- This article is about the celestial body. "Neutron Star" was a 1966 Hugo award winning short story by Larry Niven
A neutron star is a type of degenerate star composed mostly of densely packed neutrons, generally about 25 km in diameter and as massive as an average star. Stars that are more than about 8 times more massive at birth than the Sun collapse into neutron stars when they become supernovae. Neutron stars thus represent a middle ground between white dwarfs and black holes. Neutron stars were among the first major astronomical objects whose existence was first predicted from theory (1933) and later (1967) found to exist, at first as radio pulsars.
Neutron stars have a typical mass about 1.4 times the mass of the Sun. Their size (radius) is of order 12 km, about 60,000 times smaller than the Sun. So a neutron star's mass is packed in a volume 60,0003; or approximately 2x1014 times smaller than the Sun and the average mass density can be 1014 times higher than the density in the Sun. Such densities have yet to be produced in the laboratory. In fact, the density of a neutron star is about the density of an atomic nucleus.
Due to its small size and high density, a neutron star possesses a surface gravitational field about 2×1011 times that of Earth. One of the measures for the gravity is the escape velocity, the velocity one would need to give an object, such that it can escape from the gravitational field into infinity. For a neutron star, such velocities are typically 150,000 km/s, about 1/2 of the velocity of light. Conversely: an object falling onto the surface of a neutron star would strike the star also at 150,000 km/s. To put this in perspective, if an average human were to encounter a neutron star, he or she would impact with roughly the energy yield of a 100 megaton nuclear explosion.
Neutron stars are typically about 25 km in diameter, have greater than 1.4 times the mass of our Sun (the Chandrasekhar limit, below which they would be white dwarfs instead) and less than about 3 times the mass of our Sun (otherwise they would be black holes), and spin very rapidly (one revolution can take anything from thirty seconds to one six-hundredth of a second).
Current understanding of the structure of neutron stars is defined by existing mathematical models, which of course are subject to revision. Based on current models, the matter at the surface of a neutron star is composed of ordinary 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 we have 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 that 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. However so far observations have neither indicated nor ruled out such exotic states of matter.
History of discoveries
In 1933 Walter Baade and Fritz Zwicky (Phys. Rev. 45 "Supernovae and Cosmic rays") 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 new 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 E=mc², is the equivalence of 1 solar mass. It is ultimately this energy that powers the supernova.
In 1967 Jocelyn Bell and Anthony Hewish discover 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.
In 1971 Riccardo Giacconi, Herbert Gursky, Ed Kellogg, R. Levinson, E. Schreier, and H. Tananbaum discover 4.8 second pulsations in an X-ray source in the constellation Centaurus, Cen X-3. They interpret this as resulting from a rotating hot neutron star in orbit around another star. The energy source is gravitational and results from a rain of gas falling onto the surface of the neutron star.
Some neutron stars that can be observed
- 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.
- Magnetar - a neutron star with an extremely strong magnetic field; some magnetars are observed as soft gamma repeaters.
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 thousand 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 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 also have very intense magnetic fields - typically about 1012 times stronger than Earth's. 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.
When pulsars were first discovered, the fast time scale of radio pulses (about 1 s, uncommon to astronomy in the 1930s) 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.
Another class of neutron star, known as the magnetar, exists. These have a magnetic field of about 100 gigateslas, strong enough to wipe a credit card on Earth from 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 them to slow down.
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- Norman K. Glendenning, R. Kippenhahn, I. Appenzeller, G. Borner, M. Harwit (2000). Compact Stars, 2nd ed.
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