Neutron star: Wikis

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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]

Contents

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. If this acceleration were constant, a falling object would be accelerated to nearly the speed of light in the order of 100 microseconds. 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 5.5×1012 kg, about 900 times the mass of the Great Pyramid of Giza.[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 <106 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. One of the closest known neutron stars is PSR J0108-1431 at a distance of about 130 parsecs (or 424 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 at 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

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

References

  1. ^ a b c d Paweł Haensel, A Y Potekhin, D G Yakovlev (2007). Neutron Stars. Springer. ISBN 0387335439. http://books.google.com/books?id=iIrj9nfHnesC&printsec=frontcover#PPA12,M1. 
  2. ^ A neutron star's density increases as its mass increases, and, for most Equations of State (EOS), its radius decreases in a non-linear way. For example, EOS radius predictions for a 1.35 M star are: FPS 10.8 km, UU 11.1 km, APR 12.1 km, and L 14.9 km. For a more massive 2.1 M star radius predictions are: FPS undefined, UU 10.5 km, APR 11.8 km, and L 15.1 km. (NASA mass radius graph)
  3. ^ 3.7×1017 kg/m3 derives from mass 2.68 × 1030 kg / volume of star of radius 12 km; 5.9×1017 kg m-3 derives from mass 4.2×1030 kg per volume of star radius 11.9 km
  4. ^ "Calculating a Neutron Star's Density". http://heasarc.gsfc.nasa.gov/docs/xte/learning_center/ASM/ns.html. Retrieved 2006-03-11.  NB 3 × 1017 kg/m3 is 3×1014 g/cm3
  5. ^ a b c "Introduction to neutron stars". http://www.astro.umd.edu/~miller/nstar.html. Retrieved 2007-11-11. 
  6. ^ a b c Zahn, Corvin (1990-10-09). "Tempolimit Lichtgeschwindigkeit" (in German). http://www.tempolimit-lichtgeschwindigkeit.de/galerie/galerie.html. Retrieved 2009-10-09. "Durch die gravitative Lichtablenkung ist mehr als die Hälfte der Oberfläche sichtbar. Masse des Neutronensterns: 1, Radius des Neutronensterns: 4, ... dimensionslosen Einheiten (c, G = 1)" 
  7. ^ The average density of material in a neutron star of radius 10 km is 1.1×1012 kg cm−3. Therefore, 5 mL of such material is 5.5×1012 kg, or 5 500 000 000 metric tons. This is about 15 times the total mass of the human world population. Alternatively, 5 mL from a neutron star of radius 20 km radius (average density 8.35×1010 kg cm−3) has a mass of about 400 million metric tons, or about the mass of all humans.
  8. ^ Miscellaneous Facts
  9. ^ a b NASA. Neutron Star Equation of State Science Retrieved 2008-09-15
  10. ^ a b c V. S. Beskin (1999). "Radiopulsars". УФН. T.169, №11, p.1173-1174
  11. ^ neutron star
  12. ^ Chadwick, James (1932). "On the possible existence of a neutron". Nature 129: 312. doi:10.1038/129312a0. 
  13. ^ Staff (1935). "James Chadwick, The Nobel Prize in Physics 1935". Nobel Foundation. http://nobelprize.org/nobel_prizes/physics/laureates/1935/chadwick-bio.html. Retrieved 2008-07-17. 
  14. ^ Baade, Walter and Zwicky, Fritz (1934). "Remarks on Super-Novae and Cosmic Rays". Phys. Rev. 46: 76–77. doi:10.1103/PhysRev.46.76.2. 
  15. ^ Shapiro, Stuart; Teukolsky, Saul (1983). Black Holes, White Dwarfs, and Neutron Stars. United States: John Wiley & Sons, Inc. pp. 241–242. ISBN 0-471-87317-9. 
  16. ^ Even before the discovery of neutron, in 1931, neutron stars were anticipated by Lev Landau, who wrote about stars where "atomic nuclei come in close contact, forming one gigantic nucleus" (published in 1932: Landau L.D.. "On the theory of stars". Phys. Z. Sowjetunion 1: 285–288. ). However, the widespread opinion that Landau predicted neutron stars proves to be wrong: for details, see P. Haensel, A. Y. Potekhin, & D. G. Yakovlev (2007). Neutron Stars 1: Equation of State and Structure (New York: Springer), page 2.
  17. ^ Hewish and Okoye (1965). "Evidence of an unusual source of high radio brightness temperature in the Crab Nebula". Nature 207: 59. doi:10.1038/207059a0. 
  18. ^ Shklovsky, I.S. (April 1967), "On the Nature of the Source of X-Ray Emission of SCO XR-1", Astrophys. J. 148 (1): L1–L4, doi:10.1086/180001 
  19. ^ Alpar, M Ali (January 1, 1998). "Pulsars, glitches and superfluids". Physicsworld.com. http://physicsworld.com/cws/article/print/1756. 
  20. ^ [astro-ph/0601337] A Radio Pulsar Spinning at 716 Hz
  21. ^ University of Chicago Press - Millisecond Variability from XTE J1739285 - 10.1086/513270
  22. ^ Posselt, B.; Neuhäuser, R.; Haberl, F. (March 2009). "Searching for substellar companions of young isolated neutron stars". Astronomy and Astrophysics 496 (2): 533–545. doi:10.1051/0004-6361/200810156. Bibcode2009A&A...496..533P. 
  23. ^ Tauris & van den Heuvel (2006), in Compact Stellar X-ray Sources. Eds. Lewin and van der Klis, Cambridge University Press http://adsabs.harvard.edu/abs/2006csxs.book..623T
  24. ^ Compact Stellar X-ray Sources (2006). Eds. Lewin and van der Klis, Cambridge University
  25. ^ Neutrino-Driven Protoneutron Star Winds, Todd A. Thompson.
  26. ^ Binary Sub-Millisecond Pulsar and Rotating Core Collapse Model for SN1987A, Nakamura, T., 1989.

External links

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Simple English

File:Neutron star cross
A model that shows what a neutron star would look like on the inside.

A neutron star is a very small and dense star made almost completely of neutrons. It is a very large nucleus held together by gravity.

Neutron stars have a radius of about 10 kilometres (6.2 mi) and a mass from about 1.4 to 5 times the mass of the Sun.

They are usually what is left of very big stars that have exploded (these are called supernovas). Some are what happens to white dwarfs (small stars) that have got a lot of extra mass.

Neutron stars usually turn very fast, taking from 0.001 second up to 30 seconds to turn.

They come in different types. One kind is the pulsar which spins and sends a beam of radio waves (like a lighthouse in space). When this beam moves past Earth, it shows up as a pulse (like the light from a lighthouse is seen as a flash on the horizon). This 'pulse' is why they are called pulsars.

Contents

Starting as a supernova

In a supernova, a star runs out of nuclear fuel and explodes.

If the star is bigger than 5 times the mass of the Sun, it will become a black hole. If it has less than 1.4 times the mass of the Sun, it will form a white dwarf.

All other stars that go supernova become neutron stars.

Starting as a White Dwarf

A white dwarf whose mass is nearly 1.4 times the mass of the Sun will sometimes get bigger (but no one knows why). When this happens, it will collapse into a neutron star.

Properties

Neutron stars have the same properties:

Rotation

Neutron stars turn a lot faster than other stars.

Density

Neutron stars are very dense. They are much denser than anything we find naturally on the Earth.

In a neutron star the electrons have joined with the protons in the nucleus to make neutrons. All the neutrons are joined together by gravity, with not much space in between them.

To imagine how dense a neutron star is, take all of the mass of our sun (which has a diameter of 1,392,000 kilometres (865,000 mi)) and pushing it down into a size that would fit into a ball with a 19 kilometres (12 mi) diameter.

Magnetic Field

Neutron stars have very strong magnetic fields. They are some of the strongest magnetic fields that occur naturally.

All stars have a magnetic field. When a star collapses, it becomes smaller. This means that the magnetism is pushed into a smaller area, which means that the magnetic field is stronger nearer the star.


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