Neutron star Wednesday, June 24, 2009

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For the story, see Neutron Star (short story).

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 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 to 5.9 × 1017 kg/m³ (2.6 to 4.1 × 1014 times Solar density),[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 or 8 × 1017 kg/m³ deeper inside.[5]

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 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 1.4 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

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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). The radius of the star depicted here is double the size of its Schwarzschild-Radius.[6]

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 millilitres) 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 kilometres per second, or 4.3 million miles per hour.[8]

The temperature inside a newly formed neutron star is from around 1011 to 1012 Kelvin.[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 Kelvin.[5][6]. 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 equation of state (EOS) for a Neutron star is still not known as of 2009. 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.[6] 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.[9][1] 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 as well as electrons. It is possible that atomic cores at the surface are iron nuclei, 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 Kelvin (as in the case of a young pulsar), the surface should be fluid instead of the solid phase observed in cooler neutron stars (temperature <106 Kelvin)[10].

The "atmosphere" of the star is roughly one meter thick, and its dynamic is fully controled by the stars 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 1933, Walter Baade and Fritz Zwicky proposed the existence of the neutron star,[14] only a year after Chadwick's discovery of the neutron.[15] 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".[16] This source turned out to be the Crab Nebula neutron star that resulted from the great supernova of 1054 CE.

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.[17]

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 a spinning ice skater 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, 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 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 rapid and unexpected increase of its rotation speed (of a similar, 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.[18]

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.[19] 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.[20] 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 RX J185635-3754. Observations in 2000 using the Hubble Space Telescope show it to be about 200 light-years away in the southern constellation Corona Australis. While its surface temperature is 600,000 K, it radiates primarily in the X-Ray region. That and its small diameter makes it a very dim object of 26th magnitude in visible light.[21]

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.[22] 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.[23]

Subtypes

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.

Examples of neutron stars

See also

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