An Etymological Dictionary of Astronomy and Astrophysics

A prefix meaning thick.

Etymology (EN): From Fr. pachy-, from L., from Gk. pachys “thick,” akin to Av. bazah “high, deep,” Baloci baz “thick, dense,” ON bingar “heap,” Latvian biezs “dense, thick.”

Etymology (PE): Bazâ-, from Av. bazah, Baloci baz, as above; cf. Waxi bâj “thickness,” variant dabz, → concentrated.

The difference between the isotopic mass of a nuclide and its mass number divided by its mass number. The packing fraction is a measure of the stability of the nucleus.

Etymology (EN): Packing, from the verb pack “to put together in a pack,” from the noun pack, from M.E. pak, packe, from M.D. pac or perhaps M.L.G. pak; → fraction.

Etymology (PE): Barxé, → fraction; anbâštegi quality noun of anbâštan, anbârdan “to fill, to replete;” Mid.Pers. hambāridan “to fill;” from Proto-Iranian *ham-par-, from prefix ham- + par- “to fill;” cf. Av. par- “to fill,”
parav-, pauru-, pouru- “full, much, many;”
O.Pers. paru- “much, many;” Mid.Pers. purr “full;” Mod.Pers. por “full, much, very;”
PIE base *pelu- “full,” from *pel- “to be full;” cf. Skt. puru- “much, abundant;”
Gk. polus “many,” plethos “great number, multitude;” O.E. full.

1. Something consisting of or regarded as having two parts or pieces joined together. → pair annihilation; → pair production.
   
2. Two identical, similar, or corresponding things.

Etymology (EN): M.E., from O.Fr. paire, from L. paria “equals,” neuter plural of par (genitive paris) “a pair, counterpart, equal.”

Etymology (PE): 1) Joft “pair, couple,” Lori, Laki jeft, Qâyeni jof, Tabari jeft, Mid.Pers. yuxt “pair, couple,” Av. yuxta- “a team of horses,” from yaog- “to yoke, harness, put to; to join, unite,” infinitive yuxta, Mid.Pers. jug, ayoxtan “to join, yoke,” Mod.Pers. yuq “yoke;” cf. Skt. yuga- “yoke,” Gk. zygon “yoke,” zeugnyanai “to join, unite,” L. jugare “to join,” from jugum “yoke,”
P.Gmc. *yukam, E. yoke; PIE *yeug- “to join.”
Hamâl, from ham- “together, with; same, equally, even” (Mid.Pers. ham-, like L. com- and Gk. syn- with neither of which it is cognate. O.Pers./Av. ham-, Skt. sam-; also O.Pers./Av. hama- “one and the same;” Skt. sama-; Gk. homos-; originally identical with PIE numeral *sam- “one,” from *som-. The Av. ham- appears in various forms: han- (before gutturals, palatals, dentals) and also hem-, hen-) + suffix -âl, → -al.

Mutual destruction of a particle and its antiparticle, such as an electron-positron pair, when they collide. The total energy of the two particles is converted into energy as gamma rays. It is the inverse process to → pair production.

See also: → pair; → annihilation

An instability arising from the → pair production inside a → massive star leading to energetic → supernova explosions. The pair instability occurs when, late in the star’s life, the core reaches a sufficiently high temperature after → carbon burning, a condition in which the pair production can take place. The pairs of electron and positron annihilate to form a neutrino and an anti-neutrino. Consequently, the pressure drops and the outer layers fall in onto the core. The temperature and pressure increase rapidly and eventually titanic nuclear burning causes an extraordinary explosion with energies higher than 10⁵¹ erg. See also → pair-instability supernova and → pulsational pair-instability supernova.

See also: → pair; → instability.

The creation of an → elementary particle and its → antiparticle from a → boson. For example, the formation of an → electron and a → positron in the interaction of high-energy → gamma ray photons, having at least 1.02 Mev, with an → atomic nucleus (γ → e^-

• e⁺). The → rest masses of the electron and positron being 0.51 MeV each, the excess energy will be carried away by these two particles. Pair production is the inverse process to → pair annihilation.

See also: → pair; → production

→ pair instability

See also: → pair; → instability.

A special type of → supernova that would result from the → pair instability in → supermassive stars with a mass range between 140 and 260 Msun in a low → metallicity environment. Such objects descended from the → Population III stars in the early history of the Universe. Such supernovae are the most powerful thermonuclear explosions in the Universe.
Pair-instability supernovae may have played an important role in the synthesis of → heavy elements. Moreover, the energetic feedback of the processed elements to their surroundings could have affected the structure and evolution of the early Universe (See, e.g., Fryer et al. 2001, ApJ 550, 372;
Heger & Woosley 2002, ApJ 567, 532). See also → pulsational pair-instability supernova.

See also: → pair; → instability; → supernova.

In nuclear physics, the extra binding energy associated with pairs of nucleons of the same kind. This quantity 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.

See also: Pairing, verbal noun of → pair; → energy.

The roof of the → mouth, separating the oral and nasal cavities. → hard palate; → soft palate.

Etymology (EN): M.E., from O.Fr. palat and directly from L. palatum “roof of the mouth.”

Etymology (PE): Kâm “roof of the mouth,” of unknown origin.

Lacking in color, not bright, weak in color or shade.

Etymology (EN): M.E., from O.Fr. paile “pale, light-colored,” from L. pallidus “pale, pallid, colorless,” from pallere “be pale, grow pale.”

Etymology (PE): Pâl, from p(ox) + âl, a combination of (Delijân) pox “pale” + (Kurd) âl “pale.”

A prefix meaning “old, ancient” especially in reference to former geologic time periods, e.g. → paleoclimatology, → paleolithic, and
→ paleomagnetism.

Etymology (EN): From Gk. palaio-, combining form of palaios “old, ancient,” from palai “long ago, far back,” from PIE root *kwel- “to turn, move about,” also “far”

Etymology (PE): Pârin, pâriné “ancient,” also “last year” (contraction of pâr sâl), related to pir “old;” Mid.Pers. pir “old, aged, ancient;” Av. parô (adv.) “before, before (of time), in front (of space);” cf. Skt. puáh, combining form of puras “before (of time and place), in front, in advance.”
Dirin, diriné “old, ancient, of times past,” from dir “old, antique; a long while” + suffix -in. The first component
from Mid.Pers. dêr, variants dagr, drâz “long”
(Mod.Pers. derâz “long,” variant Laki, Kurdi derež);
O.Pers. darga- “long;” Av. darəga-, darəγa- “long,” drājištəm “longest;” cf. Skt. dirghá- “long (in space and time);” L. longus “long;” Gk. dolikhos “elongated;” O.H.G., Ger. lang; Goth. laggs “long;” PIE base *dlonghos- “long.” The second component -in adj. suffix, from Mid.Pers. -ên (as in âhênên, zarrên), from Av. -aēna- (ayanhaēna- “made of iron,” zaranaēna- “golden”); cf. Skt. -ēna-.

The climate during a past geologic time or before historical records.

See also: → paleo-; → climate.

The study of past → climates throughout → geologic and → historic time (paleoclimates), and the causes of their variations.

See also: → paleo-; → climatology.

A period of → geologic time lasting about 42 million years, roughly from 65 to 23 million years ago. The Paleogene is most notable as being the time in which mammals evolved from relatively small, simple forms into a large group of diverse animals in the wake of the → Cretaceous-Paleogene extinction event that ended the preceding → Cretaceous period. Birds also evolved considerably during this period, changing into roughly modern forms.

See also: Literally “ancient birth,” from → paleo- + -gene, → gene.

Of, pertaining to, or characteristic of the cultures of the Old Stone Age, marked by the earliest known chipped stone tools. The period continued from about 750,000 years ago, until the beginning of the Mesolithic Age, about 15,000 years ago.

See also: → paleo-; lithic, from Gk. lithos “stone.”

The study of natural remanent magnetization in order to determine the intensity and direction of the Earth’s magnetic field in the geologic past.

See also: → paleo-; → magnetism.

The study of ancient life through → fossils.

Etymology (EN): From Fr. paléontologie, from paléo-, → paleo-, + onto-, from Gk. ont- “being,” pr.p. of einai “to be,” + → -logy.

Etymology (PE): Pârin-šenâsi, from pârin-, → paleo- + šenâsi, → -logy.

A technical scale that categorizes the → impact hazard of a → near-Earth object (NEO). It compares the threat of a given NEO to the so-called background threat of all NEOs of the same size or larger. In this way, the probability of the → impact itself as well as the time until the predicted impact are considered. The scale is → logarithmic and continuous. A Palermo scale of -2 indicates that the predicted event is only 1% as likely as a random background event between now and the time of predicted impact. A value of 0 indicates that the risk is the same as the risk from the background threats. A value of +2 indicates an event that is 100 times more likely than the background hazard. The Palermo scale is defined in the paper “Quantifying the risk posed by potential Earth impacts” by Chesley et al. (2002), Icarus 159n 423. See also → Torino scale.

See also: Named after Palermo, in recognition of the Palermo Observatory, where in 1801 the first and largest asteroid (→ Ceres) was discovered by the Italian astronomer Giuseppe Piazzi (1746-1826); → scale.

A silvery white metal which belongs to the → platinum group elements, symbol Pd. → Atomic weight 106.4, → atomic number 46, → melting point 1554.9 °C, → boiling point 2963 °C. It is used in alloys and as a catalyst.

See also: Named 1803 by discoverer William Hyde Wollaston (1766-1828),
after the asteroid → Pallas, which was discovered at about the same time.

A class of → iron meteorite containing → olivine crystals.

See also: Named after the German naturalist Peter Pallas (1741-1811), who first studied such a type of meteorites.

An observatory located atop Palomar Mountain about 65 km north-northeast of San Diego, California. It is a center of astronomical research owned and operated by the California Institute of Technology (Caltech). The Observatory is home to three active research telescopes: the 200-inch (5.1-meter) Hale Telescope, the 48-inch (1.25-meter) Samuel Oschin Telescope, and the 60-inch (1.5-meter) telescope. Research at Palomar Observatory is pursued by a broad community of astronomers from Caltech and other domestic and international partner institutions. The famous Hale Telescope proved instrumental in cosmological research. It was the largest instrument of its kind until 1976.

See also: Palomar, a mountain ridge in the Peninsular Ranges in northern San Diego County whose highest elevation is 1,871 m; → Observatory.

A photographic atlas of the northern hemisphere and a portion of the southern hemisphere created at Mount → Palomar Observatory in southern California. The original survey was completed in 1954 using the 48-in Schmidt (Oschin) Telescope. The square photographic plates were 35.5 cm (14-inch) on a side, each encompassing roughly 6 × 6 degrees of the sky. The survey was originally intended to cover the entire sky from +90 degrees declination down to -24 degrees (plate centers) in 879 regions, using both red and blue sensitive emulsions, and including stars to magnitude +22.

Ultimately the survey was extended to -30 degrees (both red and blue), an additional 57 regions. Finally, the Whiteoak Southern Extension was added in 1962 (red plates only), with another 100 plates which extended the set down to a declination of -42 degrees plate center.

See also: → Palomar Observatory; → sky; → survey.

The innermost of Saturn’s known satellites, orbiting within the Encke Division in the A Ring at a distance of 133,583 km. Also know as Saturn XVIII. It orbits Saturn every 0.575 days and its diameter is about 20 km. Pan was discovered in 1990 from Voyager photos taken in 1981.

See also: In Gk. mythology, Pan was the god of woods, fields, and flocks, having a human torso and head with a goat’s legs, horns, and ears.

A prefix meaning “all, whole,” used as a general formative (panorama; pantelegraph; pantheism; pantonality), and especially in terms implying the union of all branches of a group (Pan-Christian; Pan-Hellenic; Pan-Slavism).

Etymology (EN): From Gk. pan-, combining form of pas (neuter pan) “all, every,” of unknown origin.

Etymology (PE): Pân- loan from Gk., as above.
Sarâsar- “all, entirely, the whole,” literally “from beginning to end; from one end to the other,” from sar “head” + -â- epenthetic vowel + sar. The main word sar is related to soru, sorun “horn”
(karnâ “a trumpet-like wind instrument,” variant sornâ “a wind instrument”);
Mid.Pers. sar “head,” sru “horn;” Av. sarah- “head,” srū- “horn, nail;” cf. Skt. śiras- “head, chief;” Gk. kara “head,” karena “head, top,” keras “horn;”
L. cornu “horn,” cerebrum “brain;”
P.Gmc. *khurnaz (Ger. Horn, Du. horen; cognate with E. horn, as above, from PIE *ker- “head, horn;”
O.E. horn “horn of an animal,” also “wind instrument;”
E. horn); PIE base *ker- “head, horn, top, summit.”
Hamé-, → all.

A system for wide-field astronomical imaging developed and operated by the Institute for Astronomy at the University of Hawaii. Its goal is to survey the sky for moving or variable objects on a continual basis, and also produce accurate astrometry and photometry of already detected objects. It is situated at Haleakala Observatories near the summit of Haleakala in Hawaii. Pan-STARRS1 (PS1) was the first part of Pan-STARRS. The survey used a 1.8 meter telescope and a 1.4 Gigapixel camera to image the sky in five broadband filters (g, r, i, z, y). The PS1 consortium is made up of astronomers and engineers from 14 institutions and six countries. The survey was completed in April 2014. The Pan-STARRS Project is now focusing on building PS2.

See also: Short for Panoramic Survey Telescope and Rapid Response System.

A thin, flat cake of batter fried on both sides on a griddle or in a frying pan (Dictionary.com).

Etymology (EN): From M.E., from pan “a usually broad, shallow, and open container for domestic use, as for cooking;” O.E. panne + cake, from M.E., from O.N. kaka “cake,” cognate with M.E. kechel “little cake;” G. Kuchen;

Etymology (PE): Lavâš “a sort of thin flattened bread.”

A model of galaxy formation in which regions of primordial gas as massive as clusters of galaxies began to collapse into thin sheets (pancakes). Within the pancakes, smaller regions of gas later collapsed to form individual galaxies.

See also: → pancake; → model.

A star strongly compressed due to the → tidal force of a → massive black hole. The intense → gravity of the → black hole pulls harder on the nearest part of the star, creating an imbalance. When the star penetrates the → tidal radius, first it becomes cigar-shaped, then the squeezing of the tidal forces flattens the star in its orbital plane to the shape of a → pancake. Next the star rebounds, and as it leaves the tidal radius, it starts to expand. A little further on its orbit the star finally breaks up into gas fragments. This flattening would increase the → density and → temperature inside the star enough to trigger intense nuclear reactions that would tear it apart (Brassart & J.-P. Luminet, 2008, Astron. Astrophys. 481, 259).

See also: → pancake; → star.

One of the inner moons of Saturn and the outer shepherd moon for the F-ring. It was discovered in 1980 from Voyager 1 photos and is also known as Saturn XVII.

See also: In Gk. mythology Pandora was the very first woman who was formed out of clay by the gods.
She was bestowed upon humankind by Zeus as a punishment for Prometheus’ theft of fire. Entrusted with a box containing all the ills that could plague people, she opened it out of curiosity and thereby released all the evils of human life.

1. One of the divisions of a window or the like, consisting of a single plate of glass in a frame. A plate of glass for such a division.
   

2. A panel, as of a wainscot, ceiling, door, etc (Dictionary.com).

Etymology (EN): M.E. pane, pan “strip of cloth, section,” from M.Fr. pan,
from O.Fr. pan “section, piece, panel,” from L. pannum “piece of cloth, garment;” cf. Goth. fana “piece of cloth,” Gk. penos “web,” O.E. fanna “flag.”

Etymology (PE): Poš, from Baluci poc “cloth, clothing,” from puš-, pušidan “to cover, to wear,” → envelope.

1. A distinct portion, section, or division of a wall, wainscot, ceiling, door, shutter, fence, etc., especially of any surface sunk below or raised above the general level or enclosed by a frame or border.
   

2. A comparatively thin, flat piece of wood or the like, as a large piece of plywood.
   

3. A group of persons gathered to conduct a public discussion, judge a contest, serve as advisers, be players on a radio or television game, or the like (Dictionary.com).

Etymology (EN): M.E., from O.Fr. panel “a piece (of anything),” diminutive of pan “piece of cloth or the like,” ultimately from L. pannus “piece of cloth.” The sense of “a small group of people called on to discuss, judge, advice on a particular matter” is from 1570s.

Etymology (PE): Pošel, from poš, → pane, + -el, → -al.

Hypothetical super-continent that existed from about 300 to 200 million years ago. It has since broken up and the fragments have drifted to become the configuration of Earth’s present-day continents.

Etymology (EN): From Gk. → pan- “all” + gaia, variant ge “earth, land, ground, soil.”

Etymology (PE): Pânzam, from → pan- + zam, variants
zamin, zami “earth, ground,” from Mid.Pers. zamig “earth;”
Av. zam- “the earth;” cf. Skt. ksam; Gk. khthôn, khamai “on the ground;” L. homo “earthly being” and humus “the earth” (as in homo sapiens or homicide, humble, humus, exhume);
PIE root *dh(e)ghom “earth.”
Pânžé, loan from Fr.

The hypothesis that life exists and is distributed throughout the Universe in the form of “seeds” that develop in the right environment. The oldest record of this idea may be traced back to the ancient Greek philospher Anaxagoras, who lived in the fifth century B.C.

Etymology (EN): N.L., from Gk. panspermia “mixture of all seeds,” from → pan- + -sperm, a combining form of sperma “seed” + -ia a noun suffix.

Etymology (PE): Pân-dâne-vari, sarâsar-dâne-vari, from pân-, sarâsar-, → pan-, + dâné “seed, grain” (Mid.Pers. dân, dânag “seed, corn,” Av. dânô- in dânô.karš- “carrying grains; an ant,” Skt. dhânâ- “corn, grain,” Tokharian B tâno “grain,” cf. Lith. duona “corn, bread”)

• -var suffix of possession, variant -ur (Mid.Pers. -uwar, -war; from O.Pers. -bara, from bar- “to bear, carry”)
• -i noun suffix.

1. The doctrine that God is the transcendent reality of which the material universe and human beings are only manifestations: it involves a denial of God’s personality and expresses a tendency to identify God and nature.
   

2. Any religious belief or philosophical doctrine that identifies God with the universe (Dictionary.com).

See also: → pan-; → theism.

1. A thin sheet made from fibrous material (wood pulp, rags, straw, etc.) suitable for writing or printing on.
   

2. A written or printed document or the like.

Etymology (EN): From M.E. papire, from L. papyrus “paper,” from Gk. papyros “any plant of the paper plant genus,” may be of Egyptian origin.

Etymology (PE): Kâqaz “paper,” probably a transliteration of old Chinese gu zhi; cf. Sogd. kāγaδā “paper,” Skt. kakali, kakari, Marathi kagad, Tamil kagidam, Malayalam kayitam (Y. Kumar, 2005, A History of Sino-Indian Relations).

1. A prefix occurring in loanwords from Gk. with the meanings “beside, alongside of, by, beyond.”
   
2. Chemistry: A combining form designating the para (1, 4) position in the benzene ring.
   Short for → parahydrogen.

Etymology (EN): From Gk. para-, from para (preposition) “beside, near, from, against, contrary to,” cognate with Av. parā, as below;
L. pro “before, for, in favor of,” per- “through;” Goth. faur “along;” O.E. for- “off, away.”

Etymology (PE): Pârâ-, parâ-, from O.Pers. parā (adv.) “along; forth;” Av. parā (adv.) “at first, in the first place; in former times, formerly;” also “away, aside;” cf. Skt. pur&#257: “before, formerly;” cognate with Gk. para, as above.

The → water molecule in which the → nuclear spins of the constituent → hydrogen atoms are → antiparallel (→ parahydrogen). See also: → ortho-water.

See also: → para-; → water.

A plain curve obtained by slicing a cone with a plane parallel to one side of the cone. A parabola can be considered an ellipse with an infinite major axis. It is one of the types of conic sections.

Etymology (EN): N.L., from Gk. parabole “comparison, application,” literally “a throwing beside,” from → para- + bole “throwing,” related to ballein “to throw.”

Etymology (PE): Sahmi, of unknown origin.

Having the form of a parabola.

See also: of or pertaining to → parabola.

An antenna comprising a parabolic reflector with a receiving and/or transmitting element positioned at or near its focal point.

See also: → parabolic; → antenna.

A concave mirror that has the form of a paraboloid of revolution.

See also: → parabolic; → mirror.

An orbit whose overall shape is like a parabola; it is the limiting case between an elliptical orbit (eccentricity less than 1) and a hyperbolic orbit (eccentricity larger than 1). The speed necessary to form a parabolic orbit is known as the escape velocity.

See also: → parabolic; → orbit.

The speed necessary to form a parabolic orbit around a gravitational center. It is also the minimum speed necessary to escape from the gravitational pull of a body.

See also: → parabolic; → velocity.

A solid formed by the revolution of a parabola about its axis.

See also: From parabol(a), → parabola, + → -oid.

1. General: A typical example of something.
   

2. Grammar: A set of word forms giving all of the possible inflections (declensions, conjugations) of a word.
   

3. Philosophy of science: A framework of concepts, results, and procedures within which subsequent work is carried out, as defined by the influential Thomas Kuhn (1922-1996) in his classic The Structure of Scientific Revolutions (1962). “Normal science” proceeds within such a framework or paradigm. When a paradigm has to be changed (→ paradigm shift),
   there comes about a crisis and then scientific revolution. For example, Newton’s mechanical paradigm
   was replaced by the paradigm of Einstein’s relativistic universe.
   Each paradigm is an interpretation of the world, rather than an objective explanation.
   For criticism of Kuhn’s views, see, e.g., Stanford Encyclopedia of Philosophy.
   

4. Term now occurring frequently in every kind of discourse, usually to mean something like “way of thinking” or “approach to a problem.”

Etymology (EN): L.L. paradigma “pattern, example,” especially in grammar, from Gk. paradeigma “pattern, model,” from paradeiknynai “to exhibit, show side by side,” from → para- “beside” + deiknynai “to show,” related to
dokein “to appear, seem, think,” cognate with Av. daēs- “to show” (Mod.Pers. andišé, andiš “thought, think,” see below); Skt. diś- “to show, point out,” diśati “he shows;” L. dicere “to utter;” PIE base *deik- “to show, to pronounce.”

Etymology (PE): Parâdiš, pârâdiš, from parâ-, pârâ, → para- + diš “to show,” as in andiš, andišidan “to → think” (related to dis, disé, → form);

Sogd. andiš “to seem,” andêš “to show,” andêšik “appearing;” ultimately from
Av. daēs- “to show,” s-aorist forms dāiš-, dôiš-, diš-, akin to Gk. deiknynai “to show,” as above, dike “manner, custom;” Skt. diś- “to show, point out;” L. dicere “to utter, say;” O.H.G. zeigon, Ger. zeigen “to show;” O.E. teon “to accuse,” tæcan “to teach.”

1. Philosophy of science: A process of revolutionary change in scientific → paradigms, whereby established scientific ideas are replaced by new ones. For instance, Copernicus’ evidence that the Earth revolved around the Sun caused a paradigm shift in astronomy.
   

2. The idea of paradigm shift has been transferred from the sciences to other areas of society and culture, referring to a fundamental reorganization of how people think about an entire topic.

See also: → paradigm; → shift; → change.

A statement, proposition, or situation that seems self-contradictory or absurd but in reality is or may be true. → Fermi paradox; → faint early Sun paradox; → twins paradox; → paradox of youth.

Etymology (EN): From L. paradoxum “contrary to expectation,” from Gk. paradoxon, from neuter of adj. paradoxos “contrary to common opinion, unbelievable,” from → para- “contrary to” + dox(a) “opinion, belief” + -os adj. suffix. The main component dox, from dokein “to appear, seem, think,” is cognate with Av. daēs- “to show;” Skt. diś- “to show, point out,” diśati “he shows;” L. dicere “to utter;” PIE base *deik- “to show, pronounce solemnly.”

Etymology (PE): Pârâdaxš (on the model of Gk. paradoxos), from pârâ-, → para-, + daxš, from Av. daxš- “to reveal, instruct, point out,” fradaxštar- “teacher,” *daxšārə “revelations;” Mod.Pers. daxš “task, effort;” cf. Skt. daks- “to be able,” dáksa- “able, expert.”

The observed presence of young stars in the immediate vicinity of the → supermassive black hole (SMBH), → Sgr A*, residing in the center of our Galaxy. The stellar population within 1 pc of the SMBH contains a variety of young and → massive stars orbiting the SMBH. Some of them are only about 20 Myr old and get as close as a few light-days to the SMBH, while from 0.1 to 0.4 pc even younger stars are found with ages of 3-7 Myr. The presence of these stars so near to the SMBH is a paradox. Their → in situ formation should be almost impossible,
since the environment is too hostile for these stars to form. Indeed the strong → tidal influence of the SMBH should hamper their formation. On the other hand, the scenario considering their → migration from other places does not seem to be adequate. The time required for the migration
from > 1 pc by dynamical friction would exceed their inferred ages unless the migration rate were somehow accelerated. This apparent contradiction was termed “paradox of youth” by Ghez et al. (2003, ApJ 586, L127). See also Genzel et al. (2010, Rev.Mod.Phys. 82, 3121, also at astro-ph/1006.0064).

See also: Youth, the condition of being → young; → paradox.

Molecular hydrogen in which the nuclei (protons) of the two hydrogen atoms contained in the molecule have spins in opposite directions. → orthohydrogen

See also: → para-; + → hydrogen.

Of or pertaining to a parallax.

See also: Adj. form of → parallax.

Of an object in the sky, the angle between the → celestial pole, the object, and the → zenith. Since parallactic angle
describes the orientation on the sky of the object for a particular observer, it can be an important quantity in some observations.

See also: → parallactic; → angle.

The path on the sky of the apparent position of a star as seen from the Earth, due to the Earth’s annual motion around the Sun.

See also: → parallactic; → ellipse.

An irregularity in the Moon’s motion caused by the Sun’s gravitational attraction, which sets the Moon ahead or behind its normal orbital position. The Moon is about 2 arcminutes ahead of its expected position at first quarter, and a similar amount behind at last quarter.

See also: → parallactic; → inequality.

The proper motion of a star due to the effect of the Sun’s motion relative to the → local standard of rest.

See also: → parallactic; → motion.

The apparent → shift of a nearby object’s → position in relation to more distant ones when the nearby object is observed from different → viewing angles. See also → stellar parallax.

Etymology (EN): From M.Fr. parallaxe, from Gk. parallaxis “change, alteration,” from parallassein “to alter, make things alternate,” from → para- “beside” + allassein “to change,” from allos “other;” → alias.

Etymology (PE): Didgašt, literally “view change,” from did “sight, view; eye,” from didan “to see” (Mid.Pers. ditan “to see, regard, catch sight of, contemplate, experience;” O.Pers. dī- “to see;” Av. dā(y)- “to see,” didāti “sees;” cf.
Skt. dhī- “to perceive, think, ponder; thought, reflection, meditation,” dādhye; Gk. dedorka “have seen”) + gašt “change, alteration,” past stem of gaštan, gardidan “to turn, to change” (Mid.Pers. vartitan; Av. varət- “to turn, revolve;” Skt. vrt- “to turn, roll,” vartate “it turns round, rolls;” L. vertere “to turn;” O.H.G. werden “to become;” PIE base *wer- “to turn, bend”).

The angular displacement associated with → parallax.

See also: → parallax; → angle.

1. Said of two or more things, such as lines or planes, that are equally distant from one another at all points.
   
2. Electricity: An arrangement of the components in an electric circuit so that the same voltage is applied to each component. Compare → series.
   

Etymology (EN): From M.Fr. parallèle, from L. parallelus, from Gk. parallelos “parallel,” from para allelois “beside one another,” from → para- “beside” + allelois “each other,” from allos “other,” → alias.

Etymology (PE): Parâsu, from parâ-, → para-, + su “direction, side,” from Mid.Pers. sôk “direction, side.”

The → moment of inertia of a body about any given axis is the moment of inertia about a parallel axis through the center of mass, plus the moment of inertia about the given axis if the mass were located at the center of mass. same as → Steiner’s theorem.

See also: → parallel; → axis; → theorem.

A small circle on the celestial sphere whose plane is parallel to the celestial horizon. Same as → almucantar.

Etymology (EN): → parallel; → altitude.

Etymology (PE): → almucantar.

A solid figure whose six bases are → parallelograms, opposite pairs being identical and parallel.

Etymology (EN): From Gk. parallelepipedon, from parallelos, → parallel + epipedon “plane surface,” from neuter of epipedos “flat,” from → epi- + pedon “ground,” cognate with L. ped-, pes, → foot.

Etymology (PE): Parâsuruyé, from parâsu, → parallel, + ruyé, → surface. Lowzivâr, from lowzi, → rhombus + -vâr, → -oid.

A four-sided → polygon whose opposite sides are parallel. A parallelogram all of whose angles are right angles is a → rectangle.

Etymology (EN): From Fr. parallélogramme, from L. parallelogrammum, from Gk. parallelogrammon “bounded by parallel lines,” from parallelos, → parallel, + gramme “line,” related to graphein “to write, draw” → -graph.

Etymology (PE): Parâsubar, from parâsu, → parallel, + bar, → side.

A paramagnetic substance, which possesses → paramagnetism.

See also: → para- + → magnet.

Relative to or characterized by → paramagnetism.

See also: → para- + → magnetic

The property of a substance that possesses a → magnetic permeability greater than that of a vacuum but significantly less than that exhibited by
→ ferromagnetism. In the absence of an external magnetic field the atomic → magnetic moments of the substance are randomly oriented and thus cancel each other out with no net total magnetic moment. Moreover the coupling between neighboring moments is weak.
However, when a magnetic field is applied magnetic moments align with the direction of the field and so the magnetic moments add together. Therefore paramagnetic substances affect external fields in a positive way, by attraction to the field resulting in a local increase in the magnetic field.
The → magnetization vanishes when the field is removed.

See also: → para- + → magnetism

General: Any of a set of physical properties whose values determine the characteristics or behavior of something. → impact parameter; → ionization parameter.
Math.: A constant or variable term in a function that determines the specific form of the function but not its general nature, as a in f(x) = kx, where k determines only the slope of the line described by f(x).

Etymology (EN): Mod.L. parametrum, from Gk. → para- + metron “measure,” → meter.

Etymology (PE): Parâmun, from parâ-, → para-,

• mun/mân “measure,” as in Pers. terms pirâmun “perimeter,” âzmun “test, trial,”
  peymân “measuring, agreement,” peymâné “a measure; a cup, bowl,”
  from O.Pers./Av. mā(y)- “to measure;” cf. Skt. mati “measures,” matra- “measure;” Gk. metron “measure;” L. metrum; PIE base *me- “to measure.”

Any of a set of equations that defines the coordinates of the dependent variables
of a curve or surface in terms of one or more independent variables or parameters.

See also: → parametric; → equation.

An → ESO observatory, located on Cerro Paranal in the Atacama Desert, northern Chile, at 2,635 m altitude. It is about 120 km south of the town of Antofagasta and 12 km inland from the Pacific Coast. The Paranal Observatory hosts the
→ Very Large Telescope (VLT) with four 8.2 m telescopes. Each telescope provides one → Cassegrain and two → Nasmyth focus stations for facility instruments. One Nasmyth focus is available for visitor instruments. In addition each telescope is equipped with a → coudé focus station from which the light can be coherently combined in the interferometric focus. ESO also operates four 1.8 m Auxiliary Telescopes (ATs), that are used as an
interferometric array (VISA) and a 4 m infrared survey telescope (VISTA).
Currently, more than 10 instruments including two
interferometric instruments (MIDI, AMBER) are operational and offered for science observations.

See also: Paranal, the name of the mountain, in the Quechua language meaning “whirlwind;” → observatory.

An astronomical and meteorological calendar written by ancient Greeks from 450 B.C. during at least three centuries. The parapegma was an inscribed stone for public use. It had holes beside the inscription, in which a peg could be inserted next to the appropriate day. The term was later applied to purely written forms of such calendars, or almanacs; plural form parapegmata.

See also: From → para- “next to” + pegma “something that fastens something else together,” from pegnyein “to stick.”

Characteristic of optical analyses that are limited to infinitesimally small apertures. Also called first-order or Gaussian optics.

Etymology (EN): → para-; → axial.

Etymology (PE): From par(â)-, → para-, + âseyi→ axial.

A ray that lies close to and almost parallel to the optical axis and behaves according to paraxial equations.

See also: → paraxial, → ray.

A catalog of stars in the → Orion Nebula created by P. P. Parenago in 1954 (Publ. Astr. Inst. Sternberg, Band 25, p. 393-437, Moskau).

See also: Pavel Petrovich Parenago (1906-1960), a Soviet astronomer.

1. General: A father or a mother. A precursor, or
   → progenitor.
   

2. Physics: A → nuclide that upon → radioactive decay or → disintegration yields a specific nuclide (the → daughter). See also → parent cloud, → parent element, → parent galaxy, → parent molecule.

Etymology (EN): From O.Fr. parent, from L. parentem (nominative parens) “father or mother, ancestor,” from parere
“to bring forth, give birth to, produce,” from PIE base *per- “to bring forth”

Etymology (PE): Permâr, literally “father-mother” (as in Sogd. māt-pitri “parent”),
from Gilaki per, → father, + Gilaki mâr, → mother.

Usually of a newborn star, the molecular cloud in which the star has formed.

See also: → parent; → cloud.

A radioactive element that spontaneously decays into a new substance. The product of this decay is known as a “daughter” element.

See also: → parent; → element.

Of a high redshift supernova, the galaxy in which the event has occurred.

See also: → parent; → galaxy.

The molecule initially produced when a comet nucleus sublimates, soon changed to different daughter molecules because of solar radiation.

See also: → parent; → molecule.

An atmospheric optical phenomenon, seen as a bright spot sometimes appearing at either side of the → Sun, often on a luminous ring or → halo and at the same angular elevation as the Sun. Parhelia are caused by the → refraction and → reflection of → sunlight by → ice crystals suspended in the Earth’s → atmosphere. Also called mock Sun or sundog.

Etymology (EN): From Gk. parhelion “a mock Sun,” from → para- “beside” + helios “sun,” → helio-.

Etymology (PE): Parâhur, from parâ-, → para-,

• hur “sun,” → helio-.

1. General: Equality, as in amount, status, or character; equivalence; correspondence; similarity; analogy. Opposite of disparity.
   

2. Physics: The principle of space-inversion invariance.
   An operation that reverses the algebraic sign of the coordinate axes used to describe a system, i.e. (x, y, z) → (-x, -y, -z). The parity principle is important in quantum mechanics because the wave functions which represent particles can behave in different ways upon transformation of the coordinate system.
   The parity is 1 (or even) if the wave function of the system is unchanged by an inversion of the coordinate system; it is -1 (or odd) if the wave function is changed
   only in sign. Parity is conserved in strong interactions, but not in weak ones.
   

3. Math.: The attribute, of an integer, of being even or odd. Thus, it can be said that 8 and 12 have the same parity, whereas 5 and 16 have opposite parity.
   

See also: → charge-parity symmetry, → even parity, → parity conservation, → parity symmetry, → parity violation.

Etymology (EN): From M.Fr. parité, from L.L. paritas “equality,” from L. adj. par “equal.”

Etymology (PE): Hamâli, quality noun of hamâl, → pair (equivalent 2).

In quantum mechanics, the condition of parity in strong and electrodynamic interactions, where it remains constant and does not change with time. In other words, parity conservation implies that Nature is symmetrical and makes no distinction between right- and left-handed rotations or between opposite sides of a subatomic particle. Thus, for example, two similar radioactive particles spinning in opposite directions about a vertical axis should emit their decay products with the same intensity upward and downward. Same as → parity symmetry.

See also: → parity; → conservation.

The invariance of physical laws under a transformation that changes the sign of the space coordinates. Parity symmetry is sometimes called mirror symmetry. It is known that the parity symmetry is violated in some weak interactions, while it is well preserved in all other three interactions (gravitational, electromagnetic, strong). Same as → P-symmetry and → parity conservation.

See also: → parity; → symmetry.

In quantum mechanics, the condition of → parity in the → weak interaction. For example, the emitted → beta particles
in → radioactive decay of → cobalt-60 nuclei are not equally distributed between the two poles of cobalt-60. More beta particles emerge from one pole than the other, and it would be possible to distinguish the mirror image nuclei from their counterparts.

See also: → parity; → violation.

A type of instability found in some astrophysical phenomena involving → magnetic fields; it arises if a gas layer is supported by the horizontal magnetic fields against → gravity. Also called → magnetic buoyancy. Briefly, this instability works as follows. Consider a uniform disk of gas which is coupled to a magnetici field that is parallel to the disk. Suppose that the disk is gravitationally stratified in the vertical direction, and is in dynamical equilibrium under the balance of gravity and pressure (thermal and magnetic). Now consider a small perturbation which causes the field lines to rise in certain parts of the disk and sink in others. Because of gravity, the gas loaded onto the field lines tends to slide off the peaks and and sink into the valleys. The increase of mass loads in the valleys makes them sink further, while the magnetic pressure causes the peaks to rise as their mass load decreases. Consequently, the initial perturbation is amplified, causing the production of density fluctuations in an initially uniform disk. The characteristic scale for the Parker instability is ~4πH, where H is the scale height of the diffuse component of the disk. For the Milky Way, where H ~ 150 pc, this scale is about 1-2 kpc. Numerical simulations show that the density contrast generated by the Parker instability is generally of order unity before the instability saturates. This implies that the Parker instability on its own may not be able to drive collapse on large scales. Nevertheless, it may trigger gravitational instability in a marginally unstable disk and/or induce strong motions in the medium, thereby acting as a source of turbulence on large scales (see, e.g., Houjun Mo, Frank van den Bosch, Simon White, 2010, Galaxy Formation and Evolution, The University Press, Cambridge, UK).

See also: First studied by E. N. Parker, 1966, ApJ 145, 811; → instability.

A NASA space mission launched on August 12, 2018 to study the outer corona of the Sun at very close distances. Parker Solar Probe is the first space mission to penetrate into solar corona as close as about 10 solar radii. It will approach the Sun to this distance 25 times. Approaching the Sun to such distances is a big technological challenge.

The main goals of the mission are to answer these questions: Why is the solar corona so hotter than the solar surface? How is the solar wind accelerated? How are the energetic particles produced and transported?

See also: Named after the physicist Eugene Newman Parker (1927-), who proposed the existence of the solar wind and did pioneering work for its interpretation.

A catalog of 397 radio sources between declinations +20° and +27° which were compiled from a finding survey made at 635 MHz with the 64m radio telescope at the Australian National Radio Astronomy Observatory, Parkes, N.S.W. and published in 1968. Originally abbreviated PSR, this catalog, which is also called PKS,
replaces and improves on four earlier lists (1964 to 1966).

See also: Parkes, proper noun; selected, p.p. of → select; → region.

A basic unit of astronomical distances, corresponding to a → trigonometric parallax of one second of arc. In other words, it is the distance at which one → astronomical unit (the mean radius of the Earth’s orbit) subtends an angle of 1 arcsecond.
1 pc = 3.2616 → light-years = 206 265 astronomical units = 30.857 x 10¹² km.

See also: From parallax + second.

An optical phenomenon resulting from the refraction and reflection of moonlight within ice crystals in cirrus cloud; also known as paraselene,
mock Moon or moondog. It is the lunar counterpart of the → parhelion.

Etymology (EN): From Gk. para- “beside,” → para-, + selene “moon,” from Gk. selene “moon,” related to selas “light, brightness, flame.”

Etymology (PE): Pârâ-, → para-, + mâh, → moon.

A theorem relating the → Fourier coefficients to the function that they describe. It states that:

(1/L) ∫ |f(x)|²dx (integrated from x₀ to x₀ + L) = (a₀/2)² + (1/2) Σ (a_r² + b_r²) (summed from r = 1 to ∞).

In other words, the sum of the moduli squared of the complex Fourier coefficients is equal to the average value of |f(x)|² over one period.

See also: Named after Marc-Antoine Parseval (1755-1836), French mathematician; → theorem.

1. A portion or division of a whole that is separate or distinct; piece, fragment, fraction, or section; constituent.
   

2. A section or division of a literary work (Dictionary.com).

Etymology (EN): M.E., from O.Fr. part “share, portion; character; dominion; side, path,” from L. partem (nominative pars) “a part, piece, a share, a division; a party or faction,” related to portio “share, portion,” from PIE root *per- “to assign, allot;” cf. Pers. pâr, pâré “piece, part, portion, fragment;” as below.

Etymology (PE): Pâr, variant pâré “piece, part, portion,” parré “portion, segment (of an orange),” pargâlé, “piece, portion; patch;” (dialects Kermâni pariké “portion, half;” Tabari perik “minute quantity, particle;” Lârestâni pakva “patch;” Borujerdi parru “patch”);
Mid.Pers. pârag “piece, part, portion; gift, offering, bribe;” Av. pāra- “debt,” from par- “to remunerate, equalize; to condemn;” PIE *per- “to sell, hand over, distribute; to assign;”
Gk. peprotai “it has been granted;” L. pars, as above; Skt. purti- “reward;” Hitt. pars-, parsiya- “to break, crumble.”

Being such in part only; not total or general; incomplete.
See also:
→ partial derivative, → partial differential equation, → partial eclipse, → partial ionization zone, → partial lunar eclipse, → partial solar eclipse, → partial truth.

Etymology (EN): M.E. parcial, from O.Fr. parcial, from M.L. partialis “pertaining to a part,” from L. pars, → part; → -al.

Etymology (PE): → part; → -al.

The derivative of a function of two or more variables, e.g., z = f(x,y), with respect to one of the variables, the others being considered constants (denoted ∂z / ∂x).

See also: → partial; → derivative.

A type of differential equation involving an unknown function (or functions) of several independent variables and its (or their) partial derivatives with respect to those variables.

See also: → partial; → differential; → equation.

An eclipse that is not total. → partial lunar eclipse, → partial solar eclipse.

See also: → partial; → eclipse.

One of several zones of the stellar interior where increased → opacity can provide the → kappa mechanism to drive → pulsations. See also → Kramers’ law. In these zones where the gases are partially ionized, part of the energy released during a layer’s compression can be used for further ionization, rather than raising the temperature of the gas. Partial ionization zones modulate the flow of energy through the layers of the star and are the direct cause of → stellar pulsation. The partial ionization zones were first identified by the Russian astronomer Sergei A. Zhevakin (1916-2001) in the 1950s. In most stars there are two main ionization zones. The hydrogen partial ionization zone where both the ionization of neutral hydrogen (H ↔ H⁺ + e^-) and the first ionization of helium (He ↔ He⁺ + e^-) occurs in layers with a characteristic temperature of 1.5 x 10⁴ K. The second, deeper zone is called the He⁺ partial ionization zone, and involves the second ionization of helium (He⁺↔ He⁺⁺ + e^-), which occurs deeper at a characteristic temperature of 4 x 10⁴ K. The location of these ionization zones within the star determines its pulsational properties. In fact if the → effective temperature of the star is ≥ 7500 K, the pulsation is not active, because the ionization zones will be located very near to the surface. In this region the density is quite low and there is not enough mass available to drive the oscillations. This explains the blue (hot) edge of the instability strip on the → H-R diagram. Otherwise if a star’s surface temperature is too low, ≤ 5500 K, the onset of efficient convection in its outer layers may dampen the oscillations. The red (cool) edge of the instability strip is believed to be the result of the damping effect of convection. He⁺ ionization is the driving agent in → Cepheids. See also → gamma mechanism.

See also: → partial; → ionization; → zone.

A → lunar eclipse when the Earth’s → umbra passes over only part of the Moon, causing only moderate darkening of the full Moon. See also → penumbral lunar eclipse.

See also: → partial; → lunar; → eclipse.

A → solar eclipse when only the → penumbra of the Moon touches the Earth.
The → umbra passes either just above the North Pole or just below the South Pole, missing the Earth.

See also: → partial; → solar; → eclipse.

A → truth value in → fuzzy logic where it can range between “completely true” and “completely false.”

See also: → partial; → truth.

To take part, be or become actively involved.

Etymology (EN): From L. paticipatus p.p. of partcipare “to share,” from particeps “partaking, sharing,” from part-, pars “part,” → partial, + capere “to take,” → concept.

Etymology (PE): Pârgertidan, from pâr “part,” → partial, + gertidan “to take,” → concept.

An act or instance of participating. The fact of taking part.

See also: Verbal noun of → participate.

A lexical form derived from a verb, that has some of the characteristics and functions of both verbs and adjectives. In most Indo-European languages participles are used to express participation in an action (present participle) or relate to a completed action (past participle). They can also appear in attributive form as adjectives.

Etymology (EN): M.E., from O.Fr. participle, variant of participe, from L. participium, literally “a sharing, partaking,” from particeps “sharing, partaking,” → participate.

Etymology (PE): Pârgerté, from pârgert present stem of pârgertidan, → participate, + nuance suffix -é.

1. A unit of → matter smaller than the → atom or its main components.
   The term particle also includes any (currently hypothetical) new particles that might be discovered, such as the supersymmetric partners of the → quarks and → leptons and → bosons.
   

2. Mechanics: A material body (a body possessing mass) the size of which can be neglected in investigating its motion.
   

3. Grammar: In some languages, a word that has a grammatical function but does not fit into the main parts of speech (i.e. → noun, → verb, → adverb, → preposition). For example, in English, off, on, out, and up in call off, pass on, rule out, and grow up, respectively.
   

See also:
→ alpha particle, → antiparticle, → astroparticle physics, → beta particle, → charged particle, → elementary particle, → energetic solar particle, → exchange particle, → Lagrangian particle, → nanoparticle, → particle horizon, → particle nature, → particle physics, → relativistic particle, → resonance particle, → sink particle, → Smoothed Particle Hydrodynamics, → strange particle, → subatomic particle, → test particle, → virtual particle, → wave-particle duality.

Etymology (EN): From L. particula “little bit or part,” diminutive of pars (genitive partis), from PIE base *per- “to assign, allot;” cf. Mid.Pers. pârag “gift, offering, bribe;” Mod.Pers. pâreh “gift” (→ partial); Gk. porein “to provide, give, grant,” peprotai “it has been granted;” Skt. purtá- “gift, pay, reward.”

Etymology (PE): Zarré, from Ar. dharrat “particle.”
Pârul, from pâr, → part, + -ul, → -ule.

For an observer at a given epoch t₀, the boundary between the observable and the unobservable regions of the → Universe. Therefore, the distance to the particle horizon at t₀ defines the size of the → observable Universe. Same as → cosmic horizon.

See also: → particle; → horizon.

A general term to describe → light involving the following phenomena: → reflection, → refraction, and → photoelectric effect. Compare → wave nature.

See also: → particle; → nature.

The branch of physics that deals with the smallest known structures of matter and energy
in order to understand the fundamental particles and forces of nature.

See also: → particle; → physics.

1. (adj.) Of or pertaining to a single or specific person, thing, group, class, occasion, etc., rather than to others or all; special rather than → general.
   

2a) (n.) An individual or distinct part, as an item of a list or enumeration.

2b) (n.) Logic: An individual or a specific group within a general class (Dictionary.com).

Etymology (EN): M.E., from O.Fr. particuler and directly from L.L. particularis “of a part, concerning a small part,” from L. particula, → particle, + -ar, → -al.

Etymology (PE): Pâruli, adj. from pârul, → particle; pârulé, from pârul + nuance adj. -é.

An → astrolabe that serves only a limited number of → latitudes.

See also: → particular; → astrolabe.

Of partial differential equations, the solution which can be obtained from the general solution by particular choice of the arbitrary functions. → general solution; → singular solution.

See also: → particular; → solution.

1. Of or relating to minute separate → particles.
   

2. A particulate → substance.

Etymology (EN): From Mod. L. particulatus, from particula, → particle.

Etymology (PE): From pârul, → particle, + suffix -é or -mand.

Meteorology: A complex → mixture of → microscopic → particles and → liquid droplets suspended in the → atmosphere, especially pollutants.

See also: → particulate; → matter.

1a) A division into or distribution in portions or shares.

1b) A separation; something that separates or divides; a part, division, or section.

2a) The act or process of dividing something into parts. The state of being so divided.

2b) Math.: → integer partition; → set partition.

2c) Computers: → disk partition.

3. To divide into parts or portions. To divide or separate by a partition.

Etymology (EN): M.E., from O.Fr. particion, from L. partition- “division, partition, distribution,” from p.p. stem of partire “to part, divide,” from PIE root *per- “to grant, allot,” related to → part and akin to pake, as below.

Etymology (PE): Parke, from Kermâni parke “piece, part, fragmant,” related to pâr “→ part;” parkidan, infinitive from parke; parkeš, verbal noun of parkidan.

See → integer partition.

See also: → partition; → function.

The ratio of the → concentration of a → solute in a single definite form in the stationary phase to its concentration in the same form in the other phase at equilibrium.

See also: → partition; → ratio.

A chemical separation process whereby a solute is distributed between two phases.

See also: → partition; → -ing.

The act or fact of being partitioned.

See also: → partition; → -ment.

In particle physics, a constituent of the hadron originally postulated in the theoretical analysis of high-energy scattering of particles off hadrons. In modern usage, the term parton is often used to mean a quark or a gluon.

See also: Coined by the American physicist Richard Feynman (1918-1988), from part, from → particle + → -ion

A fraction of a whole number in units of 1/1000,000.
It is usually used to describe chemical concentrations, very small amounts of pollutants in air, water, body fluids, and uncertainty. For example 30 ppm is 3 x 10^-5 or 0.003%.

See also: → part; → per; → million.

The → SI unit of → pressure, that of one → newton per → square → meter. Since 1 Pa is a small pressure, hPa (→ hectopascals) are more widely used. 1 Pa = 10 dyn cm^-2, = 1.02 x 10^-5 kgf cm^-2 = 10^-5 bars = 9.87 x 10^-6 atm = 7.50 x 10^-3 torr (mm Hg).

See also: In honor of Blaise Pascal (1623-1662), French mathematician, physicist, and religious philosopher for his contribution in the study of hydrodynamics and hydrostatics, in particular establishing the principle of the barometer.

An experiment carried out by Blaise Pascal in 1646 to demonstrate the hydraulic pressure. A long and narrow vertical pipe was connected to the content of a closed wooden barrel already full of water. He poured a small quantity of water into the pipe, whereby the height of the fluid within the pipe sharply increased. Due to the increase in hydrostatic pressure and → Pascal’s law, the barrel could leak and even burst.

See also: → pascal (Pa); M.E. barel, from M.Fr. baril, O.Fr. barril; → experiment

A change in the pressure of an enclosed incompressible fluid is conveyed undiminished to every part of the fluid and to the surfaces of its container.

See also: Named after Blaise Pascal (1623-1662), French mathematician, physicist, and religious philosopher for his contribution in the study of hydrodynamics and hydrostatics, in particular establishing the principle of the barometer.

An array of numbers in the shape of a triangle, having a 1 at the top and also at the ends of each row. Each number is obtained by summing the two adjacent numbers to it in the preceding row. Each row is a set of → binomial coefficients. In the expansion of (x + y)ⁿ, the coefficients of x and y are given by the n-th row of Pascal’s traingle.

See also: → pascal; → triangle.

The spectral series associated with the third energy level of the hydrogen atom. The series lies in the infrared, with Pα at 18,751 Å, and
Paschen limit at 8204 Å.

See also: In honor of Friedrich Paschen (1865-1947), German physicist; → series.

An effect on spectral lines obtained when the light source is located in a strong magnetic field. The strong field disrupts the coupling between the orbital and spin angular momenta, resulting in a different pattern of splitting.

See also: Named for the German physicists Friedrich Paschen (1865-1947) and Ernst E. A. Back (1881-1959); → effect.

The fifteenth of Jupiter’s known satellites, orbiting at 23,660,000 km from Jupiter; also known as Jupiter VIII. Its diameter is 36 km and orbital period 744 days.

See also: In Gk. mythology, Pasiphae was the wife of Minos and mother, by a white bull, of the Minotaur.

An act or instance of passing from one place, condition, etc., to another; transit. Same as → transit.

Etymology (EN): M.E, from O.Fr. passage, from passer “to go by;” originally “a road, passage.”

Etymology (PE): Gozar “passage, transit, passing,” from gozaštan “to pass, cross, transit,” variant gozâštan “to put, to place, let, allow;” Mid.Pers. widardan, widâštan “to pass, to let pass (by);” O.Pers. vitar- “to pass across,” viyatarayam “I put across;” Av. vi-tar- “to pass across,” from vi- “apart, away from” (O.Pers. viy- “apart, away;” Av. vi- “apart, away;” cf. Skt. vi- “apart, asunder, away, out;” L. vitare “to avoid, turn aside”) + O.Pers./Av. tar- “to cross over;” → trans-.

The range of wavelengths that are transmitted by a filter. Same as → band-pass.

Etymology (EN): Pass from O.Fr. passer, from V.L. *passare “to step, walk, pass,” from L. passus “step, pace;” cf. Pers. pâ “foot,” pey “step;” → band.

Etymology (PE): Gozar “passage, transit, passing,” from gozaštan “to pass, cross, transit,” variant gozâštan “to put, to place, let, allow;” Mid.Pers. widardan, widâštan “to pass, to let pass (by);” O.Pers. vitar- “to pass across,” viyatarayam “I put across;” Av. vi-tar- “to pass across,” from vi- “apart, away from” ( O.Pers. viy- “apart, away;” Av. vi- “apart, away;” cf. Skt. vi- “apart, asunder, away, out;” L. vitare “to avoid, turn aside”) + O.Pers./Av. tar- “to cross over”); bând, → band.

Tending not to participate actively; not working or operating.

Etymology (EN): From L. passivus “submissive; capable of feeling or suffering,” from passus, p.p. of pati “to experience, undergo, suffer.”

Etymology (PE): Akâr “not working, not doing,” from → a- negation prefix + kâr “work,” varaint of kar, present stem of kardan “to do, to make” (Mid.Pers. kardan; O.Pers./Av. kar- “to do, make, build;” Av. kərənaoiti “he makes;” cf. Skt. kr- “to do, to make,” krnoti “he makes, he does,” karoti “he makes, he does,” karma “act, deed;” PIE base k^wer- “to do, to make”).

A cumulus cloud that is no longer dynamically connected with the atmospheric boundary layer via updrafts or downdrafts.

See also: → passive; → cloud.

An electronic component which contains no source of power, in contrast to active components.

See also: → passive; → component.

A galaxy lacking optical emission-line activity (e.g., [O II], Hα, [O III]) and showing only stellar absorption lines (e.g., the 4000 Å → calcium break, Mg I, Na I). Also called passively evolving galaxy.

See also: → passive; → galaxy.

An electronic system which emits no energy, and is not detectable.

See also: → passive; → system.

Same as → passive galaxy.

See also: → passive; → evolve; → galaxy.

An arbitrary string of characters chosen by a user or system administrator and used to authenticate the user when he attempts to log on, in order to prevent unauthorised access to his account (foldoc.org).

See also: Pass, → passage; → word.

Of, having existed in, or having occurred during a time previous to the present; bygone (Dictionary.com).

Etymology (EN): M.E.; variant spelling of passed, p.p. of pass, from O.Fr. passer, from V.L. *passare “to step, walk, pass,” from L. passus “step, pace;” cf. Pers. pâ “foot,” pey “step,” → foot.

Etymology (PE): Gozašté, p.p. of gozaštan “to pass,” → passage.

A → participle that indicates a completed action or state.

See also: → past; → participle.

1. A small piece of material used to mend a tear or break, to cover a hole, or to strengthen a weak place.
   

   2. A piece of material used to cover or protect a wound, an injured part, etc.
      

   3. A small piece, scrap, or area of anything (Dictionary.com). → bald patch; → patchy turbulence.

Etymology (EN): M.E. pacche; of uncertain origin.

Etymology (PE): Pac, from Bašgaridi (Giroft) pac “patch; fragment,” probably from *parka-, from *par- “piece” + diminutive/relation suffix -ka; cf. pâré “piece, part, portion;” parré “portion, segment (of an orange);” pargâlé “piece, portion; patch;” Kermâni pariké “portion, half;” Tabari perik “minute quantity, particle;” Lârestâni pakva “patch;” Borujerdi parru “patch;” (Fin Bandar Abbâs) park “patch;” (Tâti) pajina “patch, darn;” → part.

A → turbulence that is not continuous in space, but is separated by regions of stability and → laminar flow.

See also: Patchy adj. of → patch; → turbulence.

A shallow dish-like crater with irregular, sometimes scalloped rims, on
the surface of a planet.

See also: From L. patera “abroad, shallow dish” (used for drinking, primarily in a ritual context).

1. A route, course, or track along which something moves.
   

2. A course of action, conduct, or procedure.
   

3. Math.: A continuous curve that connects two or more points.
   

4. In → graph theory, a → graph whose → vertices can be arranged in a sequence.

Etymology (EN): O.E. paþ, pæþ; cf. O.Fris. path; M.Du. pat; Du. pad;
O.H.G. pfad; Ger. Pfad “path.”

Etymology (PE): Pah “path, way,”
from O.Pers. paθi- “path, way;” Av. paθ-, variants paθi-, paθā-, pantay- (with conversion of -θ- to -h-, as in ciθra-/cehr, xšaθra-/šahr/šâh, vərəθraγna-/bahrâm, → Mars);
Mid/Mod.Pers. pand “path, advice, counsel;” Khotanese pande “road, path;” Ossetic fœndœg “path, road;”
cf. Skt. pánthā- “road, path, course;” Gk. patos “path, way,” pontos “sea;” L. pons “bridge, path;” P.Gmc. *finthanan “to find;” E. find; PIE base *pent- “to go, to tread.”
Râh, → way.

The path followed by an individual particle of fluid over an interval of time. It contrasts with the → streamline that represents an instantaneous picture of the motion of particles.

See also: → path; → line.

The path (up to 320 km wide) that the Moon’s shadow traces on the Earth during a total solar eclipse.

See also: → path; → totality.

1. A person who makes or finds a way, especially through unexplored areas or fields of knowledge.
   

2. An airplane, or a person dropped from a plane, sent into a target area to illuminate the area for succeeding aircraft (Dictionary.com).
   

3. → LISA pathfinder.

See also: → path; → finder.

1. A regular or repetitive form, order, or arrangement; a distinctive form. → diffraction pattern; → interference pattern.
   

2. In the → density wave model, a region of enhanced stellar density (arm or bar) in the → Galactic disk.
   → pattern speed.

Etymology (EN): M.E. patron, from O.Fr. patron, from M.L. patronus
“patron saint, lord, master, model, pattern,” from pater “father.”

Etymology (PE): Olgu “pattern,” from Turkish ülgü “form, mold, model.”

In models of → galactic structure, the → angular velocity of a → spiral arm or a → bar, assumed to be a pattern rotating as a solid body in the galaxy. Pattern speed, Ω_P , is one of the most important parameters of the → density wave theory responsible for the spiral arms. Pattern speed determines the location of the → corotation resonance.

See also: → pattern; → speed.

A quantum mechanical principle according to which no two identical
→ fermions can share the same → quantum state. Also known as → exclusion principle.

See also: In honor of Wolfgang Pauli (1900-1958), Austrian theoretical physicist, who formulated the principle in 1925; → exclusion; → principle.

The Peacock. A → constellation in the → southern hemisphere, at about 19h 30m → right ascension, 65° south → declination. Abbreviation: Pav; genitive: Pavonis.

Etymology (EN): From L. pavo “peacock;” paupulo “peacock’s sound;” Gk. taos “peacock;”

Etymology (PE): Tâvus, from Ar., ultimately from Gk. taos “peacock.”