons.

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.