Glossary Pp

P-TYPE SEMICONDUCTOR - Semiconductor consisting of semiconductor material, such as Si or Ge, doped with atoms of an acceptor element, such as Ga. The unfilled valence shells of the acceptor atoms provide empty energy states in the band gap, just above the valence band. Valence electrons can move into these "extra" states, leaving behind holes in the valence band, as shown in the energy diagram to the right. Replacing even one in every 106 Si atoms with a Ga atom can increase the conductivity of the solid by a factor of five million. Because the density of holes provided by the acceptor is much larger than the density of conduction electrons contributed by the semiconductor atoms, conduction in this material is primarily due to the motion of (positively-charged) holes. Thus it is called a p-type semiconductor.

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PAIR PRODUCTION - Creation of a particle and its antiparticle from some form of energy, such as photons. An example is the creation of an electron and a positron from two γ-ray photons with energies greater than the rest mass energy of the electron:

PALLASITE - One of two main classes of stony-iron meteorite, the other being mesosiderites. Pallasites are igneous in nature and characterized by crystals of olivine, often peridots (clear, green gem quality olivine crystals), embedded in a matrix of Ni-Fe metal. The type specimen, weighing 680 kg, was found in the mountains near Krasnojarsk, Siberia, and first documented by the German naturalist Peter Pallas in 1772, though it was some decades later before people recognized the extraterrestrial nature of what became known as the Pallas iron. Pallasites are believed to have come from the core/mantle boundary of differentiated asteroids that were broken apart by impact. In most cases, they have chemical, elemental, and isotopic features that link them to specific chemical groups of iron meteorites, suggesting that they come from the same parent bodies as these irons.

Finmarken pallasite. Image source:

PANCAKE VOLCANO - Volcanic feature on Venus formed by high viscosity lava.

Pancake volcanoes on Venus. Image source:

PARALLAX - Small periodic shift of the apparent positions of nearby stars due to the changing position of the Earth as it orbits the Sun. The nearer the star is, the larger the shift. The distance to stars in parsecs is simply 1/parallax, (or in light years it is 3.2616 divided by the parallax) where the parallax is in arcseconds.

Image source: Figure 3.2, Introduction to Modern Astrophysics, 2nd ed., Carroll & Ostlie, 2006.

PARSEC (pc) - Unit of distance commonly used by astronomers. A star one parsec away has a parallax angle of one second of arc. A star with a parallax shift of 0.1 arcseconds is at a distance of 10 parsecs, and so forth. A parsec is equal to ~3.262 lightyears (3.09 x 1016 m) or 206,265 AU. Multiples of this unit are kiloparsecs (kpc) = 103 pc and megaparsec (Mpc) = 106 pc.

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PARTITION COEFFICENT - Constant ratio that is found when a heterogeneous system of two phases is in equilibrium (also called the “distribution coefficient”). The ratio of the concentrations (or strictly, the activities), K, of the same molecular species in the two phases is constant at constant temperature and pressure:

This concept can be applied between phases in the same state (i.e., solid-solid, gas-gas) or between different states (solid-liquid).

PASCAL - Standard unit of pressure abbreviated Pa, which is equivalent to 1 kg/m2. The pressure at the surface of the Earth is 100,000 Pa. Pressures inside planets are very large numbers, usually expressed as GPa.

PASCHEN SERIES - Hydrogen series that describes the emission spectrum when electrons jump to the third orbital. All of the lines are in the infrared portion of the spectrum.

PATERA - Irregular or complex crater with toothed edges (pl. paterae).

PAULI EXCLUSION PRINCIPLE - First proposed by Wolfgang Pauli to explain the arrangement of electrons in atoms, the Exclusion Principle asserts that no two fermions of the same type can exist in the same state (having the same quantum numbers) at the same place and time. Bosons do not obey the Pauli Exclusion Principle.

PEAK RING - Central uplift characterized by a ring of peaks rather than a single peak. Peak rings are typical of larger terrestrial craters above ~50 km in diameter.

Barton Crater on Venus. This 54-km (32-mi) diameter crater is the size at which craters on Venus begin to possess peak-rings instead of a single central peak. The floor of the crater is flat and radar-dark, indicating possible infilling by lava flows sometime following the impact. Barton's central peak-ring is discontinuous and appears to have been disrupted or separated during or following the cratering process. Image Source:

PENTLANDITE - Fe-Ni sulfide, (Fe,Ni)9S8, often associated with troilite, found in the matrix and chondrules of CO, CV, CK and CR chondrites.

PERIAPSIS - Point of closest approach for an object moving in an elliptical orbit about another celestial body. At this point in the orbit, the object is traveling at its greatest speed (Kepler's Second Law). The periapsis is equivalent to the perihelion (celestial body orbiting the Sun), the perigee (celestial body orbiting the Earth), and the periastron (celestial body orbiting a star; e.g. in a binary star system). The point of maximum distance in the orbit is called the apoapsis.

PERIASTRON - Point of closest approach between the two stars in a binary star system. This position is the same as the periapsis of the orbit, but specifically refers to orbits around other stars. The point of maximum separation between the two stars is the apastron.

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PERIGEE - The point in a satellite’s or the Moon’s orbit when it is closest to Earth. At this point, the object is moving at its maximum speed (Kepler's Second Law). The perigee refers specifically to orbits around the Earth, and is equivalent to the periapsis of a general orbit.

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PERIHELION - The point in an object’s orbit when it is closest to the sun. At this point in the orbit, the planet is moving at its maximum speed (Kepler's Second Law). The perihelion refers specifically to orbits around the Sun, and is equivalent to the periapsis of a general orbit. In a strong gravitational field, the location of perihelion may advance on successive orbits. Within the Solar System, this is most easily seen in the orbit of Mercury and provides an important test of General Relativity.

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PERIOD - Horizontal row in the periodic table of the elements. Whereas groups are characterized by the number of electrons present in the outer shell, periods are characterized by the number of energy levels (shells) of electrons surrounding the nucleus.

PERIOD-LUMINOSITY RELATION - Relation between the pulsation period of a variable star and its absolute brightness. Measurement of the pulsation period allows the distance of the star to be determined.

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PEROVSKITE - Term applied to X2+Y4+O3 minerals with perovskite structure. Each Y4+ ion is octahedrally coordinated with six oxygen atoms; the X2+ ion sits in the center. Although ideally a cubic structure, the actual mineral is slightly monoclinic because one unit cell angle is 90.67º. MgSiO3 perovskite is the principal mineral in the Earth's lower mantle. CaTiO3 perovskite is a minor constituent of some igneous rocks in the Earth's crust and mantle and is found in CAIs in carbonaceous chondrites.

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PETROLOGY - Science dealing with the origin, history, occurrence, chemical composition, structure and classification of rocks.

PETROLOGIC TYPE - Measure of the degree of aqueous alteration (Types 1 and 2) and thermal metamorphism (Types 3-6) experienced by a chondritic meteorite. Type 3 chondrites are further subdivided into 3.0 through 3.9 subtypes.

PFUND SERIES - Hydrogen series which describes the emission spectrum produced when an electron jumps to the fifth orbital. All of the lines are in the infrared portion of the spectrum.

PHASE1 - Part of a thermodynamic system having uniform properties (e.g., density, crystal structure, index of refraction). The most familiar examples of phases are solids (minerals), liquids, and gases. Less familiar phases include plasmas, quark-gluon plasmas, Bose-Einstein condensates and fermionic condensates, strange matter, liquid crystals, superfluids and supersolids.

PHASE2 - Amount of illumination by sunlight of a planetary body as observed from Earth. The Moon goes through a cycle of phases that repeats every 29.531 days (synodic month). We see these phase changes occur with the Moon rising between 20-70 minutes later each day.

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A complete cycle of phases is observable for the inferior planets, Mercury and Venus. Observations of the phases of Venus by Galileo were used to support the Copernican model of the Solar System.

PHONON - Quantum of vibration excited by the acoustic mode of a crystal lattice; the vibration is usually thermally excited.

PHONON EMISSION - Generation of phonons in response to electron scattering within the crystal lattice, anharmonic lattice forces due to the interaction with other phonons, or X-ray or high-energy particle bombardment.

PHOSPHOR - Substance that emits light when excited by incident radiation or particles.

PHOTODISINTEGRATION - Process that occurs when a high-energy photon is absorbed by an atomic nucleus. The nucleus splits to form lighter elements, releasing a neutron, proton or α particle (4He) in the process. During core-collapse of a supernova, photodisintegration undoes hundreds of thousands of years of nuclear fusion by splitting Fe nuclei into He nuclei and neutrons.

The He nuclei are, in turn, split into protons and neutrons also through photodisintegration.

PHOTODISSOCIATION - Breakup of molecules resulting from the absorption of photons, typically occurring where energetic radiation from massive stars is present. Photodissociation is not limited to visible light, but to have enough energy to breakup a molecule, the photon is likely to be an electromagnetic wave with the energy of visible light or higher, such as ultraviolet light, x-rays and γ-rays.

PHOTOELECTRIC EFFECT - Ejection of a bound electron by an incident photon (x-ray or γ-ray). The photon disappears and the incident energy is shared between the ejected electron and the atom. The photon energy must exceed the atomic binding energy. The probability for the photoelectric effect is approximately proportional to Z5 of the absorber and falls of by ~Eγ3.5.

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PHOTOEVAPORATION - An interstellar cloud can be photoevaporated when an ionization front plows through and heats the gas, which expands and escapes in an ionized supersonic wind.

PHOTON - Discrete bundle of light energy. Light of a given energy (frequency) cannot be broken up indefinitely. Rather for a given frequency it comes in discrete bundles with energy (h = Planck's constant and ν= frequency):

It is often useful to think of light as a bunch of particle photons; whereas, at other times it is useful to think of light as a wave. This is termed the "wave-particle duality." The idea of the photon was advanced by Albert Einstein in his theory of the photoelectric effect.

PHOTOSPHERE - Point where a star's atmosphere appears to become completely opaque. Thus, the photosphere may be thought of as the imaginary surface from which the starlight that we see appears to be emitted. The photosphere is not a thin surface; the Sun's photosphere has a thickness of ~100 km. (Within that 100 km, the temperature drops from 6000 K at the bottom to 4000 K at the top.) Diameters quoted for stars are the diameter of the photosphere.

PHOTOSYNTHESIS - Major energy harvesting reaction that introduces energy into biological systems. Photosynthesizing organisms have chloroplasts and are referred to as autotrophs (self feeders). They take CO2 and H2O and using light make glucose and release O2 and H2O. Plants can be classified according to their photosynthetic pathways into C3 plants, C4 plants, and CAM plants.

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Differences in photosynthetic pathways yield distinct carbon isotopic variations. Most plants (85%) follow the C3 photosynthesis pathway and have lower values of δ13C, between -22‰ and -30‰. The remaining 15% of the plants are of type C4 and have higher values of δ13C, between -10 ‰ and -14 ‰.

PHYLLOSILICATES - Class of hydroxyl-bearing silicate minerals with a sheet-like structure. They result from aqueous alteration are dominantly serpentine and smectite in meteorites; found in the matrixes of carbonaceous chondrites. Phyllosilicates consist of repeating sequences of sheets of linked tetrahedra (T) and sheets of linked octahedra (O). The T sheet consists of linked silica tetrahedra with a hydroxyl group, (OH), at center of 6-fold tetrahedral rings.

The O sheet is formed of sheets of OH- anionic groups with the spaces between OH- sheets occupied by divalent cations (Mg2+, Fe2+) and trivalent cations (Al3+, Fe3+). If all three octahedral cation sites are occupied by divalent cations, the sheet is termed trioctahedral. If sites are occupied by trivalent cations, one of three sites remains empty to maintain charge balance, and it the layer is termed dioctahetral.

The T sheet is always attached to at least one O sheet. Phyllosilicates consisting of alternating TO units are termed 1:1 layer silicates; those with alternating TOT units are termed 2:1 layer silicates.

Substitution of Al3+ for Si4+ in T layer yields a net negative charge, which is balanced by positively charged interlayer cations (K+, Na+, Ca2+) in the true micas.

Important phyllosilicate minerals are given in the table below.

PIGEONITE - Low-Ca clinopyroxene found as a major mineral in eucrites and shergottites.

PLAGE - Bright cloud-like feature found around sunspots that represent a region of higher temperature and density within the chromosphere. Plages are particularly visible when photographed through filters passing H or Ca spectral lines.

PLAGIOCLASE - Common rock-forming series of feldspar: (Na1-x,Cax)(Alx,Si1-x)Si2O8, where x = 0 to 1. The Ca-rich end-member is called anorthite; the Na-rich is albite.

PLANAR DEFORMATION FEATURES (PDFs) - Microscopic features in grains of quartz or feldspar (for example) consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain's crystal structure.

Photomicrograph, crossed polarizers, of planar deformation features in quartz form the Ries impact structure. Field is 460 μm wide. Image source:

PLANCK CONSTANT (h) - Fundamental constant of physics, which sets the scale of quantum-mechanical effects. It has a value of:

where uncertain values in the decimal place are contained in brackets. Planck's constant has the units of action (energy x time, which can be shown to be the same as momentum x length). It was first identified as part of Max Planck's description of blackbody radiation. Later, it was shown by Albert Einstein to be the constant of proportionality between the energy (E) and frequency (ν) of photons:

A closely-related quantity (usually pronounced "h-bar") is:

PLANCK CURVE - Characteristic intensity distribution of radiation emitted by a blackbody. The frequency at which the emitted intensity is highest is an indication of the temperature of the radiating object. It is referred to as the "blackbody curve."

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PLANCK UNITS - Set of units for measuring length, mass and time derived (dimensionally) from combinations of the fundamental constants. These units are named after Max Planck, the physicist who first introduced them. They require us to measure physical quantities in multiples of the following quantities:

Energy - Unit of energy found using Einstein's formula relating energy and mass:

Length – Unit of length is derived dimensionally using combinations of these fundamental constants:

The Planck length and associated Planck time define the scale at which the currently accepted theory of gravity fails. On this scale, the entire geometry of spacetime as predicted by general relativity breaks down.

Mass – Mass is derived dimensionally using combinations of these fundamental constants:

Temperature – Temperature found using the relationship between energy and temperature:

where k = Boltzmann constant.

Time – Time it takes for a photon to travel a distance equal to the Planck length:

With its associated Planck length, the Planck time defines the scale at which current physical theories fail. On this scale, the entire geometry of spacetime as predicted by general relativity breaks down. For this reason, current descriptions of the early evolution of the Universe start at tp = 5.39 × 10-44 seconds after the Big Bang.

PLANET - Spherical celestial body orbiting the Sun, held by the solar gravitational field and reflecting solar light. The definition originally proposed by the International Astronomical Union was:

"A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."

This definition includes Ceres, Charon (co-orbiting with Pluto) and the Kuiper Belt object, 2003 UB313. The prerequisite of roundness essentially specifies objects with masses >5 × 1020 kg and diameters >800 km. However, the definition eventually agreed upon is:

"A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals."

This definition excludes Pluto, Charon, Ceres, and most Trans-Neptunian [Kuiper Belt] Objects (TNOs). Round non-planets are termed "dwarf planets." The definition of a planet also should be applied to extrasolar planets.

Charon and Pluto comprise a multiple object system. The primary object (Pluto) is designated a dwarf planet because it independently satisfies the conditions above. The secondary object (Charon) also satisfies these conditions and is also designated a dwarf planet because the system barycenter (center of mass) resides outside Pluto. Thus, Pluto's companion Charon is a planet, making Pluto-Charon a double planet. Secondary objects that do not satisfy these criteria are “satellites”. For example, in the Earth-Moon system the barycenter occurs within the Earth, thus the Moon is not a planet.

Smaller, non-round, objects are called "small solar system bodies." This class currently includes most of the Solar System asteroids, near-Earth objects (NEOs), Mars-, Jupiter- and Neptune-Trojan asteroids, most Centaurs, most Trans-Neptunian [Kuiper Belt] Objects (TNOs), and comets. In the new nomenclature, the concept "minor planet" is not used.

One can speak of classical (or historical) planets (those known before 1900), terrestrial planets (Mercury, Venus, Earth, Mars), gas-giant planets (Jupiter, Saturn, Uranus, Neptune), and “dwarf planets” (Ceres, Pluto, Charon, 2003 UB313). A new category, “Plutons,” includes all planets with orbital periods >200 years (Pluto, Charon, and Kuiper Belt objects). In contrast to the classical planets, plutons typically have highly inclined orbits with large eccentricities.

The number of candidate planets is large and will doubtless continue to grow as more Kuiper Belt objects are discovered. At present the list includes the asteroids Vesta, Pallas, and Hygiea; and Kuiper Belt objects Sedna, Quaoar, Orcus, 2003 EL61, 2005 FY9, 2002 TX300, Varuna and Ixion. For these to be considered planets, it will be necessary to document that they are in hydrostatic equilibrium.

PLANET MIGRATION - Major reduction in the orbit of planet likely to take place in very young planetary systems. Favored explanation for the small orbits of most extrasolar planets found to date. Two hypotheses have been suggested to account for such drastic orbital shifts.

In the first, near encounters with other large worlds modifies the orbits. Migration requires some interaction between the planet and fairly large bodies or the gravitational forces are too weak. According to this model, early in the formation of the Solar System, there were lots of Moon-sized to Mars-sized bodies, especially in the outer Solar System. A large planetesimal that crosses near Neptune will lose some energy, fall down near Jupiter and gain energy from it to be ejected into the Oort Cloud. This decreases the size of Jupiter's orbit, and expands the orbits of Saturn, Uranus and Neptune. As Neptune moves outward, it will perturb the orbits of the trans-Neptunian objects (large ice covered bodies of which Pluto is a member). This pushes Pluto/Charon into a highly eccentric, inclined 3:2 resonant orbit that it occupies today.

In the second model, which does not exclude operation of the first, a planet experiences viscous drag as it plows through the remnants of the dusty nebula from which it formed, losing energy as it goes. Both theory and observation suggest that the migration time to a small orbit for a giant planet formed at a radius of 5 AU is <1 Ma. A migrating planet must avoid terminal orbital decay and a disastrous collision with its central star. It may be that the magnetic field of the star sweeps an inner belt or region clear of gas and dust so that the drag on the planet abruptly stops when it reaches this open zone. The planet may remain parked at the outer edge of that hole.

PLANETARY NEBULA - Emission nebula formed when a red giant star blows off its outer envelope. A planetary nebula results when the ejected material is heated by strong UV radiation from the hot central star. Planetary nebulae, so named because of their round shapes, disperse in a few tens of thousands of years into the interstellar medium.

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PLANETARY RINGS - Rings composed of relatively small particles (from a few cm to a few 100 m across). All jovian planets in the solar system have rings. They may form by the breakup of a moon that came within the planet’s Roche limit or from particles that were originally there. Another possibility is that collisions on moons outside the Roche limit spewed material into the region inside the Roche limit.

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The grooved patterns observed in ring systems probably result from spiral density waves resulting from the mutual gravitational attraction of the ring particles. Narrow gaps in the rings are most likely swept clean of particles by small moonlets embedded in the rings. The tiny moons can also act as shepherd satellites maintaining a ring.

Two density waves in Saturn's A ring. Image source:

Larger gaps in rings (such as Saturn's Cassini division) result from gravitational resonances with the larger moons of Saturn. Resonance occurs when one object has an orbital period that is a small-integer fraction of another body's orbital period, e.g., 1/2, 2/3, etc. The object gets a periodic gravitational tug at the same point in its orbit. For example, particles at the inner edge of the Cassini division are in a 1:2 resonance with Mimas – they orbit twice for every one orbit of Mimas. Repeated pulls by Mimas, always in the same direction, force them into new orbits outside the Cassini gap. Other resonances with Mimas produce the boundary between the C and B ring (1:3 resonance) and the outer edge of the A ring (2:3 resonance).

PLANETESIMALS - Hypothetical solid celestial body that accumulated during the last stages of accretion. These bodies, from ~1-100 km in size, formed in the early solar system by accretion of dust (rock) and ice (if present) in the central plane of the solar nebula. Most planetesimals accreted to planets, but many – such as the asteroids– never combined to form large bodies. The very largest asteroids can be considered protoplanets.

PLANITIA – Term applied to a low plain, or a large level expanse of lowlands on a planetary surface (pl. planitiae).

PLASMA - Fourth state of matter: a gas in which many or most of the atoms are ionized. In the plasma state the atoms have split into positive ions and negative electrons, which can flow freely, so the gas becomes electrically conducting and a current can flow.

PLATINUM GROUP ELEMENTS (PGE) - Elements with geochemical properties similar to Pt (Ru, Rh, Pd, Os, Ir, and sometimes Au). These occur in nature in close association with one another and with Ni and Cu. They are among the least abundant of the Earth’s elements.

PLESSITE - A fine-grained intergrowth of kamacite and taenite that fills in the wedges between wide kamacite and taenite bands in octahedrites. The name derives from the Greek word for "filling."

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PLUTINO - Kuiper Belt object (KBO) that has an orbital periods very similar to that of Pluto. Found in the inner parts of the Kuiper Belt, the orbits of a plutino is stabilized against gravitational perturbations by the 3:2 mean-motion resonance with Neptune, meaning that it orbits the Sun twice for every three orbits of Neptune.

PLUTON1 - Igneous intrusive body formed when magma is injected into host rocks and solidifies.

PLUTON2 - Category of planet including all planets with orbital periods >200 years (Pluto, Charon, and Kuiper Belt objects). In contrast to the classical planets, plutons typically have highly inclined orbits with large eccentricities.

PLUTONIC - Igneous rocks that form from the cooling of magma in the interior of a planet or asteroid.

POLYMERIZATION - Process of reacting molecules together to form three-dimensional networks or chains. Silicon tetrahedra are usually polymerized in silicate minerals and melts. Classification of silicate minerals is based upon their degree of polymerization.

POLYMICT BRECCIA - General term for all breccias that are neither monomict nor dimict.

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POLYMORPHISM - Situation in which a single chemical composition can exist with two or more different crystal structures. Examples of polymorphs include: silica (SiO2) with α quartz, β quartz, coesite, stishovite, tridymite, and cristobalite (see below); aluminosilicates (Al2SiO5) with kyanite, andalucite, and sillimanite; and carbon (C) with graphite, diamond, and lonsdalite (among others). Transformations between crystal structures of the same chemical compound are called polymorphic transformations. These are of three types: displacive, reconstructive, and order-disorder.

POPULATION I STARS - Relatively young stars found mainly in the disk of the Galaxy. Population I stars are the most metal-rich, with metallicities ranging from ~0.1 to 3 times that of the Sun (i.e., [Z/H] from -1.0 to +0.5). This indicates that the gas from which Population I stars formed must have been recycled (incorporated into, and then expelled) from previous generations of stars a number of times, and that Population I stars are relatively young compared to Population II stars. The Sun ([Z/H] ~ 1.6) is a fairly typical Population I star, as are most of the stars in the immediate solar neighborhood. In fact, the majority of the stars contained within the thin disks of galaxies are Population I stars, but Population I stars can also found in the bulge.

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Many Population I stars in the galactic bulge have higher abundance ratios for the lighter elements (e.g. C, O). These elements are produced primarily in Type II supernova explosions (the explosions of massive stars), and indicate that the Population I stars in the bulge formed early in the star formation history of the Milky Way. In contrast, the chemical composition of Population I stars in the disk reveal that these stars are also enriched in the heavier elements that can only be produced in Type Ia supernova explosions. This indicates that Population I disk stars formed at least a billion years after star formation began in the Galaxy.

POPULATION II STARS - Relatively old stars found in the halos of galaxies and in globular clusters.  Population II stars are metal-poor, with metallicities ranging from ~0.001 to 0.1 that of the Sun (i.e., [Z/H] from -3.0 to -1.0). This indicates that the gas from which Population II stars formed could only have been recycled (incorporated into, and then expelled) from previous generations of stars a few times at most, and that Population II stars form very early in the star formation history of the galaxy. Further evidence to support the early origins of Population II stars comes from the fact that the lighter elements (e.g. C, O) dominate heavier elements (e.g. Fe, Ni). Light metals are produced primarily in Type II supernova explosions whereas heavier elements are produced only in Type Ia supernova explosions. The relative lack of heavy metals in Population II stars indicates that these stars formed in the first billion years or so of a galaxy's star formation history. Population II stars are mainly found in the bulge and halo of galaxies (globular clusters).

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POSITRON - Anti-particle of the electron, it has the same properties as the electron with the exception of charge - the electron has a negative charge while the positron has a positive charge. The combination of an electron and a positron results in an annihilation which transforms both particles into high-energy photons. The existence of the positron was predicted by Paul Dirac in 1928, but the particle itself was not observed until 1932 (by Carl Anderson). It was the first example of a particle predicted to exist by quantum mechanics later discovered to actually exist.

POSTPEROVSKITE - High-pressure form of MgSiO3 with a stacked SiO6-octahedral sheet structure that is a major component of the D" layer in Earth. Unlike perovskite, the octahedra formed by oxygen ions around the 4+ cation have differing orientations. Postperovskite forms from perovskite at conditions exceeding 125 gigapascals and 2500 K, which corresponds to a depth of ~2700 km (near the base of the mantle).The transition results in an increase in density of 1.0 to 1.2%. Incorporation of Fe into the structure greatly reduces the pressure needed for the transition. The perovskite-postperovskite phase transition probably produces the D" seismic discontinuity located just above the core-mantle boundary.

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POTENTIAL ENERGY - Energy possessed by something by virtue of its location in a potential field. AS an example, consider gravity. The higher an object is above the ground, the greater its gravitational potential energy. This potential energy is converted into kinetic energy as it falls.

P-PROCESS - Photodisintegration (hence "p") reactions are responsible for the production of proton-rich isotopes with masses >100. The relevant reactions are (γ, n) and (γ, α). A supernova explosion produces flood of γ-rays that can disintegrate the seed nuclei produced by s- and r-processes. (The p-process is sometimes called the "γ-process.") P-process contribution to isotopic abundances of elements that are also produced by s- and r-processes is usually very small. There are p-only isotopes that cannot be produced by the s- or r-process (190Pt, 168Yb); these isotopes have very small abundances compared to neighboring nuclei.

PRECESSION - Change in the orientation of an object's axis of rotation due to a torque produced by an external object. The Earth has a nonspherical shape, being oblate spheroid, bulging outward at the equator. The gravitational tidal forces of the Moon and Sun apply torque as they attempt to pull the equatorial bulge into the plane of the ecliptic. Earth' axis of rotation slowly changes its direction, maintaining a constant tilt with respect to the ecliptic and making a complete rotation once every 25700 years. In consequence, the equinoxes on the intersections of the celestial equator and the ecliptic slowly drift westward.

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PRESOLAR GRAINS - Tiny crystalline grains found in the fine-grained matrix of primitive meteorites. They are assumed to be older than the solar system, probably formed in supernovae or the stellar outflows of red giant (AGB) stars before incorporation in the molecular cloud from the solar system formed. Presolar grains survived the collapse of the solar nebula, and also the subsequent formation of planetesimals because they consist of refractory minerals.

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PRESSURE - Force applied per area covered:

Pressure is a scalar quantity and has no direction. The unit of pressure is the pascal (Pa), which is a newton/m2. The pascal is also a unit of stress and the topics of pressure and stress are connected. One pascal is such a small unit of pressure that the kilopascal (1 kPa = 1000 Pa) is more commonly used. One hundred thousand pascals are called a bar (1 bar = 105 Pa; 1 kbar = 108 Pa). The unit atmosphere is 101.325 Pa by definition.

PRIMORDIAL ELEMENTS - Elements and isotopes formed in the Big Bang; specifically, 1H, 3He, 4He, most D (deuterium = 2H) and 3H (tritium), and some 7Li.

PRIMITIVE ACHONDRITE - Achondrite with an almost chondritic composition with age similar to the primordial chondrites. These should be better classified as "metachondrites".

PROMINENCE - Structure that occurs above the photosphere of the Sun. Prominences may reach high into the corona, often as graceful loops that may hang suspended for many days. Prominences are usually associated with regions of sunspot activity; they tend to lie on the boundary of regions having opposite magnetic polarity. Streaming arches and their stability for days at a time are associated with magnetic forces acting on the charged particles in the loops. The violently eruptive prominences that are sometimes observed are associated with corresponding sudden changes in the magnetic field of the Sun.

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PROPER MOTION (μ) - Slow steady change in the apparent position of a nearby star over many years because of its independent motion within the Galaxy. Even the nearest and fastest stars require centuries to move a degree or more. Proper motion is usually quoted in units of arcseconds per year.

PROPLYD - Contrived acronym for protoplanetary disk. Proplyds are found around low mass young stars close to massive OB stars, whose radiation photoevaporates the outer layer regions of their gas-dust envelopes, which are blown by impinging stellar winds into a teardrop shape.

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PROTON - Particle of the hadron family which is one of the two particles that makes up an atomic nucleus. The proton has a positive electrical charge equivalent to the negative charge on the electron and a mass similar to that of a neutron.

PROTOPLANETARY DISK - Flattened disk of solids and gas orbiting a young star from which planets can form.

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PROTOSTAR - Star in the process of formation, which has not yet become hot enough in the core to initiate hydrogen burning (~107 K) to halt its gravitational collapse. Its luminosity results from the release of gravitational potential energy from the infall of nebula material from its accretion disk.

PROXIMAL EJECTA - Ejecta found up to 5 crater radii from the rim of the impact crater. About 90% of all ejecta is proximal. The limit of proximal ejecta scales with the crater size. Ejecta found at greater distances is called distal ejecta.

PSEUDOTACHYLITE - Rock formed by frictional melting of rocks during faulting and impact cratering as a result of high rates of deformation and nearly total transformation of kinetic energy to heat. After deformation ceases, rapid quenching of the frictional melt leads to the formation of glass. The term was first applied to veins in the Vredefort impact structure, which are found in rocks of widely varying composition. The compositions of associated pseudotachylites are the same as those of host rocks, with some systematic variations, thus indicating in situ formation. One mystery of pseudotachylite formation is how frictional processes can form large amounts of melt, because thick masses ought to preclude melting by reducing the friction between sliding rock masses. However, J. Melosh (U. Arizona) has suggested that if melt produced by sliding friction in narrow shear zones is extruded into adjacent country rock, the shearing zone can be kept narrow and will continue to produce melt.

Vredefort structure pseudotachylite. Image source:

PULSAR - Extra-terrestrial source of radiation that has a regular periodicity, usually in the form of short bursts of radio emission. Most known pulsars are radio pulsars, but a small number of pulsars emit at optical, X-ray and gamma ray wavelengths. The first pulsar was discovered in 1967 by Jocelyn Bell at the Mullard Radio Astronomy Observatory in Cambridge. The current number of known pulsars is around 1500.

Radio pulsars are rapidly rotating neutron stars with an extremely strong magnetic field, which generates a strong electric field. Electrons are torn from the surface and accelerated along the magnetic poles, resulting in twin beams of radiation shining out of the magnetic poles. Rotation beams energy into space, creating a pulsar. Most pulsars spin at a rate of about once per second, but the fastest pulsars can rotate at up to ~650 times a second. Anything spinning faster than around 50 milliseconds is referred to as a millisecond pulsar. Some radio pulsars are associated with supernova remnants, and it is generally accepted that pulsars are the collapsed cores of stars that were once more massive than 6x to 10x Msun.

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X-ray pulsars emit x-rays at regular intervals, either because of magnetospheric emission in neutron stars, or through the accretion of matter from a companion. Accretion-powered x-ray pulsars usually rotate relatively slowly (some as slowly as once every 20 minutes), because the magnetic field exerts a slow-down torque on the neutron star due to the presence of ionized material. Some x-ray pulsars rotate very rapidly, with spin frequencies in excess of 400 Hz. The luminosity of x-ray pulsars varies over at least 5 orders of magnitude, from near the Eddington limit of 1031 J/s to less than 1026 J/s.

Optical pulsars form a very small subset of known pulsars. The most famous optical pulsar is the Crab pulsar, the remnant of a supernova explosion that was visible in 1054 AD (shown in the images on the right).

Gamma ray pulsars are quite rare, and most are young neutron stars with strong magnetic fields. A few of these are also visible as radio and optical pulsars. At gamma ray wavelengths near 100 MeV, the Vela pulsar is the strongest point source in the sky.

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Pulsars are born in core-collapse supernova explosions. The rotation rate of the collapsing core increases enormously through conservation of angular momentum, and new-born pulsars typically spin >60 times a second (60 Hz). Over millions of years, they emit magnetic dipole radiation which causes their rotation to slow. Eventually, the rotation rate reaches a point where the pulsar ceases to emit radio emission and is no longer detectable from Earth. The pulsar is now said to be 'extinct'.

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PYROXENE - Most important group of rock-forming ferromagnesian silicates. Pyroxenes are divided into two groups: Orthorhombic orthopyroxenes (opx) and monoclinic clinopyroxenes (cpx). Terrestrial pyroxenes are found in mafic and ultramafic igneous rocks and high-temperature granulite facies metamorphic rocks. The pyroxene, diopside also occurs in amphibolite facies calc-silicates and marbles; whereas, pigeonite mainly restricted to meteorites and terrestrial volcanic rocks. Na-rich pyroxenes occur in high pressure eclogite-facies metamorphism of mafic igneous rocks. Pyroxene transforms into a garnet structure at high pressure to form majorite, (MgVIII)3(SiVI)2(SiIV)3O12; note that Si is octahedrally coordinated. This probably occurs at the transition zone (at ~400+ km depth).

Pyroxenes are single chain silicates with a structure consisting of linked (SiO4)4– tetrahedra, each sharing 2 oxygens. They have two distinct octahedral sites. There is a smaller relatively regular M1 site coordinated by oxygens of two opposing chains, yielding at tetrahedral-octaehral-tetrahetral (T-O-T) strip running parallel to the c-axis. The second is a larger irregular M2 site that serves to cross-link T-O-T strips.

The T-O-T strips sometimes called 'I-beams' and their bonds' resistance to breaking produces the typical near 90° {110} cleavage of pyroxenes (dashed lines below).

Pyroxenes have the general formula of XYZ2O6, where X is the regular 6- to 8-fold M2 site, Y the distorted 6-fold M1 site, and Z is the tetrahedral site. M2 cations are normally larger than M1 cations. In the diopside-hedenbergite series, the M2 site is filled by Ca2+ (1.12 Å) and M1 site by randomly distributed Fe2+ (0.78 Å) and Mg2+ (0.72 Å). Site occupancies are as follows: on the tetrahedral Z site - Si4+, Al3+, Fe3+ (rare); on the octahedral Y site (M1) - Al3+, Fe3+, Ti4+, Cr3+, Mg2+, Fe2+, Mn2+; and on the octahredral X site (M2) - Mg2+, Fe2+, Mn2+, Ca2+, Li+, and Na+.

Augite is closely related to diopside-hedenbergite series. Pigeonite has higher Ca contents than the orthorhombic enstatite-ferrosilite series. Non-quadrilateral components are accommodated by coupled substitutions. Three are important in augite, the most common terrestrial pyroxene: R2+(M2) + R2+(M1) → Na+(M2) + ½ Fe2+(M1) + ½ Ti4+(M1) yielding titanian augite; R2+(M1) + R4+(T) → R3+(M1) + R3+(T) yielding aluminous augite; and R2+(M2) + R2+(M1) → Na+(M2) + Fe3+(M1) yields aegirine augite.

The major end members are enstatite (MgSiO3), ferrosilite (FeSiO3), and, compositionally, wollastonite (CaSiO3). However, pyroxenes do not have CaSiO3 contents exceeding 50 mol%, so compositions are often displayed on a quadrilateral that is the lower half of the MgSiO3-FeSiO3-CaSiO3 ternary. Solid solutions in quadrilateral pyroxenes (shown below) are dominated by Ca2+, Fe2+ and Mg2+. Enstatite and ferrosilite form almost complete solid solution (Mg2+ substitutes for Fe2+ up to about 90 mol%). Ferrosillite is raely found in nature, because under most geological conditions it breaks down: Fe2Si2O3 (ferrosilite) → Fe2SiO4 (fayalite) + SiO2 (quartz).

Pyroxenes are divided into two crystallographic groups: Pyroxenes divided into two groups: orthorhombic (orthopyroxenes: opx) and monoclinic (clinopyroxenes: cpx). Once intermediate pyroxene compositions had individual names (e.g., hypersthene, bronzite), but modern nomenclature rulings have disallowed these. Now, general names are used with modifiers to indicate unusual attributes (e.g., chromian diopside, titian augite). Na-rich pyroxenes are classified using a ternary diagram with quadrilateral components (Ca, Mg, Fe) at the apex and jadeite (NaAlSi2O6) and aegirine (NaFe3+Si2O6) at the bottom two corners.

PYROXENOIDS - Single-chain silicates like pyroxene, but the tetrahedra composing chains are rotated and twisted. Octahedrally coordinated cations occur between chains as in pyroxenes. The interval of repetition is different for each pyroxenoid (below). The twisting results in lower symmetry than pyroxenes (all pyroxenoids are triclinic) and a splintery cleavage and sometimes fibrous habit.

The most common pyroxenoid is wollastonite (CaSiO3), which has Ca2+ in irregular octahedral coordination linking the chains. It is common in metamorphosed limestones. Other pyroxenoids include bustamite, (Mn,Ca,Fe)SiO3, with a repeat of ~14 Å, rhodonite, MnSiO3, with a repeat of 12.5 Å, and pyroxmangite, (Mn,Fe)SiO3, with a repeat of ~17 Å. There is significant chemical variation and solid solution in the pyroxenoids, except for wollastonite, which is usually very close to pure in composition (below).

PYRRHOTITE - Fe sulfide, Fe1-xS, found as an accessory mineral in CM chondrites, which is deficient in Fe relative to S. This deficiency results from the substitution 3Fe2+ ↔ 2Fe3+ + ⌈.