Neutron star

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 compact star in which the weight of the star is carried by the pressure of free neutrons. It is also called a degenerate star. The neutron is an elementary particle and one of the building blocks of atomic nuclei. Neutrons are electrically neutral (hence the name) and in contrast to protons, can be packed to form extremely large "nuclei", up to several times the mass of the Sun. Neutron stars are the first major astronomical object whose existence was first predicted from theory (1933) and later (1968) found to exist, at first as radio pulsars.

Neutron stars have a mass of the same order as the mass of the Sun. Their size (radius) is of order 10 km, about 70,000 times smaller than the Sun. So a neutron star's mass is packed in a volume 70,000³; or approximately 10¹⁴ 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 impact 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 one of the few possible endpoints of stellar evolution, therefore sometimes called a dead star. They are formed in a supernova as the collapsed remnant of a massive star (a Type II or Ib supernova) or as the remnant of a collapsing white dwarf in a Type Ia supernova.

Neutron stars are typically about 20 km in diameter, have greater than 1.4 times the mass of our Sun (the Chandrasekhar limit, below which they'd be white dwarfs instead) and less than about 3 times the mass of our Sun (otherwise they'd be black holes), and spin very rapidly (one revolution can take anything from thirty seconds to a hundredth of a second).

The matter at the surface of a neutron star is composed of ordinary nuclei as well as ionized electrons. The "atmosphere" of the star is roughly one metre 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. Some researchers refer to this theoretical substance as neutronium, though this term can be misleading and is more frequently used in science fiction. It could be a superfluid mixture of neutrons with a few protons and electrons, other high-energy particles like pions and kaons may be present, and even sub-atomic quark matter is possible. However so far observations have not indicated nor ruled out such exotic states of matter.

History of discoveries

In 1932 Sir James Chadwick discovered (Nature Vol 129, p. 312 "on the possible existence of a neutron") the neutron as an elementary particle, good for a Nobel Prize in Physics in 1935.

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 type of Soft gamma repeater with an extremely strong magnetic field.

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 or minutes 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^-12 and 10^-19 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.¹

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 internal re-organizations of the matter composing the neutron star, something similar to starquakes. Such a starquake would register as grade 20 or 25 on the Richter scale.

Neutron stars also have very intense magnetic fields - about 10¹² times stronger than Earth's. Neutron stars may "pulse" due to electrons accelerated near the magnetic poles, which are not aligned with the rotation axis of the star. These electrons travel outward from the neutron star, until they reach the point at which they would be forced to travel faster than the speed of light in order to still co-rotate with the star. At this radius, the electrons must stop, and they release some of their kinetic energy in the form of X-rays and gamma-rays. External viewers see these pulses of radiation whenever the magnetic pole is visible. 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 at those days) was considered to be caused by terrestrial intelligence (such as the farmer's electric fence signals) or by extraterrestrial intelligence, later jokingly referred to as LGM-1, for "Little Green Men." The highly regular pattern of pulses revealed after a few weeks of observations quickly excluded this option. The continuing regularity after many months was the most compelling argument for the rotating neutron star explanation.

Another class of neutron star, known as the magnetar, exists. These have a magnetic field of above 10 gigateslas, strong enough to wipe a credit card from the distance of the Sun and strong enough to be fatal from the distance of the Moon. By comparison, Earth's natural magnetic field is 50 microteslas, and on Earth a fatal magnetic field is only a theoretical possibility; some of the strongest fields generated are actually used in medical imaging. 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.

The processes in a magnetar involve the rotation of the neutron star tangling field lines until they become exceptionally dense, giving rise to a resonant magnetic field.

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Neutron star

History of discoveries

Some neutron stars that can be observed

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