Glossary Nn

N-TYPE SEMICONDUCTOR - Semiconductor that has excess of electrons. For example a semiconductor, such as Si, which has been doped with atoms of a donor element, such as P. The extra valence electrons of the donor atoms provide filled energy states in the band gap, just below the conductionband. Electrons from these “extra” states can easily move into the conduction band and become conduction electrons. Replacing even one in every 106 Si atoms with P can increase the conductivity of the solid by a factor of five million. Because the density of conduction electrons provided by the donor is much larger than the density of holes contributed by the semiconductor atoms, conduction in this material is primarily due to the motion of (negatively-charged) electrons.

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NAKHLA METEORITE - Achondrite that exploded over the Egyptian town of El-Nakhla, on June. 28, 1911. The Nakhla meteorite broke into ~40 fragments, one of which allegedly (though this has never been proved) killed a dog. The meteorite was later identified as belonging to an exclusive group of objects, known as SNC meteorites, which are believed to have come from the surface of Mars.

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NAKHLITE (NAK) - One of the SNC meteorites believed to have come from Mars. Nakhlites probably formed as lava flows with unusual compositions. The nakhlites consist mainly of green augite crystals with some olivine in a very fine-grained matrix of plagioclase, feldspar, clinopyroxene, Fe-Ti oxide, sulfide, and phosphate. There are traces of pre-terrestrial aqueous alteration products in the form of hydrated minerals, including clay minerals and carbonates. Nakhlites have relatively young ages (1.3-1.4 Ga) and, based on the age of clay found inside the specimens examined, are thought to have been exposed to water at ~600 Ma.

NEAR-EARTH ASTEROIDS (NEAs) - Asteroids with orbits that bring them within 1.3 AU (195 million km) of the Sun. NEAs are a dynamically young population whose orbits evolve on 100-million-year time scales because of collisions and gravitational interactions with the Sun and the terrestrial planets. These asteroids are probably ejected from the main belt by a combination of asteroid collisions and the gravitational influence of Jupiter. Some NEAs may be the nuclei of dead, short-period comets. Approximately 250 NEAs have been found to date, probably only a few percent of their total population. The largest presently known is 1036 Ganymed, with a diameter of ~25.5 miles (41 kilometers). The NEA population appears to represent most or all asteroid types found in the main belt. NEAs are grouped into three categories, named for famous members of each: 1221 Amor, 1862 Apollo, and 2062 Aten. The Amors cross Mars' orbit but do not quite reach the orbit of Earth. The Apollos cross Earth's orbit and have periods >1 year.  The Atens cross Earth's orbit and have periods <1 year. With expected lifetimes of around 10 million years, the ultimate fate of NEAs may be gravitational ejection from the Solar System or collision with one of the terrestrial planets.

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NEAR-EARTH ASTEROID RENDEZVOUS (NEAR SHOEMAKER) - First mission to orbit an asteroid (see The spacecraft carried instruments, including a multi-Spectral Imager (MSI) used to determine Eros's shape and surface features, and mineral distributions. Other instruments determines the distribution and abundance of surface minerals like olivine and pyroxene, searched for induced magnetic fields, and used laser ranging to construct topographic profiles. The NEAR spacecraft passed within 1225 km of the asteroid 253 Mathilde, a C-type carbon-rich asteroid, on its trip to Eros and acquired images and determined its mass to an accuracy of ~5%. After a year of orbiting Eros, the spacecraft croshed on the asteroid's surface in February 2001, after transmitting close-up images of the surface during its final descent.

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NEBULA - Interstellar cloud of gas and dust. The properties of nebulae vary enormously and depend on their composition as well as the environment in which they are situated. Emission nebula are powered by young, massive stars and emit their own light, reflection nebulae shine by reflecting light from nearby massive stars, and dark nebulae, as the name suggests, are dark and can only be seen when silhouetted against a bright background.

NEUTRAL HYDROGEN - Electrically neutral hydrogen atom with one proton and one electron. It is commonly referred to as HI (pronounced H-one), and is located throughout galaxies as HI clouds or external to galaxies as part of the intercloud gas. Neutral H is detected via the spin-flip transition at 21 cm, and HI clouds were used to determine the structure of our Galaxy from our location within it.

NEUTRALINO - Candidate for non-baryonic dark matter particle. Supersymmetry, a hypothetical symmetry relating bosons to fermions, predicts the existence of hitherto undetected "partners" for each type of particle in the Standard Model of particle physics. Under certain assumptions, the lightest such partner particle would be stable, and if it is neutral (a "neutralino"), would make a good dark matter candidate. Reasonable neutralino masses range from 30 GeV to 10 TeV. If they make up the dark matter of the Milky Way, then they should have nonrelativistic velocities and hence their annihilation would give rise to gamma rays with unique energies.

NEUTRINO (ν) - One of three electrically neutral leptons that experience only the weak nuclear force and gravitional force, and pass easily through matter. Three types of neutrinos are known and there is strong experimental evidence that no additional types of neutrinos exist. Each type or "flavor" of neutrino is related to a charged particle (which gives the corresponding neutrino its name):

Charger Partnerelectron (e-)muon (μ)tau (τ)

Electron neutrinos (νe) are generated in the interior of the Sun and similar stars during hydrogen burning. normous numbers of neutrinos are formed during supernova explosions as electrons are squeezed into protons, producing a neutron core. The potential energy liberated by such a collapse is radiated mainly in a burst of neutrinos. Such a neutrino burst was first observed of the supernova of 1987.

Recent solar and atmospheric neutrino experiments have established that at least some neutrinos have mass and that they mix (oscillate) between themselves. Based on these experiments, it appears that the cosmic density of massive neutrinos is at least as large as that of luminous matter. Neutrino experiments as well as the large-scale clustering of galaxies indicate that light stable neutrinos have a mass less than a few electron volts. Best mass estimates as of 2006 are that the heaviest neutrino must be at least 0.05 eV, but no more than 0.18 eV.

In addition to neutrinos produced in nuclear reactions, a large number of "relic" neutrinos originated in the Big Bang. These should have a local present density of ~110 per nerutrino species per cm3. However, no viable method of direct detection exists.

NEUTRON (n) - Charge-neutral hadron with a mass of 1.6748 x 10-27 kg, equivalent to 939.573 MeV, and an intrinsic angular momentum, or spin, of ½ (in units of h/2π). The neutron is a nucleon, one of the two basic constituents of all atomic nuclei (apart from 1H, which consists of a single proton). The total number of protons and neutrons in an atomic nucleus defines its mass number, A. Neutrons are stable within the nucleus, but unstable in isolation (t½ = 886±2 s), decay being mediated by a W boson:

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NEUTRON CAPTURE - Primary mechanisms by which very massive nuclei are formed in stars and supernova explosions, principally, the s-process and r- process. Instead of fusion of like nuclei, heavy elements are created by the addition of more and more neutrons to existing nuclei.

NEUTRON DECAY - Nuclear decay by emission of a neutron (decreasing A by 1).

NEUTRON DEGENERACY PRESSURE - Pressure produced by quantum mechanic constrains on neutron packing. Quantum mechanics restricts the number of neutrons that can have low energy. Each neutron must occupy its own energy state. When neutrons are packed together, the number of available low energy states is too small and many neutrons are forced into high energy states that produce pressure. Because the pressure arises from a quantum mechanical effect, it is insensitive to temperature (pressure doesn't go down as the star cools). Similar to electron degeneracy pressure but, because neutrons are much more massive than electrons, neutron degeneracy pressure is much larger and can support stars more massive than the Chandrasekhar mass limit.

NEUTRON STAR - Dense ball of neutrons that remains at the core of a star after a supernova explosion has destroyed the rest of a star with mass 8-18 (?) Msun. A neutron star has mass ~2-3 Msun, density ~1014 g/cm3, and is supported by neutron degeneracy pressure. Typical neutron stars are 10-20 km across and have escape velocities ~70% speed of light. Neutron stars rotate extremely rapidly as a consequence of the conservation of angular momentum, and have incredibly strong magnetic fields due to conservation of magnetic flux. The relatively slowing rotating core of the massive precursor star increases its rotation rate enormously as it collapses to form the much smaller neutron star. At the same time, the magnetic field lines of the massive star are pulled closer together as the core collapses. This intensifies the magnetic field of the star to around 1012 times that of the Earth. The result is that neutron stars can rotate up to at least 60 times per second when born. If they are part of a binary system, they can increase this rotation rate through the accretion of material, to over 600 times per second!

Neutron stars that have lost energy through radiative processes have been observed to rotate as slowly as once every 8 seconds while still maintaining radio pulses, and neutron stars that have been braked by winds in X-ray systems can have rotation rates as slow as once every 20 minutes. Observations also reveal that the rotation rate of isolated neutron stars slowly changes over time, generally decreasing as the star ages and rotational energy is lost to the surroundings through the magnetic field (though occasionally glitches are seen). An example is the Crab pulsar, which is slowing its spin at a rate of 38 nanoseconds per day, releasing enough energy to power the Crab nebula.

The interior of a neutron star contains exotic matter such as superfluid neutrons, superconducting protons, and stranger subatomic particles (e.g., pion condensates, λ hyperons, Δ isobars, and quark-gluon plasmas). Rapidly rotating neutron stars are called pulsars.

NEWMAN LINES - Fine striations seen in some high-Ni iron meteorites.

NEWTON’S LAW OF GRAVITATION - Two bodies, with masses m1 and m2, separated by distance, r, have a mutual force of attraction, F, of:

where, G = universal gravitational constant (6.670 × 10-11 N m2/kg2). An equal and opposite force acts on each mass, but the force acting on m1 is in the opposite direction of that on m2.

NEWTON’S LAWS OF MOTION - Isaac Newton's theory of mechanics, describing the motions of objects, and the law of gravitation. First Law: An object in a state of uniform motion will remain in motion unless acted upon by an external force. Second Law: Net applied force on an object produces acceleration, a, proportion to the mass, m:

Third Law: If A exerts a force on B, then B will exert an equal and oppositely directed force on A. For every action, there is an equal and opposite reaction.

NITROGEN (N) - Principal constituent of the Earth’s atmosphere (78.08 vol. % at ground level). Nitrogen is the fifth most abundant element in the universe by atom abundance. Nitrogen comprises only 3.5 vol. % of the atmosphere of Venus and 2.7 vol. % of Mars’s atmosphere. Nitrogen has two isotopes: 14N (99.632 %) and 15N (0.368 %). Nitrogen isotopic variations are reported as δ15N in parts per thousand (‰) deviations from the nitrogen isotopic composition of Earth's atmosphere (15N/14N = 0.003676).

Large N isotopic variations are observed in the solar system from -250 ‰ (lowest values in lunar soils) to 1600 ‰ (bencubinites). Comets Hale-Bopp and LINEAR have essentially identical δ15N values of ~940 ‰. Values of δ15N in the lunar regolith show a very large range from ~100 ‰ to -250 ‰ with ratios appearing to vary with age. The lunar results can be explained by secular variation in δ15N of the solar wind over time, although this hypothesis conflicts with the apparent stability of ratios in other solar wind volatile elements. Alternatively, the variation could result from mixing in a non-solar wind component (meteorite or comet material deposited on the moon during impacts). Extreme δ15N variations have been documented in organic material from meteorites (image of insoluble organic matter extracted from the CR chondrite EET 92042).

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NO HAIR CONJECTURE - Conjecture proved in the 1970s and 1980s that within general relativity a black hole has only three salient external characteristics: mass, angular momentum, and electric charge. All other properties (including baryon number, lepton number, strangeness, etc.) are destroyed as matter falls into the event horizon.

NOBLE GASES - Element occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive.

NODE - Point where the orbit of a planet, as viewed from the sun, intersects the ecliptic. The ascending and descending nodes refer respectively to the points where the planet moves from S to N and N to S.

NONMETAL - Elements that do not exhibit the characteristics of metals. These elements differ markedly from metals in respect to electronegativity and thermal and electrical conductivity. These elements, in general, are poor conductors and have high electronegativity. This series includes halogens, noblegases and some of the metalloids (B, Si, As and Te).

NONTHERMAL RADIATION - Radiation the characteristics of which do not depend on the temperature of the source. There are three common types non-thermal radiation in astronomy. Synchrotron emission is generated by charged particles spiraling around magnetic field lines at relativistic speeds. Bremsstrahlung is produced by the acceleration of free electrons after interaction with positively charged hydrogen ions. Stimulated emission is produced when electrons in a metastable state are prompted to decay to the ground state by a passing photon with the same energy as the difference between the two levels.

NORITE - Igneous rock found in the lunar highlands composed of plagioclase and pyroxene.

NOVA - Star that, over a period of a few days, becomes 103-104 times brighter than it was previously. Novae are observed ~10-15 times per year in the Milky Way.

NUCLEAR FORCES - Two of the fundamental forces, or interactions, the strong interaction and the weak interaction. These are not necessarily confined to the nucleus, despite the name. The strong interaction not only holds nucleons together in the nucleus, but also binds quarks into hadrons. The weak interaction is involved in some nuclear processes such as radioactivity, but also causes free neutrons to decay.

NUCLEON - Either of the two baryons, the proton and the neutron, which form the nuclei of atoms.

NUCLEOSYNTHESIS - Process by which nuclear reactions produce the various elements of the periodic table. H and He were largely created in the Big Bang; most of the heavier elements were formed inside stars and during supernova explosions by the r-process and s-process.

NUCLEOTIDE - Subunits of nucleic acids and the fundamental structural unit of the nucleic acid group of organic macromolecules. Nucleotides are composed of a phosphate, a sugar such as ribose, and a nitrogen-containing base such as adenine. Nucleotides are involved in information storage (as nucleotides in DNA), protein synthesis (as nucleotides in RNA), and energy transfers (e.g. as single nucleotide ATP).

NUCLEUS - Core of an atom, where most mass and all positive charge is concentrated. It consists of protons and neutrons.

NULL EXPERIMENT - Experiment which, after being executed, yields no result. Null experiments are just as meaningful as non-null experiments. If current theory predicts an observable effect (or predicts there should be no observable effect) and experimentation (within the required accuracy) does not yield said effect, then the null experiment has told us something about the theory.

NUMBER DENSITY - Number of a particular type of object found in each unit volume. For example, if 2500 garnets are fairly uniformly spread through a volume of 100 cm3, the number density of garnets is 25/cm3.

NUTATION – Nodding motion of the earth's pole superposed on the slow precession.