N-Magnitude	A magnitude derived from observations made at a wavelength of 10 microns. [H76]
N Galaxy	A galaxy with a small, bright, blue nucleus superposed on a considerably fainter red background. (In the Yerkes 1974 system, a galaxy with a small nucleus containing a considerable fraction of the luminosity; N-, less pronounced N galaxies; N+, extreme examples of N galaxies.) Also, a type of radio galaxy having a brilliant, starlike nucleus containing most of the luminosity of the system. N galaxies are compact galaxies, and as a class are intermediate between Seyfert galaxies and quasars in properties of form, color, spectra, redshift, and optical and radio variability. [H76]
N Lines	Two green forbidden lines of doubly ionized oxygen [O III]. N1 is at 5007 Å; N2 at 4959 Å. [H76]
N Star	(a) Stars of spectral type N are very red stars similar to M stars except that bands of C2, CN, and CH are present instead of those of TiO. N stars are strongly concentrated toward the Galactic plane. [H76] (b) In the old terminology, the cooler C-type stars. [JJ95]
Nadir	The point on the celestial sphere diametrically opposite to the zenith. [S92]
Naked Singularity	A singularity that will be visible and communicable to the outside world. [H76]
nano-	A prefix meaning 10-9. [H76]
Nanosecond	A billionth of a second. [LB90]
NASA	National Aeronautics and Space Administration US government body set up in 1958, under which the Space Center at Houston, Texas, and the Space Center at Cape Canaveral, Florida, are responsible for manned and unmanned space flights. [A84]
NASDA	National Air and Space Development Agency (Japan). [LLM96]
Native Oxide	The silicon dioxide layer which grows in air on the exposed backside surface of a thinned CCD. [McL97]
Natural Line Broadening	The broadening of spectral lines resulting from the fact that excited levels have certain mean lives, and these mean lives, by virtue of the uncertainty principle, imply a spread in the energy values. [H76]
Natural Philosophy	A term widely employed in the seventeenth century to mean what today is encompassed in the word science. [F88]
Natural Selection	Tendency of individuals better suited to their environment to survive and perpetuate their species, leading to changes in the genetic makeup of the species and, eventually, to the origin of new species. see Evolution [F88]
Natural Units	Units of length, time, mass, etc. in which the fundamental constants c (the speed of light), (Planck's constant) and kB (Boltzmann's constant) are equal to unity. That is, c, and kB have the numerical value 1. (For example, if we measure length in light-years and time in years, then c = 1 light-year per year.) The use of natural units allows these constants to be omitted from mathematical equations, leading to less-cluttered calculations. In natural units, E = mc2 becomes E = m and E = kBT becomes E = T, so that both mass and temperature can be expressed in units of energy. (Of course, the correct factors of c, and kB must be inserted at the end of a calculation to obtain measurable quantities.) [CD99]
Nautical Mile	Equals 1.15 statute miles. [F88]
N-body Simulations	Computer simulations of the behavior of a large number of bodies under their mutual interactions. In cosmological N-body simulations, the bodies are usually galaxies and the interactions are gravitational. Thus, the computer simulates how a group of galaxies should behave under their mutual gravitational attraction. The law of gravity and the initial positions and velocities of the hypothetical galaxies and other masses are fed into the computer. The computer then calculates the evolution of the system. [LB90]
NBS	National Bureau of Standards (USA). [LLM96]
Near Zone	The zone of a pulsar within the velocity-of-light radius. [H76]
Nebula	(a) The term "nebula" was previously applied to all kinds of hazy patches in the sky, many of which are now recognized to be clusters or galaxies. (The plural is nebulae) [H76] (b) Indistinct, nonterrestrial objects visible in the night sky. "Bright" nebulae glow with light emitted by the gas of which they are composed ("emission" nebulae) or by reflected starlight ("reflection" nebulae) or both. "Dark" nebulae consist of clouds of gas and dust that are not so illuminated. "Planetary" nebulae are shells of gas ejected by stars. Spiral nebulae are galaxies. [F88]
Nebular Hypothesis	Hypothesis, maintained in the nineteenth and early twentieth century, that the spiral nebulae are not galaxies but are instead whirlpools of gas from which new systems of stars and planets are condensing. Compare Island Universe Theory [F88]
Nebular Lines	Lines that originate from forbidden transitions. [H76]
Nebular Variable Stars	Also called T Tauri variables, a type of variable star of spectral classification F, G or K (giants above the main sequence on the Hertzsprung-Russell diagram) that loses an appreciable proportion of its mass in its (irregular) more luminous periods, and is thus surrounded by volumes of gas and dust. [A84]
NEC	Noise Equivalent Charge [LLM96]
NEF	Noise Equivalent Flux [LLM96]
Negative Hydrogen Ion	H- ion. [H76]
Negatron	A negatively charged electron, as opposed to a positron. [H76]
NELG	Narrow Emission Line Galaxy
Neodymium	A toxic silvery element belonging to the lanthanoid series of metals. It occurs in association with other lanthanoids. Neodymium is used in various alloys, as a catalyst, in compound form in carbon-arc searchlights, etc., and in the glass industry. Symbol: Nd; m.p. 1021°C; b.p. 3068°C; r.d. 7.0 (20°C); p.n. 60; r.a.m. 144.24. [DC99]
Neon	Element with atomic number 10, and the third most common metal in the Universe. Neon is produced by carbon burning in high-mass stars and released into the Galaxy when they explode. [C95]
Neon Burning	The stage in which a star burns neon into oxygen and magnesium. [C95]
NEP	Noise Equivalent Power [LLM96]
Neptune	(a) Eighth major planet out from the Sun, discovered in 1846 by Johann Galle and Louis d'Arrest following predictions calculated by Urbain Le Verrier. Similar predictions had been made a year earlier by John Couch Adams but were not followed up. [A84] (b) Mean distance from Sun 30.07 AU, e = 0.009. Orbital period 164.8 years, Vorb = 5.43 km s-1. Synodic period 367.49 days. Albedo 0.62. Maximum apparent brightness + 7.6 mag. Mass 1.03 × 1029 g; radius 24,500 ± 500 km; mean density 1.7 g cm-2. Oblateness 0.02; Vesc 25 km s-1; surface gravity 1.3 that of Earth. Rotation period 15h49m30s. Inclination 1°.8; obliquity 28°.8. The primary constituents of its atmosphere are hydrogen and methane. Two satellites, Triton and Nereid. [H76]
Neptunium	A toxic radioactive silvery element of the actinoid series of metals that was the first transuranic element to be synthesized (1940). Found on Earth only in minute quantities in uranium ores, it is obtained as a by-product from uranium fuel elements. Symbol: Np; m.p. 640°C; b.p. 3902°C; r.d. 20.25 (20°C); p.n. 93; most stable isotope 237Np (half-life 2.14 × 106 years). [DC99]
Nereid	The outer satellite of Neptune (radius 150-250 km). Period about 360 days direct. It has the most eccentric orbit (e = 0.76) of any natural satellite. Discovered by Kuiper in 1950. [H76]
Nernst Theorem	All substances have zero entropy at 0 K. (Third law of thermodynamics)[H76]
Neumann Lines	In iron meteorites, groups of very fine parallel lines that cross each other at various angles. Irons containing Neumann lines can easily be cleaved in three mutually perpendicular directions. [H76]
Neutral Current	A type of reaction between particles subject to the weak nuclear force. In this reaction, all intermediary particles that transmit the forces are electrically neutral, which is the origin of the word "neutral". [LB90]
Neutral Current	weak interaction where no change takes place in the charges of the participants. [D89]
Neutral Current Reactions	Weak-interaction reactions in which no electric charge is exchanged between the colliding particles. Observation of such reactions in 1973 provided important support for the then-developing gauge theory of the weak interactions. We now know that these reactions are mediated by the exchange of a massive, neutral gauge boson - the Z0. [CD99]
Neutral Hydrogen	A hydrogen atom that has a proton and an electron and so is electrically neutral. Neutral hydrogen produces radio waves that are 21 centimeters long. [C95]
Neutral Region (solar)	A region where the magnetic field strength approaches zero. Generally, neutral regions occur between regions of opposite polarity. [H76]
Neutrino	(a) An electrically neutral, very weakly interacting particle, with a rest energy which is either zero or very small. The particle was predicted in 1931 as a means of reconciling the measurements of beta decays with the conservation of energy, but it was not directly detected until 1956. [G97] (b) Chargeless species of particle, subject only to the weak force. [G99] (c) An electrically neutral, massless particle of spin-1/2, which interacts only by the weak force and gravity. It was first postulated by Pauli in 1930 to ensure conservation of energy and angular momentum in nuclear decay. Three different types of neutrinos are known to exist corresponding to the three massive leptons: e, µ and . [CD99] (d) A fundamental elementary particle with no electric charge and very small if any rest mass. Believed to be exceedingly abundant in the Universe. The neutrino has a very low cross-section for interaction with matter and is almost impossible to detect, hence the uncertainty over its rest mass. The Sun produces neutrinos from thermonuclear reactions in its core, and a large flux of neutrinos carries away most of the energy, of a supernova. Neutrinos are one candidate for Dark Matter. Experiments to detect cosmic neutrinos involve large masses of "stopping" material and indirect detection of the effects of neutrino absorption. [McL97] (e) A stable particle with no charge, a rest mass of zero, and a spin of 1/2, that carries away energy in the course of nuclear reactions. Its main characteristic is the weakness of its interactions with all other particles. Since the wavelengths of neutrinos at the energies at which they are normally emitted from unstable nuclei are only a few thousandths of an angstrom (compared with the wavelength of a light photon which is several thousand angstroms), they have negligible probability (10-19 that of a light photon) of interacting with matter and escape at the speed of light. Neutrinos arise only in the energy-producing regions of stars and therefore, unlike light photons, provide direct evidence of conditions in stellar cores. There are two types of neutrinos, those associated with electrons (ve) and those associated with muons (vµ). [H76] (f) An electrically neutral, massless lepton. There are three known varieties, one in each generation of particles, associated with electron, muon, and tau leptons. It only takes part in weak interactions.[D89] (g) A fundamental particle. There is one neutrino for each of the three families of particles. [K2000]
Neutrino Bremsstrahlung	The reaction in which an electron scatters from a nucleus, emitting a neutrino-antineutrino pair. [H76]
Neutrino Process	Nucleosynthesis induced by neutrinos. It may create fluorine and boron. [C95]
Neutron	(a) Uncharged particle in the nucleus of all atoms except hydrogen. Through beta decay, a neutron may become a proton and an electron; the process occurs in reverse during the formation of a neutron star. [A84] (b) One of the constituents of the atomic nucleus discovered in 1932. It is bound into atomic nuclei by the strong nuclear force. Free neutrons decay slowly via the weak nuclear force. Despite being electrically neutral, the neutron possesses both an electric dipole moment (as if it were made of positive and negative charges separated a minute distance) and a magnetic moment, indicating some internal substructure. [CD99] (c) A baryon made of one up quark and two down quarks. It possesses no electromagnetic charge and can only be found in atomic nuclei. [c97] (d) A nuclear particle with zero charge and with a mass slightly greater than that of a proton (mass of the neutron is 1.008665 amu = 1.6749 × 10-24 g). A free neutron decays, after a half-life of about 10.6 minutes, into a proton, an electron, and an antineutrino. The neutron is probably made up of still more fundamental particles having both positive and negative charges. The charges balance to give a net charge of zero, but the motions of the charges are such that their magnetic contributions do not cancel and consequently the neutron is magnetic. [H76] (e) Chargeless particle, typically found in the nucleus of an atom, consisting of three quarks (two down-quarks, one up-quark). [G99] (f) Electrically neutral, massive particles found in the nuclei of atoms. Each neutron is composed of one up quark and two down quarks; its mass is 939.6 MeV, slightly more than that of the proton. Stable within the nucleus, the neutron if isolated decays, with a ha half-life of fifteen minutes. [F88] (g) Free neutrons have a lifetime of about fifteen minutes; the y decay into a proton, an electron, and an antineutrino. When the neutrons are bound into nuclei (such as those in us), the decays are no longer possible because of subtle effects explained by quantum theory, so the neutrons in nuclei are as stable as protons. see also Hadron [K2000]
Neutron Drip	The rapid increase in the abundance of free neutrons that occurs when physical conditions are such that the neutron becomes the stable nucleon with respect to electron capture (as will happen in a degenerate assembly of electrons with sufficiently high Fermi threshold energy). [H76]
Neutron Excess ()	The excess of neutrons over protons in an atomic nucleus. = (Nn - Np) / (Nn + Np). [H76]
Neutron Lifetime	The time it takes an isolated neutron at rest to disintegrate into other elementary particles, about equal to 15 minutes. [LB90]
Neutron Matter	Degenerate matter in which the electron:proton:neutron ratio is about 1:1:8 (or perhaps 1:1:10 or 1:1:12). [H76]
Neutron Star	(a) A dead, collapsed star that consists mostly of neutrons and is only about 20 kilometers across. Neutron stars are much denser than white dwarfs. [C95] (b) A star whose core is composed primarily of neutrons, as is expected to occur when the mean density is in the range 1013-1015 g cm-3. Under current theories pulsars are assumed to be rotating magnetic neutron stars. A neutron star would probably be only 10-15 km in diameter with a magnetic field of about 1012 gauss, a density of 1013-1015 g cm-3 (compared with a white dwarf's maximum density of about 108 g cm-3) and a central temperature of about 109 K and thus would be both bluer and dimmer than a white dwarf. [H76] (c) Remnant of a star after it has exploded as a supernova. Usually optically dim, a neutron star sends out regular or irregular radio emissions and is therefore also called a pulsar. The density of such a star may be unimaginably great although the diameter is generally around only 10 km; the gravitational and magnetic forces are correspondingly vast. It is called a neutron star because in such density, protons fuse with electrons to form neutrons, of which the star is almost entirely composed. [A84] (d) A compact star in which the internal pressure support is provided by neutron degeneracy pressure. Their masses cannot exceed roughly the mass of the Sun because of the Chandrasekhar limit. Their central densities are roughly nuclear densities and neutron stars may therefore be thought of as giant nuclei. Their radii are about 10 km and the corresponding central densities are about 10 ** kg m*. Pulsars are magnetized rotating neutron stars and X-ray binaries consist of a neutron star as a member of the binary system, the X-ray emission being associated with mass transfer onto the neutron star. [D89]
New Inflation	(a) The first fully successful version of the inflationary theory, discovered independently by Andrei Linde in the Soviet Union, and by Andy Albrecht and Paul Steinhardt in the U.S. see Inflation; False Vacuum; Decay of the False Vacuum [G97] (b) A 1982 modification of the original Inflationary Universe model. While the Old Inflationary Universe model solved a number of cosmological problems, it led to the result that the Universe was very inhomogeneous during the inflationary epoch and contained bubbles of empty space surrounded by a medium filled with energy. In New Inflation, no such bubbles appear, although the Universe still undergoes a brief epoch of extremely rapid expansion. see Inflationary Universe Model [LB90]
New Inflationary Universe	a revised form of the Inflationary Universe model that provides a mechanism to avoid the gross inhomogeneities which result from the theory as originally proposed. [D89]
Newton (N)	The SI derived unit of force, equal to the force necessary to give an acceleration of 1 meter per second2 to a mass of 1 kg. N = 105 dynes. [H76]
Newton's Constant G	Newton's law of gravitation says that the gravitational for ce between any two bodies is proportional to the product of their masses and decreases as the square of the distance between them. This statement is turned into an equation by inserting the constant G, so the force F = Gmm'/r2. Because all of the particles feel the gravitational force, G is universal, so G can be used to form quantities with dimensions, giving the Planck scale. G is measured by finding the force between two objects of known masses separated by a known distance. [K2000]
Newton's Laws	(1) A body remains in a state of rest or uniform motion when left to itself. (2) The net force on a body is equal to the product of its mass and its acceleration. (3) When two bodies interact, the force on the first due to the second is equal and opposite to the force on the second due to the first. [H76]
Newton's Laws of Motion	Laws describing the motion of bodies based on the conception of an absolute and immutable space and time; these laws held sway until Einstein's discovery of special relativity. [G99]
Newton's Universal Gravitational Constant	the constant that sets the scale of gravitational forces. Its value is independent of the actual constitution of the matter producing the gravitational field.[D89]
Newton's Universal Theory of Gravity	Theory of gravity declaring that the force of attraction between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them. Subsequently supplanted by Einstein's general relativity. [G99]
Newtonian	A class of reflecting telescope developed by Sir Issac Newton with a paraboloidal primary mirror and a small, plane secondary mirror at 45° to deflect the focus of the primary to a position outside the tube near the top of the telescope. [McL97]
Newtonian Cosmology	The simplest cosmological models, including the standard Big Bang models, can be derived in the framework of Newtons classical theory of gravitation, although Birkhoff's theorem from the theory of general relativity is needed to justify the use of Newtonian theory in an infinite medium. [Silk90]
Ney-Allen Nebula	An extended infrared source in the Trapezium region of Orion, which shows a strong 10-µ emission feature assumed to result from circumstellar shells of silicate dust. [H76]
NGC	New General Catalogue -- A catalogue of 7,840 nebulae, star clusters, and galaxies that was published in 1888 by John Dreyer. [C95]
Nickel	A transition metal that occurs naturally as the sulfide and silicate. It is used as a protective coating (on other metals) and in the manufacture of various alloys, such as Nichrome and stainless steel. Symbol: Ni; m.p. 1453°C; b.p. 2732°C; r.d. 8.902 (25°C); p.n. 28; r.a.m. 58.6934. [DC99]
NICMOS	Near Infrared Camera for Multi-Object Spectrography [HST] [LLM96]
Nicol Prism	A device for producing plane polarized light, consisting of a crystal of calcite cut with a 68° angle, cleaved along the optic axis, and stuck together with a thin layer of Canada balsam. The Canada balsam, which is not birefringent, has the same refractive constant for both ordinary and extraordinary rays (n = 1.66). The latter ray passes through the prism (in calcite n = 1.66). However, the ordinary ray in calcite has a lower refractive constant (n = 1.48) than in the Canada balsam, and suffers total internal reflection at the interface. Nicol prisms are more transparent than Polaroid. [DC99]
Nicol Prism	A device made from a split crystal of Iceland spar with which plane-polarized light can be detected. [H76]
Nightglow	see Airglow [H76]
Night-Sky Light	The faint, diffuse glow of the night sky. It comes from four main sources : airglow, diffuse Galactic light, Zodiacal light, and the light from these sources scattered by the troposphere. [H76]
Niobium
NIR	Near InfraRed
Nitrogen	(a) Element with atomic number seven and the fourth most common metal in the Universe. It formed during hydrogen burning in main-sequence stars and red giants, via the CNO cycle. [C95] (b) A nonmetallic element; nitrogen accounts for about 78% of the atmosphere (by volume) and it also occurs as sodium nitrate in various mineral deposits. It is separated for industrial use by the fractionation of liquid air. Nitrogen has two isotopes; 14N, the common isotope, and 15N (natural abundance 0.366%), which is used as a marker in mass spectrometric studies. Symbol: N; m.p. -209.86°C; b.p. -195.8°C; d. 1.2506 kg m-3 (0°C); p.n. 7; r.a.m. 14. [DC99]
NLR	Narrow Line Region
NLRG	Narrow-Line Radio Galaxy
NLS1	The class of Narrow-Line Seyfert 1 galaxies was first proposed by Osterbrock and Pogge (1985, Ap.J., 297, 166). These objects have permitted linewidths (on the order of 1,000 km/sec) which are much smaller than normal Seyfert 1 galaxies (which have linewidtths on the order of 10,000 km/s). However, they differ from Seyfert 2 galaxies in that their optical spectra show several characteristics normally associated with Seyfert 1 galaxies, such as [OIII]/H_beta < 3, permitted lines somewhat broader than forbidden lines, and either blends of Fe II or very high excitation lines of [Fe VII] or [Fe X], etc. About 10% of Seyfert 1 galaxies may be Narrow-Line Seyfert galaxies. [BFM06]
NLXG	Narrow Emission Line X-ray Galaxy
NOAO	National Optical Astronomy Observatory (USA). [LLM96]
Nobelium	A radioactive transuranic element of the actinoid series, not found naturally on Earth. Several very short-lived isotopes have been synthesized. Symbol: No; p.n. 102; most stable isotope 259No (half-life 58 minutes). [DC99]
No-Boundary Proposal	An initial (boundary) condition for the Universe proposed by James Hartle and Stephen Hawking. In this proposal, the mathematics of general relativity is reformulated so that time is replaced by a space-like coordinate, in effect representing the Universe as having 4 space dimensions instead of 3 space dimensions and a time dimension. (In such a formulation, "time" does not have its usual meaning.) Hawking and Hartle suggest that the geometry of this representation of the Universe should be analogous to the geometry of the surface of a sphere, that is, a shape with no edges-hence the name no-boundary proposal. When translated back into ordinary time and space, this suggested boundary condition takes the form of a specific initial condition for the Universe. The no-boundary proposal is formulated within a quantum mechanical calculation of the behavior of the early Universe. [LB90]
Node	(a) The point at which a standing wave pattern intersects the horizontal axis and at which the wave consequently has zero amplitude. [H76] (b) Either of the points on the celestial sphere at which the plane of an orbit intersects a reference plane. The position of a node is one of the standard orbital elements used to specify the orientation of an orbit. [S92]
Nodes, Line of	The intersection between the orbital plane of the Moon or a planet and the plane of the ecliptic. [H76]
Nodical Month	The interval of time (27.2122 days) between two successive transits of the Moon through its ascending node. (also called Draconic month)[H76]
Noether's Theorem	A mathematical theorem that states that for every symmetry of the Lagrangian of a physical system (i.e. for every set of transformations under which the Lagrangian is invariant), there will be some quantity which is conserved by the dynamics of the system. [CD99]
No Hair Theorem	(a) Proposed by John Wheeler, it states that the only properties of matter conserved after entering a black hole are its mass, its angular momentum and its electrical charge; it thus becomes neither matter nor antimatter. [A84] (b) The notion that all detailed effects of the initial conditions of the Universe have been erased by subsequent physical processes. According to this view, a wide variety of initial conditions would have led to the same Universe today; the initial conditions were unimportant in determining the future state of the Universe. [LB90]
Noise	(a) Sound composed of a random mix of different frequencies. White noise is a completely random mix over a wide frequency range; it has a confusing effect on the listener. Pink noise - random frequencies in a selected range - is often used as a background to mask other sounds. (b) A general term describing any signal that impairs the efficient working of an electronic device. There are two principal types: white noise and impulse noise. The unwanted energy of white noise is distributed across a wide frequency band, in the same way that the energy of white light is distributed across a band according to color. Impulse noise is a consequence of one or more momentary electrical impulses. There are numerous types of white noise: thermal noise is caused by the random thermal motion of electrons superimposed on a steady current flow; random noise can also be caused by an irregular momentary disturbance originating in an atmospheric electrical disturbance or a similar disturbance in the Sun, etc. (c) Noise results in undesirable sounds in a loudspeaker, which mask the desired effect, and, in television, results in `snow', an unwanted pattern on a TV screen similar to falling snow. [DC99] (d) A term used to describe unwanted electronic signals, sometimes random and sometimes systematic, which contaminate the weak signal from an astronomical source. Types of noise include, 1/f noise which gets larger at lower frequencies, white noise which is independent of frequency, and reset noise which is a consequence of device readout architecture but which can be removed by correlated double sampling. [McL97]
Noise Source	An electronic device designed to generate known amounts of radio noise in order to test and calibrate the receivers of radio telescopes. [H76]
Non-Baryonic Matter	Material that consists of exotic subatomic particles. These subatomic particles can move slowly (cold dark matter) or fast (hot dark matter). Cosmologists who believe in inflation - or anyone else who thinks the mass density of the Universe () is 1.0 - believe that most of the Universe consists of non-baryonic matter. [C95]
Non-Coherent Scattering	Absorption of a photon and reemission at a different frequency (as seen by an observer) by scattering atoms. The natural width of the lines, Doppler broadening, and pressure broadening are the main processes that give rise to noncoherent scattering. [H76]
Non-Conservative Scattering	Scattering that occurs in the presence of absorption. [H76]
Non-Destructive Readout	A means of reading out an infrared array or a CCD with a skipper output in which the output voltage is sampled without resetting the detector. The output voltage can be sampled a large number of times to improve noise performance. [McL97]
Non-Euclidean Geometry	Geometry that does not follow the postulates and results of Euclidean geometry. For example, in a non-Euclidean geometry, the sum of the interior angles of a triangle differs from 180 degrees. According to Einstein's general relativity theory, gravity distorts space into a non-Euclidean geometry. [LB90]
Non-Gray Atmosphere	A model atmosphere constructed by letting the absorption coefficient vary with frequency. [H76]
Non-Linear Optics	This is concerned with the optical properties of matter in intense radiation fields. The induced electromagnetic polarization does not depend linearly on the radiation strength but is severely distorted by the strong field. Optical harmonics, frequency mixing and intensity dependent refractive indices are all produced by this nonlinear response. [D89]
Non-Perturbative	Feature of a theory whose validity is not dependent on approximate, perturbative calculations; an exact feature of a theory. [G99]
Non-Relativistic Zone	The region far from a pulsar where r M. [H76]
Non-Thermal Radiation	Radiation emitted by energetic particles for reasons other than high temperature of the source. The spectrum of nonthermal radiation is different from that predicted by Planck's law for a blackbody. [H76]
Nordtvedt Effect	A theoretical violation of the principle of equivalence for massive, self-gravitating bodies. [H76]
Normal	The perpendicular to a reflecting or refracting surface at the point of incidence of the ray concerned. Angles of incidence, reflection, and refraction are measured between the normal and the incident ray, reflected ray, and refracted ray respectively. A normal ray is one incident perpendicularly on a surface - the angle of incidence is zero. [DC99]
Normal Modes	(a) All the characteristic frequencies of an oscillating body. [H76] (b) The state of a superconducting metal above its transition temperature, in which its behavior is essentially indistinguishable from that of a metal which never becomes superconducting.[D89]
Normalization	A mathematical technique for eliminating divergent terms or for making them converge. [H76]
North Point	The point at which the meridian intersects the horizon below the North Celestial Pole. [H76]
North America Nebula	An emission nebula (NGC 7000) in Cygnus. [H76]
North Galactic Pole	A point in the constellation Coma Berenices where we look perpendicular to and above the Galactic Plane. The nearest bright star to the North Galactic Pole is Arcturus, in the neighboring constellation Bootes. [C95]
North Polar Sequence	(a) Or circumpolar stars, comprises those stars which never set, from the viewpoint of an observer on Earth. [A84] (b) A series of accurately measured magnitudes (down to mv = 20) of stars within 2° of the north celestial pole, which have been used to establish an arbitrary zero point on the magnitude scale. (Nowadays the UBV system is more frequently used to establish the zero point.) [H76]
North Polar Spur	A radio continuum feature extending from the galactic plane to the vicinity of the North Galactic Pole. It is probably about 50-200 pc distant, and is believed to be a supernova remnant. It is also an X-ray source. [H76]
North Star	Polaris. [C95]
Northern Cross	A group of bright stars in the constellation Cygnus. [H76]
Nova	(a) A star that brightens suddenly and to an unprecedented degree, creating the impression that a new star has appeared where none was before. Hence the name, from nova for "new". see Supernova [F88] (b) A star that exhibits a sudden surge of energy, temporarily increasing its luminosity by as much as 14 mag. (Since 1925 novae have been given variable star designations.) Novae are old disk-population stars. Unlike supernovae, novae retain their stellar form and most of their substance after the outburst. All known common novae are found in close binary systems with one component a cool red giant and the other a hot, less massive object which is the seat of the instability. [H76] (c) A stars that undergoes an explosion during which its brightness increases by up to ten magnitudes. Usually the following phases are distinguished (in order of time): pre-maximum, principal, diffuse enhanced, Orion, nebular and post-nova. [JJ95]
NRAO	National Radioastronomy Observatory (USA). [LLM96]
NSF	National Science Foundation (USA). [LLM96]
NTT	New Technology Telescope [LLM96]
Nuclear Chronology	A method of dating an object by measuring how many atomic nuclei have disintegrated and changed into other nuclei. Uranium dating of the earth is an example of nuclear chronology. [LB90]
Nuclear Density	The density of an atomic nucleus (about 1014 g cm-3). [H76]
Nuclear Disk	A rotating disk of about 106 M of neutral hydrogen in the inner 800 pc of our Galaxy. [H76]
Nuclear Forces	(a) There are two kinds of nuclear forces: the strong nuclear force and the weak nuclear force. These two forces, plus the gravitational and electromagnetic forces, comprise the four fundamental forces of nature. The strong nuclear force, which is the strongest of all four forces, is the force that holds protons and neutrons together in the atomic nucleus. The weak nuclear force is responsible for certain kinds of radioactivity; for example, the disintegration of a neutron into a proton, electron, and antineutrino. [LB90] (b) Although protons and neutrons carry no strong (or color) ch arge, at tiny distances near a proton or neutron, the cancellation of the strong field from its constituent quarks and gluons is incomplete, leaving a residual strong force that leaks outside the proton and neutron. This is the nuclear force that binds protons and neutrons into nuclei. [K2000]
Nuclear Fusion	Process by which the Sun (and other stars) radiates energy. The nucleus of an atom fuses with the nuclei of other atoms to form new, heavier atoms at the same time releasing large amounts of energy. In the Sun, hydrogen atoms are converted into helium by this process, with carbon and nitrogen as intermediates. Cooler stars undergo the proton-proton cycle with a similar result. [A84]
Nuclear Matter	Matter in which the numbers of protons and neutrons are roughly equal, as in atomic nuclei. Nuclear matter is probably in a liquid or a solid state. [H76]
Nuclear Statistical Equilibrium	Equilibrium with respect to strong and electromagnetic interactions. [H76]
Nuclear Time Scale	Time required for a star to evolve a significant distance off the main sequence; the time it takes a star to convert all its available hydrogen into helium. For the Sun, it is 1010 years. (Cf. Kelvin time scale.) [H76]
Nucleon	(a) The generic name for the proton and the neutron. [CD99] (b) The generic name for neutrons and protons which are the constituents of a nucleus.[D89]
Nucleosynthesis	(a) The transformation of one element or isotope into another. Nucleosynthesis occurred just after the big bang, but today most nucleosynthesis takes place in stars - for example, the Sun presently converts hydrogen into helium. [C95] (b) The production of heavy nuclei from the fusion of lighter ones. According to the big bang theory, the infant Universe consisted of only hydrogen, the lightest of all atomic nuclei, because any heavier nuclei would have come apart in the intense heat. All other elements would have to be formed later, in nucleosynthesis processes. It is believed that most of the helium, the next lightest element after hydrogen, was formed when the Universe was a few minutes old, and that most of the other elements were made much later, in nuclear reactions at the centers of stars. [LB90] (c) The nuclear processes by which the chemical elements are synthesized. The principal sites for these processes are the central regions of stars where the temperatures are sufficiently high for the synthesis of, for example, helium from protons and neutrons or carbon from three helium nuclei to take place. Other forms of nucleosynthesis include explosive nucleosynthesis which takes place during supernova outbursts and primordial nucleosynthesis in which light elements are synthesized during the hot early phases of the hot big bang.[D89]
Nucleosynthetic Era	The era following the Leptonic Era, between 1 second and 1,000 seconds after the Big Bang, when neutrons were produced and helium and deuterium were synthesized. [Silk90]
Nucleus	(a) The central part of an atom, composed of protons and neutrons (which are made of quarks) and containing nearly all of each atom's mass. [F88] (b) The central region of a galaxy. [F88] (c) The massive, positively charged central part of an atom, composed mainly of protons and neutrons, around which the electrons revolve. The radius of an atomic nucleus is directly proportional to the cube root of its mass. Density at least 1014 g cm-3. Radius 10-12-10-13 cm. [H76] (d) Although protons and neutrons are color-neutral composites of quarks and gluons, the quarks and gluons are not all at the same places, so some of their color (or strong) fields exist outside the proton or neutron, giving an attractive force that binds protons and neutrons into nuclei. The attractive effects of this residual color force are offset by the electrical repulsion of the protons, so nuclei with too many protons cannot exist. It turns out that there are ninety-two stable or long-lived nuclei in nature. They are the nuclei of the atoms of the ninety-two chemical elements. [K2000]
Nucleus (of a comet)	The stellar-appearing frozen core, containing almost the entire cometary mass, in the head of a comet. [H76]
Nuclide	A species of atomic nucleus, analogous to the word "isotope" for a species of atom. The word is also used to distinguish between atomic nuclei that are in different energy states. [H76]
Null Geodesic	The path of a light ray in curved spacetime. It is characterized by the fact that its tangent U at any point is of zero length: Uµ Uµ = 0. (same as null line) [H76]
Null Line	The path, in space-time, of a light ray or other massless object. Space-time distances measured along a null line are zero. (same as null geodesic) [P88]
Number-Count Test	A cosmological test that involves counting all galaxies down to a certain limiting magnitude and repeating this procedure for fainter and fainter limiting magnitudes. Deviations from the relation expected in Euclidean space can help ascertain whether the Universe is open or closed. In practice, this test provides a strong constraint on models of galactic evolution and luminosity at past eras. [Silk90]
Number Density	Number of particles per cm3 (cf. column density). [H76]
Numbers (Dirac):	Big Number Hypothesis: In 1937 Dirac pointed out that the ratio of the largest to the smallest natural units of length, of force and of time, each came to about 1040; a number which he called a cosmological constant; they have subsequently been often referred to as big numbers. Thus radius of the Universe / radius of an electron coulomb force between proton and electron / gravitational force between proton and electron T/t 1040 where T is the time taken for light to reach the edge of the Universe and t that required for light to cover a distance equal to the radius of an electron. Furthermore, the ratio of the mass of the Universe to the mass of an average particle of matter comes to about 1079, which is approximately (1040)2; the figure 1079 is also called the Eddington number after the British astronomer Sir Arthur Eddington (1882-1944) who predicted the number of particles in the Universe in the late 1920s. The big number hypothesis indicates that if the fundamental constants are all expressed as dimensionless ratios to rid them of man-made dimensions (e.g. by using units of length, mass and time from the hydrogen atom) the constants will tend to lie within a few orders of magnitude of unity with the notable exception of the gravitational constant G. [JM92]
Nutation	(a) Slight but recurrent oscillation of the axis of the Earth, caused by the Moon's minutely greater gravitational effect on the Earth's equatorial "bulge". [A84] (b) A small, irregular oscillation in the precessional motion of Earth's rotational axis, caused primarily by lunar perturbations. It has a principal period of 18.6 years, and moves the equinox as much as 17" ahead of or behind its mean position. [H76]
Nyquist Sampling	The minimum number of resolution elements required to properly describe or sample a signal, such as a star image, without causing erroneous effects known as aliasing. For electronic imaging, this number is generally taken as 2 pixels across the seeing disk diameter at the half intensity points (FWHM). [McL97]