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Neutron stars are collapsed stars in the universe.

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neutron star.
A model of a neutron star's internal structure

A neutron star is one of the few possible endpoints of stellar evolution. A Neutron star is formed from the collapsed remnant of a massive star, a Quark star might be created, however this is still a grey area in astronomy. In any case, a Gravitational collapse will occur on any star over 5 solar masses, inevitably producing a black hole.

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 one seven-hundredth of a second 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 1/2 of the speed of light. Matter falling onto the surface of a neutron star would strike the star also at 150,000 km/s, and then be crushed under its own weight into a puddle less than an atom thick.

Structure of a neutron star.

Current understanding of the structure of neutron stars is defined by existing mathematical models. A neutron star is so dense that one teaspoon of its material would weigh 100 million metric tons. 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 of neutron stars.

neutron star.
The first direct observation of a neutron star in visible light. The neutron star being RX J185635-3754.

In 1932, Sir James Chadwick discovered the neutron as an elementary particle, 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, 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 1967, Jocelyn Bell and Antony Hewish discovered radio pulses from a pulsar, later interpreted as originating from an isolated, rotating neutron star. The energy source is 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).

Rotation of a neutron star.

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 sudden coupling between the superfluid interior and the solid crust.

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 neutron star, catalogue number XTE J1739-285 was observed to spin at an astonishing rate of 1122 revolutions per second.

Subtypes of neutron stars

  • 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.
  • pulsar - general term for neutron stars that emit directed pulses of radiation towards us at regular intervals due to their strong Magnetic Fields. When pulsars were first discovered, the fast time scale of pulses (about 1 s, uncommon to astronomy in the 1960s) was half-seriously considered to be caused by extraterrestrial intelligence, later jokingly referred to as LGM-1, for "Little Green Men." The discovery of many pulsars, spread all over the sky with different rotation periods quickly excluded this option. The discovery of a pulsar associated with the Vela Supernova remnant, soon followed by the further discovery of a pulsar which appeared to be powering the Crab Nebula, produced compelling arguments for the neutron star interpretation.
  • Magnetar - a neutron star with an extremely strong magnetic field; some magnetars are observed as soft gamma repeaters, or Anomalous X-ray pulsars. These have a magnetic field of about 100 gigateslas, roughly a thousand times stronger than the fields of normal neutron stars. This is strong enough to erase a credit card on Earth from half the distance of the Moon's orbit. By comparison, the Earth's natural magnetic field is about 60 microteslas. A small neodymium based rare earth magnet has a field of about a tesla, and most media used for data storage can be erased with milliteslas. Magnetars occasionally produce bursts of X-ray emission. About once per decade, a magnetar somewhere in the galaxy produces a giant flare of gamma-rays. Magnetars have long rotation periods, typically 5 to 12 seconds, because their strong magnetic fields have caused their rotation to slow.
  • 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.

Giant nuclei of a neutron star.

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.

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