P-Branch	A set of lines in the spectra of molecules corresponding to unit increases in rotational energy. [H76]
p-Electron	An orbital electron whose l quantum number is 1. [H76]
P-Strong Stars	A small subgroup of B-type stars in which P lines are very strong. [JJ95]
P Cyg Stars	High-luminosity, early-type stars, in which all lines have a P Cyg type profile (an emission component on the red side of the absorption line). [JJ95]
pep Reaction	A reaction occurring in the proton-proton chain. The first step, instead of p + p -> d + e+ + ve, is p + e- + p -> d + ve. This latter reaction occurs only once in 400 p-p reactions but produces far more energetic neutrinos (1.44 MeV as against 0.42 MeV). [H76]
P-Process	The name of the hypothetical nucleosynthetic process thought to be responsible for the synthesis of the rare heavy proton-rich nuclei which are bypassed by the r- and s-processes. It is manifestly less efficient (and therefore rarer) than the s- or r-process, since protons must overcome the Coulomb barrier, and may in fact work as a secondary process on the r- and s-process nuclei. It seems to involve primarily (p, ) reactions below cerium (where neutron separation energies are high) and the (, n) reactions above cerium (where neutron separation energies are low). The p-process is assumed to occur in supernova envelopes at a temperature greater than about 109 K and at densities less than about 104 g cm-3. [H76]
P-Spot	See Sunspot. [H76]
P-Wave	Primary Wave: A longitudinal seismic acoustic wave that moves by compression. The p-waves travel faster than s-waves and can penetrate the core of the Earth. [H76]
Packing Fraction	Mass defect per nuclear particle. The term has been largely superseded by the related quantity, binding energy per nuclear particle. [H76]
PAH	Polycyclic Aromatic Hydrocarbon
Pair Annihilation	Mutual destruction (annihilation) of an electron-positron pair with the formation of gamma rays, or of a proton-antiproton pair with the formation of pions. The charges cancel, and the total mass of the pair is converted into energy (unlike nuclear fusion, in which less than 1% of the mass is converted into energy). [H76]
Pair production	(a) The production of an electron and its antiparticle (a positron) from a photon (according to the equation E = mc2) The process can occur in the field of an atomic nucleus. Since the mass of an electron or positron is equivalent to 0.511 MeV, the minimum energy of a photon that can promote pair production is 1.022 MeV. Any surplus energy becomes kinetic energy of the products. [DC99] (b) The inverse process to pair annihilation. A gamma ray of the right energy (> 1.02 MeV) is transformed into an electron-positron pair (or a pion is transformed into a proton-antiproton pair). [H76]
Pairing Energy ()	A quantity which expresses the fact that nuclei with odd numbers of neutrons and protons have less energy and are less stable than those with even numbers of neutrons and protons. [H76]
Palladium	A silvery white ductile transition metal occurring in platinum ores. It is used in electrical relays and as a catalyst in hydrogenation processes. Hydrogen will diffuse through a hot palladium barrier. Symbol: Pd; m.p. 1552°C; b.p. 3140°C; r.d. 12.02 (20°C); p.n. 46; r.a.m. 106.42. [DC99]
Pallas	The second asteroid to be discovered (by Olbers in 1802). Diameter about 560 km; a = 2.77 AU, e = 0.235, i = 34°.8. Orbital period 1,686 days; rotation period 9-12 hours. Albedo ~ 0.05; mass (1972 est.) 2.6 × 1023 g. Spectrum resembles meteorites of either low-grade carbonaceous chondrite or enstatite achondrite. [H76]
Palomar	The mountain in California upon which sits the largest telescope in the United States, 200 inches (5 meters) in diameter. The telescope itself is sometimes referred to as the Mt. Palomar Telescope. [LB90]
Pan	Unofficial name for Jupiter XI. P = 692d R, e = 0.2; i = 163°. Discovered by Nicholson in 1938. [H76]
Pancake Model	A model of galaxy formation in which the first structures to condense out of the smooth background of primordial gas were very large in size. These large masses then collapsed into thin sheets (pancakes) and fragmented into many smaller pieces the size of galaxies. A competing theory, sometimes called the hierarchical clustering model, proposes that the first structures to form were the size of galaxies. As galaxies clustered together, due to gravity, larger and larger structures were formed. (See Hierarchical Clustering Model) [LB90]
Panstellar	Pertaining to more than one star. [F88]
Paradox	A self-contradictory proposition. Paradoxes are most useful when they seem most likely to be true, for it is then that they best serve to expose flaws in the data or reasoning that led to their appearance. [F88]
Para-Hydrogen	Molecular hydrogen in which the two protons of the diatomic molecule have opposite directions of spin. It is a lower energy state than ortho-hydrogen. [H76]
Parallax	(a) Angle subtended by the apparent difference in a star's position when viewed from the Earth either simultaneously from opposite sides of the planet, or half such an angle, measured after a gap of six months from opposite sides of the planet's orbit; the nearer the celestial body, the greater the parallax. [A84] (b) The tiny shift in a star's apparent position that occurs when the star is viewed from slightly different perspectives as the Earth revolves around the Sun. The larger a star's parallax, the closer the star is to Earth. [C95] (c) The apparent displacement in the position of a star or planet occasioned by its being viewed from two different locations - e.g., by observing it from two widely separated stations on Earth, or at intervals of six months, when the earth is at either extreme of its orbit around the sun. The resulting angle can be used, by triangulation, to determine the distance of the star or planet. [F88] (d) The difference in apparent direction of an object as seen from two different locations; conversely, the angle at the object that is subtended by the line joining two designated points. Geocentric (diurnal) parallax is the difference in direction between a topocentric observation and a hypothetical geocentric observation. Heliocentric or annual parallax is the difference between hypothetical geocentric and heliocentric observations; it is the angle subtended at the observed object by the semi-major axis of the Earth's orbit. First trigonometric parallax was obtained in 1838. (eee also horizontal parallax.) [S92]
Parametric Amplifier	A device used in radio astronomy for increasing the strength of a radio signal. (paramp) [H76]
Parametric Representation	An indirect means of expressing the solution to a differential equation in terms of an arbitrary parameter. As the parameter is allowed to vary, the parametric expression takes on the various values that the actual solution would have. [Silk90]
Parasites	In radio jargon, spiral coils or gratings of wire used on dipole antennas of radio telescopes to give greater sensitivity. [H76]
Para-Spectrum	Spectrum of singlet (l = 0). [H76]
Paraxial	Describing rays incident on a surface close and parallel to the axis. Only paraxial rays pass or appear to pass through the focal point of a spherical reflecting or refracting surface. [DC99]
Parity	(a) The principle of space-inversion invariance; i.e., no experiment can differentiate between the behavior of a system and that of its mirror image. Parity is conserved in strong interactions, but not in weak ones. [H76] (b) The operation which reverses the signs of the coordinate axes used to describe a system, i.e. (x, y, z) -> (-x, -y, -z). [CD99] (c) The operation of studying a system or sequence of events reflected in a mirror.[D89]
Parsec	(a) The distance at which one astronomical unit subtends an angle of one second of arc; equivalently, the distance to an object having an annual parallax of one second of arc. (abbreviation for parallax second) [S92] (b) Astronomical unit of distance, equal to 3.26 light-years. [F88] (c) The distance at which one astronomical unit subtends an angle of 1 second of arc. 1 pc = 206,265 AU = 3.086 × 1013 km = 3.26 light-years. [H76] (d) A unit used by astronomers to describe stellar distance. It is the distance from which the radius of the earth's orbit would subtend an angle of one second of arc, i.e. 3.08572 × 1016 m, 3.26168 light years or 2.062648 × 105 astronomical units. Alternatively one parsec is the distance at which one astronomical unit subtends one second of arc. The unit was approved at the first meeting of the International Astronomical Union which took place in 1922. The name, derived from parallax second, is said to have been proposed by Professor H. H. Turner (1861-1930), Savilian Professor of Astronomy at Oxford. The idea of using star parallaxes for measuring astronomical distances was suggested about 1840 by both Bessel and Meadows. [JM92]
Particle	The term particle is used somewhat loosely and includes not only the elementary quarks and leptons and bosons, but also the composite hadrons. It also includes any (currently hypothetical) new particles that might be discovered, such as the supersymmetric partners of the quarks and leptons and bosons. [K2000]
Particle Accelerator	A device using electric and magnetic fields to accelerate beams of particles-usually electrons, positrons, protons, or antiprotons-to high energies for experimental purposes. Modern accelerators are often very large: the main ring at Fermilab, for example, is 4 miles in circumference. [G97]
Particle Distribution Function	The number of particles per unit volume of phase space. [H76]
Particle Horizon	The distance a photon of radiation could have traveled since the creation of the particle. [c97]
Particle Physics	(a) The branch of science that deals with the smallest known structures of matter and energy. As their experimental investigation usually involves the application of considerable energy, particle physics overlaps with high-energy physics. [F88] (b) That branch of physics that attempts to understand the fundamental particles and forces of nature. [LB90] (c) The physics of so-called `elementary' particles, electrons, protons, pions, W± and Z0 bosons, neutrinos etc. [D89] (d) hose engaged in this field of physics study the particles and try to understand their behavior and properties. Sometimes a distinction is made between the study of quarks, leptons, gauge bosons, and Higgs physics, and the study of hadron physics, which aims to relate the properties of the hadrons to the theory of the color force. More broadly, the goals of particle physics are to understand not only the description of the particles and their interactions but also why the laws of nature are what they are and how the universe arises from those laws. [K2000]
Particle-to-Antiparticle Ratio	Same as matter-to-antimatter ratio. [LB90]
Particles	Fundamental units of matter and energy. All may be classed as fermions, which have half-integral spin and obey the exclusion principle, and bosons, which have integral spin and do not obey the exclusion principle. The term particle is metaphoric, in that all subatomic particles also evince aspects of wave-like behavior. [F88]
Partition Function	(a) The effective statistical weight of an atom or ion under existing conditions of excitation or ionization. [H76] (b) A weighted sum extending over all the possible microscopic arrangements of a macroscopic system, whose evaluation permits the determination of the observable properties of that system. in equilibrium.[D89]
Parton	(a) A generic term used to describe any particle which may be present inside nucleons. It includes quarks, antiquarks and gluons. [CD99] (b) A conjectured constituent of hadrons, these days normally identified with quarks.[D89] (c) A hypothetical pointlike constituent of a nucleon, which contains all the charge of the nucleon. [H76]
Pascal	The derived SI unit of pressure. 1 Pa = 1 N m-2 = 10-5 bars. [H76]
Paschen-Back Effect	An effect on spectral lines obtained when the light source is located in a strong magnetic field, so that the magnetic splitting becomes greater than the multiplet splitting. H76]
Paschen Series	(a) A series of lines in the infrared spectrum emitted by excited hydrogen atoms. The lines correspond to the atomic electrons falling into the third lowest energy level and emitting energy as radiation. The wavelength () of the radiation in the Paschen series is given by 1 / = R(1 / 32 - 1 / n2) where n is an integer and R is the Rydberg constant. (See also Spectral Series) [DC99] (b) The spectral series associated with the third energy level of the hydrogen atom. The series lies in the infrared - P at 18,751 Å; Paschen limit at 8204 Å. (P of He II is at 4686 Å; He II series limit is at 2040 Å.) [H76]
Passband	The frequency band that is transmitted with maximum efficiency and without intentional loss. [H76]
Past Light Cone	See Light Cone. [H76]
Patroclus	Asteroid 617, a Trojan 60° behind Jupiter. P = 11.82 yr, a = 5.19 AU, e = 0.14, i = 22°.1. [H76]
Paul Trap	A radio-frequency quadrupole ion trap in which charged particles can be suspended by radiofrequency electric fields for times limited primarily by collisions with the background gas. [H76]
Pauli Exclusion Principle	(a) States that particles with half integer spins cannot occupy the same quantum states. This manifests itself as the reason why solid objects cannot exist in the same physical space. [c97] (b) Two identical fermions cannot occupy the same quantum state (i.e., cannot have the same charge, spin, momentum, quantum numbers etc. within the same region of space). [CD99] (c) A principle of quantum theory first enunciated by Wolfgang Pauli. It applies to spin* particles, such as quarks or leptons and states that at most one such can carry any given set of quantum numbers. It thus underwrites the electronic structure of atoms and the quark structure of hadrons.[D89]
Photino	The supersymmetric partner of the photon. [K2000]
PC	Photoconductor [LLM96]
pc	Abbreviation for Parsec. [H76]
PDA	Photodiode Array [McL97]
Peculiar A Star	Stars can be classified according to their surface temperatures, which determine, in large part, the spectrum of radiation they emit. A Stars have surface temperatures between about 7,500 and 11,000 degrees centigrade. Peculiar A Stars are A stars whose emitted radiation spectra have many of the characteristics of A stars but are peculiar in certain ways. [LB90]
Peculiar Motions
Peculiar Stars	Stars with spectra that cannot be conveniently fitted into any of the standard spectral classifications. They are denoted by a p after their spectral type. [H76]
Peculiar Velocity	(a) Velocity with respect to the Local Standard of Rest. [H76] (b) A deviation in the velocity of a galaxy from that expected on the basis of a uniform expansion of the Universe. (See Hubble law) [LB90]
Pencil Beam	The main lobe of an antenna pattern, consisting of a narrow receiving beam of the type obtained with a single parabolic instrument. [H76]
Penrose Process	A means of extracting energy from a rotating black hole. If a particle spirals into the ergosphere of a black hole in a direction counter to the rotation of the black hole, and if the particle then breaks up into two fragments inside the ergosphere, one of the fragments can escape with energy greater than the energy of the original particle. [H76]
Penrose's Theorem	A collapsing object whose radius is less than its gravitational radius must collapse into a singularity. [H76]
Penumbra	(a) Less than full shadow (umbra). [A84] (b) The portion of a shadow in which light from an extended source is partially but not completely cut off by an intervening body; the area of partial shadow surrounding the umbra. [S92]
Percolation	A random system defined mathematically on an infinite space, such as the random formation of bubbles in the early Universe, is said the percolate if the objects merge to form an infinite cluster. In the original inflationary theory the bubbles that ended the inflation were found not to percolate, implying that they could never be homogeneous enough to describe our Universe. The failure of the original inflationary theory became known as the graceful exit problem. (See Decay of the False Vacuum) [G97]
Perfect Cosmological Principle	(a) The assumption adopted by the steady-state theory, that all observers, everywhere in space and at all times, would view the same large-scale picture of the Universe. [H76] (b) A term introduced to describe the Steady-State Universe theory, in which the appearance of the Universe is assumed to be independent not only of the position of the observer (homogeneity) and the direction she is looking (isotropy), but also the time at which she is making the observations. (See also Cosmological Principle) [G97]
Perfect Gas	See Ideal Gas. [H76]
Periapsis	The point in the orbit of a satellite where it is closest to its primary. [H76]
Periastron	The point in the orbit of one component of a binary system where it is nearest the other component. [H76]
Pericenter	The point in the orbit of one component of a binary system which is closest to the center of mass of the system. [H76]
Pericynthion	The point in the orbit of a satellite around the Moon closest to the Moon. [H76]
Perigalacticon	The point in a star's orbit around the Galaxy when the star lies closest to the Galactic center. The Sun is near perigalacticon now. [C95]
Perigee	The point at which a body in orbit around the Earth most closely approaches the Earth. Perigee is sometimes used with reference to the apparent orbit of the Sun around the Earth. [S92]
Perihelion	The point in the orbit of an object orbiting the Sun where it is closest to the Sun's center of mass. Earth's perihelion occurs early in January. [H76]
Period	(a) The period of a wave is the time interval between the receipt of two successive peaks (often called crests) of the propagating disturbance. [G97] (b) The interval of time required to complete one revolution in an orbit or one cycle of a periodic phenomenon, such as a cycle of phases. [S92]
Period Doubling	the motion of a particle under the influence of a force may settle down to a regular orbit with a definite period. If the force acts nonlinearly on the particle, and is increased, then the orbit period (the time taken to return to a previous position) may suddenly double when the motion changes to a more complex pattern. This doubling from a simple motion (called a one-cycle) to the more complex form (a two-cycle) is period doubling. The process may continue until an n-cycle is produced. Period doubling is a major phenomenon in nonlinear systems, especially in lasers where the particle motion is replaced by radiation fields.[D89]
Period-Luminosity Relation	A correlation between the periods and mean luminosities of Cepheids, discovered by Henrietta Leavitt in 1912. [H76] (b) Cepheid variables obey this relation: the longer the Cepheid's pulsation period, the more luminous the star. Since measuring a Cepheid's period is easy, the period-luminosity relation allows astronomers to determine the Cepheid's intrinsic brightness and hence distance. If the Cepheid is part of another galaxy, the Cepheid's distance gives the distance to the entire galaxy. [C95]
Persei	A young open cluster with a high mean rotational velocity. [H76]
Persei stars	A class of eclipsing binaries (see Algol) with periods of from 2 to 5 days, the depth of whose secondary minimum is almost negligible. [H76]
h and Persei	A double stellar association about 2 kpc distant, visible to the naked eye as a patch of light. It contains many young 0 and B stars and also many M supergiants. (also called Perseus OB1) [H76]
Perseus A	A strong radio source (z = 0.0183; recession velocity about 5000 km s-1). Optically it is a Seyfert galaxy (NGC 1275) with a huge amount (about 108 M) of ionized gas receding from it. It is also a strong X-ray source (3U 0316+41). (also known as 3C 84, Abell 426) [H76]
Perseus Arm	The spiral arm that lies next out from the arm containing the Sun. The most famous members of the Perseus arm are the young star clusters h and Chi Persei. [C95]
Perseus Cluster	A diffuse, irregular cluster of about 500 galaxies (z = 0.0183) (richness class 2) dominated by and centered on the Seyfert galaxy NGC 1275 (Perseus A). Mass required to bind the cluster, greater than 1015 M; mass of cluster, about 2 × 1015 M. [H76]
Perseus OB1	See h and Persei. [H76]
Perseus OB2	A young association of OB stars about 350 pc distant. (also called Perseus 2) [H76]
Perseus-Pisces Region	A region of space containing a huge congregation of galaxies called a supercluster. The galaxies in this supercluster appear to be distributed in a long chain. [LB90]
Perseus X-1	The strongest known extragalactic X-ray source, centered on NGC 1275. (3U 0316+41) [H76]
Perturbation	(a) A small disturbance which makes the system deviate from its equilibrium state. It is by considering such perturbations that one determines the stability of a system: it is stable if in time the system returns to its equilibrium state: and it is unstable if some initial perturbation makes the system depart from the equilibrium state indefinitely. [H76] (b) Deviations between the actual orbit of a celestial body and an assumed reference orbit; also, the forces that cause deviations between the actual and reference orbits. Perturbations, according to the first meaning, are usually calculated as quantities to be added to the coordinates of the reference orbit to obtain the precise coordinates. [S92]
Perturbation Expansion	A method of successive approximations that is used to obtain predictions from a theory such as quantum electrodynamics, which cannot be solved exactly. [G97]
Perturbation Method	A system of successive approximations to the solution of a problem, by starting with a closely similar problem whose solution is known, applying small departures from equilibrium, and then calculating their consequences. [H76]
Perturbation Theory	(a) A mathematical approximation in which a small disturbance added to an exactly soluble system is analysed by a series expansion in powers of the small disturbance.[D89] (b) A framework for simplifying a difficult problem by finding an approximate solution that is subsequently refined as more details, initially ignored, are systematically included. [G99]
Pfund Series	A spectral series of hydrogen lines in the far-infrared, representing transitions between the fifth energy level and higher levels. [H76]
PG 1159 Stars	Very hot stars with strong O VI and C IV lines, which are X-ray emitters. Probably these stars are the central stars of planetary nebulae that have dissipated their envelopes. Also called pre-degenerates. [JJ95]
PHA	Pulse Height Analyzer
Phase	(a) A number (usually expressed as an angle between 0° and 360°) which characterizes a wave, where the phase of a wave corresponds to the position in its cycle relative to some arbitrary reference point. (b) The name given to the changing shape of the visible illuminated surface of a non-self-luminous celestial body (the Moon or a planet). The phase changes are caused by the relative positions of the Earth, Sun, and illuminated body. Conventionally, 0° phase occurs when the hemisphere facing the Earth is fully sunlit. [H76] (c) The ratio of the illuminated area of the apparent disk of a celestial body to the area of the entire apparent disk taken as a circle. For the Moon, phase designations (see Lunar Phases) are defined by specific configurations of the Sun, Earth and Moon. For eclipses, phase designations (total, partial, penumbral, etc.) provide general descriptions of the phenomena. [S92] (d) When used in reference to matter, describes its possible states: solid phase, liquid phase, gas phase. More generally, refers to the possible descriptions of a physical system as features on which it depends (temperature, string coupling constant values, form of spacetime, etc.) are varied. [G99]
Phases	Differences in the appearance of the Moon, in particular, but also of Mercury and Venus, caused by the Earth observer's seeing only a part of the body lit by the Sun. [A84]
Phase Angle	(a) The angle measured at the center of an illuminated body between the light source and the observer. [S92] (b) A complex number has a phase-angle as well as a length.[D89]
Phase Conjugation	This novel form of nonlinear mixing of optical waves generates an output wave which retraces precisely the path taken by the input wave. The phase conjugation reverses the wave front variation in the incident field and can compensate exactly aberrations and distortions in the input. The technique is a kind of holography in real time. [D89]
Phase Difference	Two identical waves of the same wavelength are said to be "in phase" when the peaks and troughs coincide perfectly. If this is not the case then a phase difference is said to occur. For a phase difference of half a wavelength, the peak of one wave coincides with the trough of the other. [McL97]
Phase Space	(a) A space whose coordinates are given by the set of independent variables characterizing the state of a dynamical system. [D89] (b) A multi-dimensional space that can be used to define the state of a system. Phase space has coordinates (q1, q2,... p1, p2,...), where q1, q2, etc., are degrees of freedom of the system and p1, p2 etc., are the momenta corresponding to these degrees of freedom. For example, a single particle has three degrees of freedom (corresponding to the three coordinates defining its position). It also has three components of momentum corresponding to these degrees of freedom. This means that the state of the particle can be defined by six numbers (q1, q2, q3, p1, p2, p3) and it is thus defined by a point in six-dimensional phase space. If the system changes with time (i.e. the particle changes its position and momentum), then the point in phase space traces out a path (known as the trajectory). The system may consist of more than one particle. Thus, if there are N particles in the system then the state of the system is specified by a point in a phase space of 6N dimensions. The idea of phase space is useful in chaos theory. (See also Attractor, Chaos Theory) [DC99] (b) A six-dimensional mathematical space that includes not only the three dimensions of ordinary space but also the three dimensions of velocity space. A point in phase space represents a given position in ordinary space and a given velocity in velocity space. [H76]
Phase Switching	A technique used in radio astronomy to suppress background noise so that the receiver records only point sources. [H76]
Phase Transition	(a) An abrupt change in the equilibrium state of a system. (b) A sudden transition between one state of matter or energy and another state. For example, when hot water turns to steam or when ice crystallizes out of a liquid that has been cooled to below freezing, a phase transition has occurred. According to the grand unified theories of particle physics, the infant Universe may have undergone one or more overall phase transitions. In this case, the energy uniformly filling all space corresponded to the supercooled liquid. [LB90] (c) A change of state such as occurs in the boiling or freezing of a liquid, or in the change between ferromagnetic and paramagnetic states of a magnetic solid. An abrupt change, characterized by a lump in an order parameter is known as `first-order'; a change in which the order parameter evolves smoothly to or from zero is called continuous. [D89]
Phasor	A rotating vector used to represent a sinusoidally varying quantity (e.g. an alternating current). The projection of the vector on a fixed axis represents the amplitude variation with time. A phase angle between two quantities (e.g. current and voltage) is represented by the angle between their phasors. [DC99]
Phillips Bands	Spectral bands of the C2 molecule in the red and near-infrared (0-0 band at 1.207 µ). [H76]
Phobos	The potato-shaped inner satellite of Mars (about 18 × 22 km), discovered by A. Hall in 1877. Orbital and rotation period 7h39m14s, e = 0.021, i = 1°.1. Visual geometric albedo 0.06. Infrared observations suggest that its surface is covered with dust. Phobos lies just outside the Martian Roche limit. [H76]
Phoebe	The outermost satellite of Saturn, discovered by Pickering in 1898. Period 550 days retrograde: radius about 100 km. [H76]
SX Phoenicis	A dwarf Cepheid (spectral type A) with the shortest known period (1h19m). [H76]
Phonon	The quantum associated with lattice vibrations in a solid. Phonons are sound quanta. [H76]
Phosphorus	A reactive solid non-metallic element. There are three common allotropes of phosphorus and several other modifications of these, some of which have indefinite structures. Symbol: P; m.p. 44.1°C (white) 410°C (red under pressure); b.p. 280.5°C; r.d. 1.82 (white) 2.2 (red) 2.69 (black) (all at 20°C); p.n. 15; r.a.m. 30.973762. [DC99]
Photocathode	A thin metallic plate housed inside an evacuated tube capable of releasing electrons through the "photoelectric effect" when illuminated by light. These surfaces are best for optical and ultraviolet light. [McL97]
Photoconductivity	Absorption of light increases the number of charge carriers. [McL97]
Photodiode	A light-sensitive device made from the junction of two differently doped species of a semiconductor such as silicon. Also known as a pn junction. An internal electric field is generated at the junction of p and n type material. Photons absorbed in the junction create electron-hole pairs which are separated by the field and create a current. [McL97]
Photoelectric Effect	(a) Phenomenon in which electrons are ejected from a metallic surface when light is shone upon it. [G99] (b) The forced ejection of electrons from certain metals due to the transfer of energy from incident "photons" of light. A demonstration (by Einstein) of the quantum effect. If the ejected electrons are collected as a charge or a current then the intensity of the incident light can be measured. Almost all modern detectors rely on some variant of the photoelectric effect. [McL97]
Photoelectric Devices	Any detector which uses the photoelectric effect to convert photons to electrons. [McL97]
Photoelectric Emulsions	Materials in which the absorption of light leads to a chemical reaction. [McL97]
Photoelectric Filtering	Means of measuring the astronomical colour index of a star, involving colour filters on photoelectric cells to define the color index between two set wavelengths. The filters correspond to the UBV photometry system. [A84]
Photoelectric Magnitude	The magnitude of an object as measured with a photoelectric photometer. (mpe) (antiquated term) [H76]
Photoelectrons	Electrons ejected from a solid, liquid, or gas by the photoelectric effect or by photoionization. [DC99]
Photoemission	The emission of photoelectrons by the photoelectric effect or by photoionization. [DC99]
Photographic Magnitude	The magnitude of an object as measured on the traditional photographic emulsions, which are sensitive to a slightly bluer region of the spectrum than is the human eye. (mph)(antiquated term) [H76]
Photoionization	(a) The ionization of atoms or molecules by electromagnetic radiation. Photons absorbed by an atom may have sufficient photon energy to free an electron from its attraction by the nucleus. The process is M + h M+ + e-. As in the photoelectric effect, the radiation must have a certain minimum threshold frequency. The energy of the photoelectrons ejected is given by W = h - I, where I is the ionization potential of the atom or molecule.[DC99] (b) The ionization of an atom or molecule by the absorption of a high-energy photon by the particle. It is an important source of opacity in stars. [H76]
Photometric Binaries	Eclipsing variables like Per whose orbital plane lies so nearly in the line of sight that eclipses, as seen from the Earth, can occur and can be detected from their light curves. [H76]
Photometric Redshift
Photometry	The measurement of the intensity of light from an astronomical object. [LB90]
Photomultiplier	Device used in photometry for the amplification of light by the release and acceleration of electrons from a sensitive surface. The result is a measurable electric current that is proportional to the intensity of received radiation. [A84]
Photomultiplier Tube	A vacuum encapsulated photocathode from which electrons are ejected by the photoelectric effect followed by multiple cathodes from which many additional electrons are emitted in a cascade. When finally collected, the original single electron may have generated a pulse of over one million electrons. [McL97]
Photon	(a) Smallest packet of the electromagnetic force field; messenger particle of the electromagnetic force; smallest bundle of light. [G99] (b) The quantum of the electromagnetic field. It is the massless spin-1 gauge boson of QED. Virtual photons mediate the electromagnetic force between charged particles. Virtual photons can also adopt a mass for a short period, in accordance with Heisenberg's uncertainty principle. [CD99] (c) The quantum particle of the electromagnetic field. [D89] (d) The photon is the particle that makes up light. It transmits the electromagnetic force. It is the gauge boson of electromagnetism. Once electrons exist, quantum theory implies that photons must exist and must have the properties they do. [K2000]
Photon Counting	The detection of individual photons. Photomultiplier tubes are capable of detecting single photons. Photon counting statistics says that if N is the total number of photoelectrons counted then the error in N is ± N. [McL97]
Photon Decoupling	The release of photons from constant collisions with massive particles as the Universe expanded and its matter density diminished. (See Decoupling) [F88]
Photon Detectors	These devices respond directly to individual photons. An absorbed photon then releases one or more bound charge carriers in the detector that may (1) modulate the electric current in the material; (2) move directly to an output amplifier; or (3) lead to a chemical change. Photon detectors are used throughout the x-ray, ultraviolet, visible and infrared spectral regions. (See also Thermal Detectors; Coherent Receivers)
Photoneutrinos	Neutrino-antineutrino pairs produced by the collision of high-energy photons with electrons: + e+ -> e- + v + vbar. [H76]
Photonics	The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon for a range of applications ranging from detection to laser energy production to communications and information processing. [McL97]
Photon-to-Baryon Ratio	The ratio of the number of photons to the number of baryons in any typical, large volume of space. (See Baryons; Photon) [LB90]
Photosphere	(a) The region of a star which gives rise to the continuum radiation emitted by the star. The visible surface of the Sun (temperature about 6000K), just below the chromosphere and just above the convective zone. The photosphere ends (and the chromosphere begins) at about the place where the density of negative hydrogen ions has dropped to too low a value to result in appreciable opacity. The spectrum of the photosphere consists of absorption lines (unlike that of the chromosphere, which consists of emission lines). [H76] (b) The visible surface of the Sun, or more generally, the layer of a star that gives rise to the continuum (as opposed to spectral-line) radiation emitted by the star.
Photosphere of the Sun	The Solar "surface". Granular in appearance, it comprises spicules of gaseous helium at an average temperature of 6,000°C. Each spicule averages 7,000 km in height but lasts for less than 8 minutes. The sunspots are cooler depressions in the photosphere. [A84]
Photovisual Magnitude	The magnitude of an object as measured photographically by filters and emulsions that are sensitive to the same region of the spectrum as the human eye. (antiquated) [H76]
Photovoltaic Effect	Absorption of a photon leads to the production of a voltage across a junction. [McL97]
Physics	The scientific study of the interactions of matter and energy. [F88]
PI	Pulse Invariant (also Principal Investigator).
Pickering Series	A spectral series of He II lines found in very hot O-type stars. It is associated with the fourth energy level - Pi at 10124 Å; Pi 6560 Å. The series limit is at 3644 Å. [H76]
Pico-	A prefix meaning 10-12. [H76]
Pinch Machine	A fusion device containing a plasma heated by a shock wave generated within the plasma as it is constricted by the rapidly increasing magnetic field. [H76]
Pinwheel Galaxy	M33, a spiral galaxy that lies 2.6 million light-years away and is the third largest member of the Local Group, after Andromeda and the Milky Way. [C95]
Pion	Pi Meson -- (a) An unstable nuclear particle of mass intermediate between that of a proton and an electron (+ and -: 273 me; 0: 264 me). The pions are believed to be the particles exchanged by nucleons, resulting in the strong nuclear force; they play a role in the strong interactions analogous to that of the photons in electromagnetic interactions. A charged pion usually decays into a muon and a neutrino; a neutral pion, into two -rays. Pions have spins of 0. (also called -meson) [H76] (b) A type of meson. There are three types of pion, having positive, negative, or zero charge. The charged pions have rest energy 139.6 MeV and mean life 2.6 × 10-8 s. Each decays to give a muon and a neutrino. The neutral pion has a rest energy 135 MeV and has mean life 8 × 10-17 s. It decays by emitting electromagnetic radiation. Pions are strongly interacting bosons with zero spin. [DC99] (c) The lightest hadron, and therefore the one most often produced in collisions. (See also Hadron) [K2000] (d) The lightest meson. Predicted by Yukawa, to explain the force binding the nucleus. It comes in three varieties distinguished by their electrical charges +1, 0, -1 labelled π+, π0, π-.
Pioneer Spaceprobes	Series of US spaceprobes the first 9 of which concentrated predominantly on Solar exploration and research. From then on, Pioneer probes have been sent to the outer planets of the Solar System. [A84]
Pitch Angle	Angle specifying the direction of electron velocity; or the angle between a tangent to a spiral arm and the perpendicular to the direction of the galactic center. [H76]
Pixel	Derived from "picture element." The smallest individual element of an array detector. Note that the size of the detector pixel does not necessarily equate with the resolution of the system (undersampling). [McL97]
Pixons	A term used to describe pixel-like elements in an image reconstruction algorithm. Pixons range in size and shape depending on the information content in that part of the image. [McL97]
P-L Relation	See Period-Luminosity relation. [H76]
Plage	The bright rim of a sunspot, observed in emission in monochromatic light of some spectral line (H[alpha] or Ca II). It is a chromospheric phenomenon associated with and often confused with a facula. (sometimes called flocculus)[H76]
Planar Spin Model	similar to the Heisenberg model, except that the spin of the atom is restricted to lie in a plane instead of being free to point in any direction in space. [D89]
Planck's Blackbody Formula	A formula that determines the distribution of intensity of radiation that prevails under conditions of thermal equilibrium at a temperature T: Bv = (2hv3 / c2)[exp(hv / kT) - 1]-1 where h is Planck's constant and v is the frequency. [H76]
Planck Constant	(a) The fundamental constant of nature = 1.05 × 10-34 kg m s-1, which characterizes quantum physics. Its simplest occurrence is in Heisenberg's uncertainty principle. For macroscopic systems, is usually negligibly small. [D89] (b) A fundamental constant; the ratio of the energy (W) carried by a photon to its frequency (v). A basic relationship in the quantum theory of radiation is W = hv. The value of h is 6.626 196 × 10-34 J s. The Planck constant appears in many relationships in which some observable measurement is quantized (i.e. can take only specific separate values rather than any of a range of values). [DC99] (c) The constant of proportionality relating the frequency of a photon to its quantum of energy: h 6.626 × 10-27 erg seconds. [H76] (d) A fundamental constant of nature that measures the magnitude of quantum mechanical effects. Visible light, for example, consists of discrete particles of light, or photons, each carrying an amount of energy equal to Planck's constant multiplied by the frequency of visible light. (The energy of one photon of visible light is approximately 10-18 or a billionth of a billionth, the energy of a penny dropped to the floor from waist high.) By combining Planck's constant with two other fundamental constants of nature - Newton's gravitational constant and the speed of light - one obtains other "Planck units" that mark critical densities and times when quantum mechanics and gravity were both extremely important. For example, the Planck density, or Planck scale, is the density of matter above which the structure, and perhaps meaning, of space and time break down due to quantum mechanical effects. Numerically, the Planck density is about 1093 grams per cubic centimeter. The infant Universe had this enormous density when it was about 10-43 seconds old, which is called the Planck time, and when it had a temperature of about 1022 Centigrade. At this temperature, the mean energy per particle was equivalent to the Planck mass, about 10 grams. (See Quantum Mechanics.) [LB90] (e) Many things are quantized, such as the energy levels of atoms. Planck's constant, h, sets the scale of quantization: Energy levels are separated by amounts proportional to h, the amount of spin a particle can have is a multiple of h, etc. Planck originally found that the energy radiated by a heated body was emitted in quanta, rather than emission of any continuous amount being possible. The amount of energy was always an integer multiple of hf where f is the frequency or color of the radiation; that is, hf of energy can be emitted, or 2hf, or 3hf and so on, but not amounts in between. By separately measuring the frequency, Planck deduced the value of h, which is 6.63 × 10-34 joule-second. (See also Planck Scale) [K2000]
Planck Distribution	The distribution with respect to wavelength of the frequency of the intensity of blackbody radiation can be expressed as Bv = 2v3c-2 exp[v / kT - 1]-1 where is Planck's constant, T is the radiation temperature, and v is the frequency. [Silk90]
Planck Energy	(a) An energy of 1.22 × 1019 GeV (billion electron volts), at which the strength of the gravitational interactions of fundamental particles becomes comparable to that of the other interactions. It is believed that the quantum effects of gravity become important at approximately this energy. [G97] (b) About 1,000 kilowatt hours. The energy necessary to probe to distances as small as the Planck length. The typical energy of a vibrating string in string theory. [G99]
Planck Epoch/Planck Time	The first instant following the beginning of the expansion of the Universe, when the cosmic matter density was still so high that gravitational force acted as strongly as the other fundamental forces on the sub-atomic scale. [F88]
Planck Era	The first 10-43 seconds of the Universe's existence. Physics can currently say very little about this time. Quantum gravity is needed before quantum cosmology can be fully realised.
Planck Length	(a) The dimension at which space is predicted to become "foamlike" and at which Einstein's theory is supposed to break down. [(G / c3)1/2 = 1.6 × 10-33 cm] [H76] (b) The size limit at which normal notions of space-time are supposed to break down. [P88] (c) About 10-33 centimeters. The scale below which quantum fluctuations in the fabric of spacetime would become enormous. The size of a typical string in string theory. [G99] (d) The fundamental quantity with the units of length that is expected to define the regime in which quantum gravity effects are of importance. It is defined as Lp : (G/c3)1/2 where is Planck's constant, G is Newton's constant and c is the speed of light. The numerical value is Lp 10-36 m. [D89]
Planck Mass	About ten billion billion times the mass of a proton; about one-hundredth of a thousandth of a gram; about the mass of a small grain of dust. The typical mass equivalent of a vibrating string in string theory. [G99]
Planck's Radiation Law	The energy radiated per unit area per unit time per unit wavelength range at wavelength from a black body at kelvin temperature T is given by E = 2 π h c2 -5 / [exp (hc / kT) - 1] where h is the Planck constant, k the Boltzmann constant and c the speed of light in a vacuum. This law was first proposed by Planck (1900). The problem of the energy distribution in black-body radiation had been studied for many years. The physical system considered is of extreme simplicity, and the failure of the existing statistical system of Maxwell and Boltzmann to solve the problem was rightly considered to be very significant (see ultraviolet catastrophe). Planck deduced his equation by assuming that electromagnetic radiation was not emitted and absorbed continuously as previously supposed but in quanta of energy hc / . Planck's formula was found to be in good agreement with experiment and it permitted the first reasonably accurate measurement of the Boltzmann constant, and hence of related quantities such as the charge of the electron and the masses of atoms. Little progress however was made with developing the ideas until Einstein (1905) argued that the quantum hypothesis would explain the photoelectric effect and Stokes' law, and later proposed other applications. This work led eventually to the development of quantum mechanics. see Bose-Einstein Statistics [DC99]
Planck Scale	The Planck scale refers to certain values of length, time, and energy or mass. To understand how these values originate, suppose you were trying to explain to an intelligent being in another galaxy how long humans typically lived. You couldn't use hours or years, because those units are defined on earth (for example, by how long it happens to take our planet to go around its sun once), so a being in another galaxy wouldn't know what you meant. However, every physicist in the universe knows the values of Planck's constant (h), the speed of light in vacuum (c), and the universal strength of the gravitational force (G). You could use those values to form ratios that define a universal unit of time called the Planck time and then tell the being from another galaxy our typical lifetime in units of Planck times. Similar units for length and mass or energy can be defined. Max Planck realized this possibility and defined these units at the beginning of the twentieth century. Because the Planck scale units are the only universal ones, we expect the fundamental laws of nature to be simple in form when expressed in those units. [K2000]
Planck Tension	About 1039 tons. The tension on a typical string in string theory. [G99]
Planck Time	(a) About 10-43 seconds. Time at which the size of the Universe was roughly the Planck length; more precisely, time it takes light to travel the Planck length. [G99] (b) An instant in the Big Bang, prior to which Einstein's theory of gravitation breaks down and a quantized theory of gravity is needed. It can be expressed as (G / c5)1/2 where G is Newton's constant of gravitation, is Planck's constant, and c is the speed of light, and it equals 10-43 second. [Silk90] (c) The time taken for a photon travelling at the speed of light to move a distance of the Planck length.[D89]
Planck Units	Fundamental units of length, time, mass, energy, etc. involving Planck's quantum constant, , Newton's gravitational constant, G, and the speed of light, c. As they incorporate both the quantum and gravitational constants, the Planck units play a key role in theories of quantum gravity. [CD99]
Plane-Parallel Atmosphere	An atmosphere stratified in parallel planes normal to the direction of gravity. [H76]
Planet	(1) Planet A celestial body that (a) is in orbit around the Sun, (b) having sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (near round) shape, and (c) has cleared the neighborhood around its orbit. The eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. (2) Dwarf Planet A celestial body that (a) is in orbit around the Sun, (b) having sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (near round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite. (3) Small Solar-System Bodies All other objects, except satellites, orbiting the Sun. These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs) comets and other small bodies. [BFM2006]
Planetary Nebula	(a) A bubble of gas surrounding a hot, dying star. The star is so hot that it makes the planetary nebula glow, which allows astronomers to see it. The star was once the core of a red giant, which ejected its outer atmosphere and created the planetary. A planetary nebula has nothing to do with a planet, but through a small telescope, it looks like a planet's disk, hence the misleading name. [C95] (b) An expanding envelope of rarefied ionized gas surrounding a hot white dwarf. The envelope receives ultraviolet radiation from the central star and reemits it as visible light by the process of fluorescence. The planetary nebula stage lasts for less than 50,000 years. During the core contraction that terminates the red-giant stage, the helium-burning shell is ejected at a velocity so high that it becomes separated from the core. Under current theories, a star with a carbon core and a mass greater than 0.6 M (but less than 4 M) will become a planetary nebula and leave behind a white dwarf. Planetary nebulae are now known to occur in stars less than 4 M whose envelope becomes unstable during the hydrogen shell burning stage. [H76]
Planetary Precession	The component of general precession caused by the gravitational coupling between the center of mass of the Earth and that of the other planets. The effect of planetary precession is to move the equinox eastward by 0".11 / year and to diminish the angle between the ecliptic and the equator by about 0".47 / year. [H76]
Planetesimals	Asteroid-sized solid bodies that are hypothesized to form when the protosolar nebula collapsed into a disk and fragmented. Most of the planetesimals subsequently accumulated into planets. [Silk90]
Planetocentric Coordinates	Coordinates for general use, where the z-axis is the mean axis of rotation; the x-axis is the intersection of the planetary equator (normal to the z-axis through the center of mass) and an arbitrary prime meridian; and the y-axis completes a right-hand coordinate system. Longitude (see Longitude, Celestial) of a point is measured positive to the prime meridian as defined by rotational elements. Latitude (see Latitude, Celestial) of a point is the angle between the planetary equator and a line to the center of mass. The radius is measured from the center of mass to the surface point. [S92]
Planetographic Coordinates	Coordinates for cartographic purposes dependent on an equipotential surface as a reference surface. Longitude (see Longitude, Celestial) of a point is measured in the direction opposite to the rotation (positive to the west for direct rotation) from the cartographic position of the prime meridian defined by a clearly observable surface feature. Latitude (see Latitude, Celestial) of a point is the angle between the planetary equator (normal to the z-axis and through the center of mass) and normal to the reference surface at the point. The height of a point is specified as the distance above a point with the same longitude and latitude on the reference surface. [S92]
Planetoids	see Asteroids [A84]
Plano-Concave Lens	A diverging lens with one plane face and once concave face. [DC99]
Plano-Convex Lens	A converging lens with one plane face and one convex face. [DC99]
Plaskett's Star	HD 47129: A very massive O-type giant with known anomalies in its spectrum. It is a spectroscopic binary in which mass exchange is occurring. Its spectrum can be interpreted to mean that each component has a mass of 75 M. [H76]
Plasma	A completely ionized gas; the so-called fourth state of matter (besides solid, liquid, and gas) in which the temperature is too high for atoms as such to exist and which consists of free electrons and free atomic nuclei. [H76]
Plasma Clouds	Clouds of electrically charged particles embedded in the Solar wind. [H76]
Plasmapause	The region in Earth's ionosphere (at about 4-7 Earth radii) where the particle density (100 particles per cm3 just below the plasmapause) drops off very rapidly. It marks the transition from high to low density. [H76]
Plate Scale	The number of seconds of arc on the sky corresponding to 1 mm in the focal plane of the telescope. Originating with photographic plates, but now being replaced by "pixel scale" for digital (generally CCD) detectors. [BFM2002]
Platinum	(a) Element with atomic number 78. It is produced almost entirely by the r-process in supernovae. [C95] (b) A silvery-white malleable ductile transition metal. It occurs naturally either free or in association with other platinum metals. Platinum is used as a catalyst for ammonia oxidation (to make nitric acid) and in catalytic converters. It is also used in jewelry. Symbol: Pt; m.p. 1772°C; b.p. 3830 ± 100°C; r.d. 21.45 (20°C); p.n. 78; r.a.m. 195.08. [DC99]
Platonic Solids	The five regular polyhedrons - the tetrahedron, octahedron, hexahedron, icosahedron, and dodecahedron - esteemed by Plato as embodying aesthetic and rational ideals. [F88]
Pleiades	A very young open cluster of several hundred stars (B6 and later) in Taurus, about 125 pc distant. Six members of the cluster (all of spectral type B or Be) are visible to ordinary sight. (M45, NGC 1432) [H76]
Pleione	A B8pe star (28 Tau), one of the brightest stars in the Pleiades, which developed an envelope or shell first observed in 1938. The shell increased in strength and attained its maximum intensity in 1945; thereafter it weakened and was scarcely visible by 1954. In 1972 it developed another shell. It is rotating so fast that it is unstable. [H76]
Plume	A rising column of gas over a maintained source of heat. [H76]
Plurality of Worlds	Hypothesis that the Universe contains inhabited planets other than Earth. [F88]
Pluto	The most distant known planet from the Sun (39.44 AU), discovered by Clyde Tombaugh in 1930. Orbital period 248.43 years, Vorb 4.7 km s-1. Its orbit has the highest eccentricity (0.249) and highest inclination to the ecliptic (17°.17) of any planet and some astronomers suggest that it may be an escaped satellite of Neptune. Synodic period 366.7 days; albedo less than 0.25: rotation period 6d9h17m49s. In the mid-1970s Pluto crosses Neptune's orbit on its way in, and for the rest of this century Pluto will be closer to the Sun than Neptune. (Pluto and Neptune, however, are never less than 2.6 AU apart.) Perihelion will occur in 1989. Effective temperature about 50-60 K. Its mass and radius have not been determined with any great certainty, but it is probably about 0.1 to 0.2 the mass of the Earth (6 × 1026 g?) and no more than 2900 km in radius. [H76]
Plutonium	A radioactive silvery element of the actinoid series of metals. It is a transuranic element found on Earth only in minute quantities in uranium ores but readily obtained, as 239Pu, by neutron bombardment of natural uranium. The readily fissionable 239Pu is a major nuclear fuel and nuclear explosive. Plutonium is highly toxic because of its radioactivity; in the body it accumulates in bone. Symbol: Pu; m.p. 641°C; b.p. 3232°C; r.d. 19.84 (25°C); p.n. 94; most stable isotope 244Pu (half-life 8.2 × 107 years). [DC99]
PM	PhotoMultiplier [LLM96]
pm	Post Meridiem [LLM96]
Pockels Cell	An electro-optic crystal used as a reversible waveplate by applying alternately high positive and negative voltage. [H76]
Pogson's Ratio/Scale	The ratio between two successive stellar magnitudes, introduced by N. Pogson in 1856. [H76]
Poincaré's Theorem	The total kinetic energy of all the stars in a cluster is equal to half the negative gravitational potential energy of the cluster. [H76]
Point-Like	If matter is probed with projectiles that are large and have energies that are less than what is needed to change the energy levels of an atom, then atoms will seem to be point-like objects. If the energy is increased, eventually the projectile will penetrate the atom but will encounter the nucleus, which will seem to be point-like. With higher energy, the nucleus will appear to be made of point-like protons and neutrons. With still higher energies, the protons and neutrons will be seen to be made of point-like quarks and gluons. As the energies of projectiles were increased still more, quarks and leptons might have been seen to be made of something still smaller, but that has not happened. Rather, they behave as point-like up to the highest energies they have been probed with - energies well beyond those for which we would have expected to find more constituents if history were to repeat itself once more. Further, the structure of the Standard Model theory suggests that quarks and leptons are the fundamental, point-like constituents of matter. [K2000]
Point Source	(a) A source whose angular extent cannot be measured (< 0".05). [H76] (b) A source of exactly spherical wave fronts. All sources can be considered to be point-like if viewed from a large enough distance. The stars provide an obvious example. In a number of practical situations, sources that are effectively point sources are required. Normally a small hole in an otherwise opaque illuminated surface is used. Holes can be made as small as 100 µm; the main problem is to arrange adequate illumination of the hole by the real source. [DC99]
Poisson Distribution	An approximation to the binomial distribution used when the probability of success in a single trial is very small and the number of trials is very large. [H76]
Poisson's Equation	An equation (2 = 4G) which relates the gravitational (or electromagnetic) potential to the mass density (or charge density). [H76]
Polar Axis	The axis of an equatorially mounted telescope that points towards the Celestial Poles and is therefore parallel to the Earth's axis of rotation. [McL97]
Polar Motion	The irregularly varying motion of the Earth's pole of rotation with respect to the Earth's crust. (See Celestial Ephemeris Pole) [S92]
Polarimeter	Device that measures the polarization of any form of electromagnetic radiation, particularly light. [A84]
Polaris	(a) The North Star, a second-magnitude star in the constellation Ursa Minor. The star is a yellow-white F-type supergiant that lies 330 light-years away. (b) A supergiant F8Ib, F3V visual binary, 120 pc distant, with an orbital period of thousands of years. The primary (a Cepheid with a pulsation period of 3.97 days) is itself a single-lined spectroscopic double with a period of 29.6 years. There are at least two more faint (12th mag) components of the system. ( UMi) [H76]
Polarization	Restriction of the vibrations in a transverse wave. Normally in a transverse wave the vibrations can have any direction in the plane perpendicular to the direction of propagation. If the directions of the vibrations are restricted in any way the radiation is said to be polarized. The simplest case is that of plane polarization. In a plane-polarized transverse acoustic wave in a solid, all the vibrations are parallel to each other. In plane-polarized electromagnetic radiation, all the electric oscillations are parallel to each other and at right angles to the magnetic oscillations. Light can be plane-polarized by reflection or on passing through certain substances (e.g. Polaroid). Light polarized in a certain plane by passing through one polarizer is totally absorbed by a second polarizer set to polarize at right angles. Electromagnetic waves such as light and those of shorter wavelength generally interact with matter by means of their electric fields (although energy is transferred equally by both). Thus, for example, the directions in which photoelectrons are ejected may depend upon the plane of polarization. Radio waves are normally emitted plane-polarized. Thus if the electric vibrations are vertical a rod-shaped antenna, which responds to these vibrations, must be set vertically to detect them. An antenna consisting of a coil with ferrite core will have to be set so that the magnetic vibrations are perpendicular to its plane. [DC99]
Polarization of Light	Reduction of light, considered to travel in three-dimensional transverse waves (vibrating in all directions perpendicular to the direction in which it is traveling), to two dimensions. To achieve this a filter is used. The results may vary from a beam of light in which the waves vibrate in one plane only (plane-polarized light) to one in which the plane rotates but the amplitude is constant (circular polarization). Because light is also polarized by reflection, investigation of polarized light reflected from, for example, the lunar surface enables that surface to be analyzed. [A84]
Polarization (polarimeter)	The property of transverse electromagnetic waves that describes the plane of vibration of the wave and its behavior as the wave progresses. Linearly polarized light implies that all the waves vibrate in the same plane. Circular polarization occurs when the plane of vibration rotates as the wave progresses. A polarimeter is used to measure these properties. [McL97]
Polarization Modulator	An optical device sensitive to the plane of vibration of electromagnetic waves, i.e. to their polarization. It is used to convert the polarization of light into a measurable brightness change. [McL97]
Polaroid (Trade name)	A synthetic doubly refracting substance, that strongly absorbs polarized light in one plane, while easily passing polarized light in another plane at right angles. Unpolarized light passed through a sheet of Polaroid is plane-polarized. Spectacle lenses made of this material normally absorb light vibrating horizontally - as produced by reflection from horizontal surfaces. They thus reduce reflected glare. The first type was announced by Land in 1934. The modern form is made from a plastic sheet, highly strained to align the molecules and make it birefringent, then stained with iodine to make it dichroic. [DC99]
Pole Star	The star - Polaris - that lies near the direction in the sky toward which the North Pole of the Earth points. [F88]
Polonium	A radioactive metallic element belonging to group 16 of the periodic table. It occurs in very minute quantities in uranium ores. Over 30 radioisotopes are known, nearly all alpha-particle emitters. Polonium is a volatile metal and evaporates with time. It is also strongly radioactive; a quantity of polonium quickly reaches a temperature of a few hundred degrees C because of the alpha emission. For this reason it has been used as a lightweight heat supply in space satellites. Symbol: Po; m.p. 254°C; b.p. 962°C; r.d. 9.32 (20°C); p.n. 84; stablest isotope 209Po (half-life 102 years). [DC99]
Pollux ( Gem)	A K0 III star 11 pc distant. [H76]
Polychromatic Radiation	Electromagnetic radiation that has a mixture of different wavelengths. Compare monochromatic radiation. [DC99]
Polysilicon	A non-crystalline form of silicon with a high conductivity like a metal; preferred in CCD manufacture to the use of metals because it keeps the entire process in silicon and is more transparent to visible light. [McL97]
Polytrope	A mathematical model of an inhomogeneous, compressible configuration in equilibrium under its own gravitation in which the relation between the pressure and the density satisfies the relation p = K(n+1)/n, where K is a constant and n is the polytropic index. [H76]
Polytropic Index (n)	The polytropic index may have any value from zero (uniform density throughout) to 5 (entire mass concentrated at the center). A polytropic index of 1.5 corresponds to a fully degenerate, nonrelativistic electron gas; it also describes a perfect-gas star in convective equilibrium. (See also Polytrope) [H76]
Population	see Stellar Population. [C95]
Population I	Younger stars, generally formed towards the edge of a galaxy, of the dusty material in the spiral arms, including the heavy elements. The brightest of this Population are hot, white stars. [A84]
Population II	Older stars, generally formed towards the centre of a galaxy, containing few heavier elements. The brightest of this Population are red giants. [A84]
Populations I and II	Two classes of stars introduced by Baade in 1944. In general, Population I (now sometimes called arm population) are young stars with relatively high abundances of metals, and are usually found in the disk of a galaxy, especially the spiral arms, in dense regions of interstellar gas. Population II (now sometimes called halo population) are older stars with relatively low abundances of metals, and are usually found in the nucleus of a galaxy or in globular clusters. The Sun is a rather old Population I star. [H76]
Population I, II, and III	The youngest observed stars are called Population I stars; older observed stars are called Population II; and it is postulated that an even older generation of stars, called Population III, existed still earlier. Population II stars formed mostly from hydrogen and helium. Population I stars, like our sun, formed from hydrogen, helium, and a large range of heavier elements (like carbon and oxygen) believed to have been created in the interiors of earlier Population II and III stars and then blown out into space. [LB90]
Population Inversion	A condition that exists when there are more molecules in an excited state than an equilibrium distribution would allow. It is necessary for masers. [H76]
Poseidon	Unofficial name for J VIII, the next outermost satellite of Jupiter. P = 737dR, e = 0.4, i = 147°. Discovered by Melotte in 1908. [H76]
Position Angle	Angular distance (in degrees, measured from north through east) between the primary and secondary components of a binary system. [H76]
Positron	(a) The antiparticle of the electron, discovered by Anderson in 1934. It has the same mass and spin as the electron, but opposite charge and magnetic moment. (also called Antielectron) [CD99] (b) Carries positive electric charge. Annihilation with electron produces energy and new varieties of hadrons and quarks. [D89]
Positronium	A fleeting combination of an electron and a positron to form an analog to a hydrogen atom. When the two particles have their spins parallel the half-life is about 1.5 × 10-7 s; when they are antiparallel the half-life is shortened to 10-10 s. A positronium `atom' decays to form two photons by annihilation. A combination of two electrons and two positrons also appears to exist, and is known as a positronium `molecule', analogous to a hydrogen molecule. [DC99]
Post-Asymptotic Branch Stars	F-type supergiants with strong sulfur lines. [JJ95]
Post-Galilean Transformation	A transformation which replaces the Lorentz transformation when first-order corrections due to general relativity are included. [H76]
Post Hoc Fallacy	The erroneous assumption that, because B follows A, B therefore was caused by A. More strictly, the fallacy of calculating, in retrospect, the odds of B's having occurred by adding up a long sequence of such putative causes. [F88]
Post-Newtonian Effects	The first nontrivial gravitational effects which go beyond the predictions of Newton's theory. [H76]
Potassium	A soft reactive metal. The atom has the argon electronic configuration plus an outer 4s1 electron. Symbol: K; m.p. 63.65°C; b.p. 774°C; r.d. 0.862 (20°C); p.n. 19; r.a.m. 39.0983. [DC99]
Potential	A quantitative measure of how much energy is associated with each possible arrangement of a physical system. An arrangement with a relatively high value of the potential is one with a relatively large amount of energy. For example, the gravitational potential of a pendulum at the top of its swing is large; the potential is small when the pendulum is at the bottom of its swing. Since systems in nature generally evolve toward arrangements of lower energy, as in the tendency of upended books to fall over, the final resting point of a system is in a configuration at the minimum value of the potential. The minimum of the potential for empty space corresponds to the vacuum. [LB90]
Potentiality	a peculiarly quantum mechanical mode of reality, which is intermediate between full actuality and bare logical possibility. When an eventuality is characterized as a potentiality, it is neither true nor false but indefinite, but it has a definite probability of turning out to be true if the system is subjected to physical conditions which suffice to make it actual. [D89]
Potts Model	a generalisation of the Ising model in which the two states for each lattice site of the ising model are replaced by n equivalent states. [D89]
Power Series	A series of the form a0 + a1v + a2v2 + . . . + anvn = n=1anvn. [H76]
Power Spectrum
Poynting-Robertson Effect	An effect of radiation pressure on a small particle orbiting the Sun that causes it to spiral slowly into the Sun. The radiation falls preferentially on the leading edge of the orbiting particle and acts as a drag force. [H76]
Poynting Vector	Symbol: S The vector product of the electric field vector E and the magnetic field vector H (= B0/µ0) in an electromagnetic wave. The Poynting vector gives, in magnitude and direction, the power radiated through unit area at any instant. The unit is the watt per sqare meter. In a simple harmonic wave, the average value is 1/2E0H0, where E0 and H0 are the amplitudes. [DC99]
ppb	Parts per Billion [LLM96]
ppm	Parts per Million [LLM96]
PPN Parameters	denotes Parametrized Post-Newtonian Parameters. dimensionless parameters that describe the first relativistic corrections beyond Newtonian gravity in the Solar System; their values depend on the theory of gravity adopted. [D89]
Praesepe	A naked-eye open cluster in Cancer, about 160 pc distant. (Beehive Cluster) (M44, NGC 2632) [H76]
Prandtl Number	(a) Ratio of the product of the viscosity coefficient and the specific heat at constant pressure to the thermal conductivity. [H76] (b) A dimensionless parameter used in fluid mechanics. It may be expressed as the ratio of the kinematic viscosity ν to the thermal diffusivity or thermometric conductivity K. The latter is given by k / c where k is the thermal conductivity, is the density and c is the specific heat. At 20°C the Prandtl number lies between 0.67 and 1.0 for gases; for water it is 6.7 and it is of the order of thousands for very viscous liquids. The unit was deduced by Nusselt in 1910, but has been named after L. Prandtl (1875-1953) in the misinformed belief that he had initially derived it in 1922. Prandtl, in a footnote in his book on fluid dynamics, states 'The author is unwilling to encourage this piece of historic inaccuracy and therefore prefers the equally brief expression v/K'. The number was recognized by the American Standards Association in 1941. [JM92]
Praseodymium
Preamplifier	A low-noise amplifier designed to be located very close to the source of weak electronic signals, but capable of delivering amplified signals down tens of meters of cables. [McL97]
Precession	(a) The slow (once per twenty-six thousand years) gyration of the Earth's axis. [F88] (b) A slow, periodic conical motion of the rotation axis of a spinning body. In the case of Earth's precession it is due to the fact that Earth's axis of rotation is not perpendicular to the ecliptic but is inclined about 23°.5 and is thus affected by gravitational perturbations from other bodies in the Solar System. The Moon and Sun pull harder on that part of the Earth's equatorial bulge nearest them than on that farthest away; this causes a torque which precesses the Earth's rotational axis. [H76] (c) The uniformly progressing motion of the pole of rotation of a freely rotating body undergoing torque from external gravitational forces. In the case of the Earth, the component of precession caused by the Sun and Moon acting on the Earth's equatorial bulge is called lunisolar precession; the component caused by the action of the planets is called planetary precession. The sum of lunisolar and planetary precession is called general precession. (See Nutation.) [S92]
Precession, Constant of	The ratio of the lunisolar precession to the cosine of the obliquity of the ecliptic. It amounts to about 54".94 per annum. [H76]
Precession of the Equinoxes	The First Point of Aries (0 hr Right Ascension) moves backwards (westward) along the equator at 50.2 arcseconds per year due to the 26 000-year conical motion of the Earth's rotation axis caused by the gravitational pull of the Sun and Moon on the Earth's equatorial bulge. Correcting for this effect yields the mean equator and mean equinox. Nutation is the wobble of the Earth's axis as it precesses. Correction for this effect gives the true equator and true equinox. Because of precession the tropical year is about 20 minutes shorter than the sidereal year. It takes about 25,800 years for Earth's axis to complete one circuit. [McL97]
Precursor Pulse	A component of a Pulsar pulse which occurs slightly before the main pulse. At energies of about 600 keV the precursor pulse becomes stronger than the main pulse. [H76]
Predict	Predict is used in the normal sense that a theory may predict some unanticipated or as-yet-unmeasured result. It is also used in another sense: A theory can be said to predict a result that is already known, because once the theory is written, it gives a unique statement about that result. Sometimes an in-between situation holds, in that the theory predicts a result uniquely in principle, but the prediction depends on our knowing some other quantity or requires very difficult calculations. [K2000]
Predictability	the ability to predict the future behavior of a dynamical system on the basis of the present knowledge available on this system. [D89]
Preflash	The technique of illuminating the CCD with a low light level flash before beginning a long exposure in order to "fill up" any charge traps. [McL97]
Pressure	The force exerted over a surface divided by its area. [H76]
Pressure Broadening	A broadening of spectral lines, particularly in white dwarfs, caused by the pressure of the stellar atmosphere, which in turn is caused by the surface gravity of the star. [H76]
Pressure Gradient	A pressure difference between two adjacent regions of fluid results in a force being exerted from the high pressure region toward the low pressure region. In a star, the hot, dense interior and the cooler, more tenuous surface layers supply an outward pressure gradient, which balances the inward attractive force of gravity and stabilizes the star. [Silk90]
Pressure Ionization	A state found in white dwarfs and other degenerate matter in which the atoms are packed so tightly that the electron orbits encroach on each other to the point where an electron can no longer be regarded as belonging to any particular nucleus and must be considered free. [H76]
Pressure Scale Height	see Scale Height. [H76]
Primary Quantum Number	Number used in electron structure notation, (e.g., 1s, 2s, etc.) Corresponds to the orbit in the Bohr atom where an electron would be found. Written "n".[SEF01]
Primary Mirror	The first mirror encountered by incident light in a telescope system. [McL97]
Primary Cosmic Rays	The cosmic rays that arrive at Earth's upper atmosphere from outer space (see also Secondary Cosmic Rays). [H76]
Primary Theory	The name used in this book for the theory sought by many particle physicists that includes not only the Standard Theory but also the theory of gravity, explains why the primary theory itself takes the form it does, explains what quarks and other particles are, explains what space and time are, and more. (See Theory of Everything) [K2000]
Prime Focus	The focal point of the large primary reflecting mirror in astronomical telescopes when the light source is extremely distant. This focus actually falls at a point just within the upper structure of the telescope itself and is therefore accessible to CCD cameras and other instruments; it provides a large field of view. [McL97]
Primeval Fireball	The hot, dense, early stage of the Universe (predicted by the Big Bang theory) when the Universe was predominantly filled with highly energetic radiation, which subsequently expanded and cooled and is now observed as the cosmic microwave background radiation. [Silk90]
Primordial Black Holes	Small black holes hypothesized to have formed during the first 10-43 seconds of the Universe, when quantum effects were very large. [LB90]
Primordial Chaos	The concept that the early Universe might have been highly irregular and inhomogeneous. It could enable us to understand the origin of structure in the Universe and why the Universe is homogeneous and isotropic on the very largest scales. [Silk90]
Primordial Density Fluctuations
Primordial Fireball Radiation	Same as cosmic background radiation. [LB90]
Primordial Nucleosynthesis	(a) The creation of elements that occurred just minutes after the Big Bang. According to standard theory, primordial nucleosynthesis gave the Universe only five nuclei, all lightweight: hydrogen-1, hydrogen-2 (or deuterium), helium-3, helium-4, and lithium-7. [C95] (b) Production of atomic nuclei occurring during the first three minutes after the big bang. [G99]
Primordial Quarks	All baryons and mesons are believed to be composed of quarks, which are elementary particles of fractional charge. In the high-density, hot-temperature phase of the very early Universe, prolific numbers of quarks would have been present in equilibrium with the other elementary particles. As the Universe expanded and cooled, some of these quarks may have been frozen out. To what extent independent free quarks could survive is an unresolved issue of elementary particle physics. [Silk90]
Principal Quantum Number	A measure of the major axis of an electronic orbital. In the case of hydrogen, the energies of bound levels are specified completely by n. [H76]
Principia Mathematica	Short form of the title of Isaac Newton's great work, published in 1687; title also of the mathematically philosophical work of Bertrand Russell and Alfred North Whitehead, published in 1910-13 [A84]
Principle of Equivalence	(a) States that inertial mass is indistinguishable from gravitational mass. [c97] (b) Core principle of general relativity declaring the indistinguishability of accelerated motion and immersion in a gravitational field (over small enough regions of observation). Generalizes the principle of relativity by showing that all observers, regardless of their state of motion, can claim to be at rest, so long as they acknowledge the presence of a suitable gravitational field. [G99]
principle of equivalence	a principle which states that all bodies should fall with the same acceleration; also denoted the `weak equivalence principle'.[D89]
Principle of Relativity	Core principle of special relativity declaring that all constant-velocity observers are subject to an identical set of physical laws and that, therefore, every constant-velocity observer is justified in claiming that he or she is at rest. This principle is generalized by the principle of equivalence. [G99]
Printed Circuit	A compact double-sided circuit board with no wires but instead fine tracks, etched on a copped-clad board, perform the same function. [McL97]
Probable Error (p.e.)	The error which will not be exceeded by 50 percent of the cases. The probable error is equal to 0.6745 times the standard error. [H76]
Procyon	(a) The brightest star in the constellation Canis Minor and one of the nearest stars, lying just 11.4 light-years from Earth. Procyon is the eighth brightest star in the night sky. It consists of two stars: Procyon A, a bright yellow-white F-type star that has just started to evolve off the main sequence; and Procyon B, a dim white dwarf. [C95] (b) ( CMi) An F5 IV-V star 3.5 pc distant (parallax 0.283). It is a visual binary; its companion is a DF8 white dwarf with an orbital period of ~ 40 yr. [H76]
Product	The result of multiplying two numbers. [G99]
Profile	See Line Profile. [H76]
Program Stars	The stars being observed or measured, as contrasted with the comparison stars. [H76]
Prograde Motion	Motion in the same direction as the prevailing direction of motion.
Projectile	To study particles and their interactions, it is necessary to probe them with projectiles. The projectiles are other particles (electrons, photons, neutrinos, and protons) because these are small enough and can be given enough energy. [K2000]
Promethium	Pm: An unstable rare earth. The longest-lived isotope, 145Pm, has a half-life of only 18 years. [H76]
Prominence	A region of cool (104 K), high-density gas embedded in the hot (106 K), low-density Solar corona. Prominences are the flamelike tongues of gas that appear above the limb of the Sun. [H76]
Propagator	The mathematical expression used to describe the propagation in space-time of virtual particles. [CD99]
Proper Mass	Rest mass. [H76]
Proper Motion	(a) Apparent angular rate of motion of a star across the line of sight on the celestial sphere. [H76] (b) The apparent movement of a star, year after year, caused by the star's velocity across the line of sight. If the star's distance is known, this velocity-called the tangential velocity, can be computed. The star with the largest proper motion is Barnard's Star, whose proper motion is 10.3 arc-seconds per year. [C95] (c) The projection onto the celestial sphere of the space motion of a star relative to the Solar System; thus the transverse component of the space motion of a star with respect to the Solar ystem. Proper motion is usually tabulated in star catalogs as changes in right ascension and declination per year or century. [S92]
Proper Time	The timelike invariant spacetime interval between the points along the trajectory of a particle. (More prosaically, time measured by an ideal clock at rest with respect to the observer.) [H76]
Proportional Counter	A device used in X-ray astronomy which counts the number of ions produced when photons come into a volume of gas and ionize the gas. The more energetic the photon, the more ions are produced. [H76]
Protogalactic Gas Cloud	A massive gas cloud that collapsed to form a galaxy. Such clouds were produced as a result of the continued growth of density fluctuations after the Decoupling Era. [Silk90]
Protogalaxy	(a) A galaxy in the process of formation. None are observed nearby, indicating that all or most galaxies formed long ago. [F88] (b) A galaxy during the early phase, before it has developed its present shape and mix of stars. [Silk90]
Proton	(a) A subatomic particle with positive electric charge. Every atom has at least one proton in its nucleus; the number of protons determines the element. For example, all atoms with one proton are hydrogen, all atoms with two protons are helium, and so on. [C95] (b) One of the constituents of the atomic nucleus. It is a spin-1/2 particle carrying positive electric charge. The proton is the lightest baryon and, as a result, is the particle into which all other baryons eventually decay. It is believed to be absolutely stable, but certain theories (GUTs) predict it will decay very, very slowly. [CD99] (c) A baryon made of two up quarks and a down quark. It possesses a positive electromagnetic charge and can only be found in atomic nuclei. A single proton is a hydrogen nucleus. [c97] (d) A massive particle with positive electrical charge found in the nuclei of atoms. Composed of two up quarks and one down quark. The proton's mass is 938.3 MeV, slightly less than that of the neutron. [F88] (e) A positively charged elementary particle; the nucleus of a hydrogen atom. Mass of proton 1.00728 amu = 1.6726 × 10-24 g = 1836.12 me. [H76] (f) Positively charged constituent of the nucleus that gives it electrical charge. Built from (three) quarks.[D89]
Proton Decay	(a) Spontaneous disintegration of the proton, predicted by grand unified theory but never observed experimentally. [F88] (b) If the Standard Model were the complete theory that described nature, protons would be stable, never decaying. If the Standard Model is part of a more comprehensive theory that unifies quarks and leptons, then possibly protons are unstable, though with extremely long lifetimes. Some basic theories imply that protons decay; others do not. Experiments that search for proton decay are very important, because knowing that it occurred (and what the proton decayed into) would provide valuable information about how to extend the Standard Model. [K2000]
Proton-Proton Chain	(a) (p-p chain) A series of thermonuclear reactions in which hydrogen nuclei are transformed into helium nuclei. The temperature and density required are about 107 K and 100 g cm-3. It is the main source of energy in the Sun, where 1038 of these reactions occur every second. All parts of this reaction have been observed in the laboratory, except for the first step 1H(p, +v)2D, which occurs only a few times in 1012 collisions of protons. But the first two reactions provide about one-third of the Sun's total energy release. The p-p chain divides into three main branches: PP I: 1H(p, +v)2D (p, )3He(3He, 2p)4He + 4 × 10-5 ergs of energy. PP II: 1H(p, +v)2D(p, )3He (4He, ) 7Be(+v)7Li(p,)4He. PP III: 1H(p, +v)2D(p,)3He(4He,)7Be(p,)8B (+v)8Be -> 2 4He. (PP III occurs once in 1000 times:) Although the neutrinos from the PP II and PP III chains are detectable, they have not been observed. [H76] (b) Process of nuclear fusion by which relatively cooler stars produce and radiate energy; hotter stars commonly achieve the same result by means of the carbon-nitrogen cycle. [A84] (c) The nuclear sequence by which the Sun and all other main-sequence stars with less than 1.5 Solar masses fuse hydrogen into helium. [C95] (d) An important nuclear fusion reaction that occurs in stars. It begins with the fusion of two hydrogen nuclei, each of which consists of a single proton. [F88]
Proto-Planets	Early stage in the formation of planets according to the theory by which planetary systems evolve through the condensation of gas clouds surrounding a young star. The theory is not, however, generally accepted. [A84]
Proto-Solar Nebula	The slowly rotating cloud of gas and dust from which the Solar System formed. [Silk90]
Protostellar Core	The smallest opaque clumps into which a collapsing interstellar gas cloud fragments. The characteristic mass of a protostellar core is only 0.01 Solar mass. It grows by accretion as the surrounding matter falls toward it, attaining stellar mass within 105 years after it forms. At this stage, it is a protostar - a large cocoon of contracting matter that is radiating predominantly in the far infrared. [Silk90]
Proxima Centauri	The faintest of the three stars that make up the Alpha Centauri star system. (See Alpha Centauri) [C95]
PSD	Pulse-Shape Discrimination [LLM96]
PSD	Power Spectral Density [LLM96]
PSF	Point Spread Function -- The size and shape of the actual image of a point source as a result of the combined effects of atmosphere, optics, guiding. [McL97]
PSI	A name by which the J- or psi- (-)meson is known. [D89]
PSR	Pulsar [LLM96]
PSR B 1257+12	A pulsar in the constellation Virgo and the site of the first Solar System to be discovered outside our own. The planets were detected in 1991. [C95]
POSS	Palomar Observatory Sky Survey [BFM02]
Ptolemaic Model of the Universe	A geocentric model in which the Earth remained stationary as the other planets the Sun, the Moon and the stars orbited it on their spheres. It was eventually replaced by the Copernican model. [A84]
Pulsar	(a) A fast-spinning neutron star that emits radiation toward Earth every-time it rotates. [C95] (b) Neutron stars that spin rapidly and have strong magnetic fields, which produce electromagnetic radiation. (See Neutron Star) [LB90] (c) An object discovered at Cambridge University in 1967 which has the mass of a star and a radius no larger than that of Earth and which emits radio pulses with a very high degree of regularity (periods range from 0.03 s for the youngest to more than 3 s for the oldest). All pulsars are characterized by the general properties of dispersion, periodicity, and short duty cycle. Pulsars are believed to be rotating, magnetic (surface magnetic fields of 1010 to 1014 gauss are estimated) neutron stars which are the end products of supernovae. Type S pulsars have a simple pulse shape: Type C, complex: Type D have drifting subpulses. [H76]
Pulsating Nova	A variable star, probably not a true nova, in which the change between more and less luminous stages is extreme. (also called recurrent novae) [A84]
Pulsating Universe	Theory that the Universe constantly undergoes a Big Bang, expands, gradually slows and stops, contracts, and gradually accelerates once more to a Big Bang. Alternative theories include an ever-expanding Universe and the Steady-State Universe. (also know as Oscillating Universe) [A84]
Pulse Counter	When an atom is ionized by collision with a charged particle, the electrons it loses can be collected by applying a voltage. The process of collection gives an electrical pulse that is proportional to the number of free electrons, which in turn is proportional to the energy of the colliding particle. [H76]
Pulse Counting	Counting each individual photon as it comes off. [H76]
Pulse Width	The interval of time between two successive pulses. [H76]
Pulse Window	see Mean Profile. [H76]
Pumping (optical)	A process of raising matter from lower to higher energy levels. In order for a maser to work continuously, there must be some mechanism that replenishes the energy depleted by the emission and that provides population inversion. Such a mechanism is known as a pump. [H76]
Puppis A	A supernova remnant 104-105 years old, about 1-2 kpc distant. It is an extended nonthermal radio source, and also a source of soft X-rays (2U 0821-42). [H76]
Pup	An extremely bright O4f star (the brightest Of star known) embedded in the Gum Nebula. It has an envelope which is rapidly accelerating outward. [H76]
Pycnonuclear	An adjective used to describe nuclear processes (such as the proton-proton chain) that take place at relatively low temperatures and that are not strongly temperature-dependent. [H76]