An Etymological Dictionary of Astronomy and Astrophysics

A → logarithmic measure of → hydrogen ion concentration, originally defined

pH = log₁₀ (1/[H⁺]),

where [H⁺] is the concentration of hydrogen ions in → moles per liter of solution.

The hydrogen ion concentration in pure water around room temperature is about 1.0 × 10^-7 moles. Therefore, a pH of 7 is considered “neutral,” because the concentration of hydrogen ions is exactly equal to the concentration of → hydroxide (OH^-) ions produced by → dissociation of the → water. Increasing the concentration of hydrogen ions above 1.0 × 10^-7 moles produces a solution with a pH of less than 7, and the solution is considered → acidic. Decreasing the concentration below 1.0 × 10^-7 moles produces a solution with a pH above 7, and the solution is considered → alkaline or → basic. The neutral pH is different for each → solvent. For example, the concentration of hydrogen ions in pure ethanol is about 1.58 &times 10^-10 moles, so ethanol is neutral at pH 9.8. A solution with a pH of 8 would be considered acidic in ethanol, but basic in water.

See also: From Ger. PH, introduced by Danish biochemist S.P.L. Sørensen (1868-1939)
in 1909, from P, for Ger. Potenz “power, potency,” and H, symbol of → hydrogen.

A → logarithmic measure of → hydrogen ion concentration, originally defined

pH = log₁₀ (1/[H⁺]),

where [H⁺] is the concentration of hydrogen ions in → moles per liter of solution.

The hydrogen ion concentration in pure water around room temperature is about 1.0 × 10^-7 moles. Therefore, a pH of 7 is considered “neutral,” because the concentration of hydrogen ions is exactly equal to the concentration of → hydroxide (OH^-) ions produced by → dissociation of the → water. Increasing the concentration of hydrogen ions above 1.0 × 10^-7 moles produces a solution with a pH of less than 7, and the solution is considered → acidic. Decreasing the concentration below 1.0 × 10^-7 moles produces a solution with a pH above 7, and the solution is considered → alkaline or → basic. The neutral pH is different for each → solvent. For example, the concentration of hydrogen ions in pure ethanol is about 1.58 &times 10^-10 moles, so ethanol is neutral at pH 9.8. A solution with a pH of 8 would be considered acidic in ethanol, but basic in water.

See also: From Ger. PH, introduced by Danish biochemist S.P.L. Sørensen (1868-1939)
in 1909, from P, for Ger. Potenz “power, potency,” and H, symbol of → hydrogen.

A blue, → main sequence star of → apparent visual magnitude 2.44 and
→ spectral type A0 Ve located in → Ursa Major. Other designations: Phecda; Phekda; Phegda; Phekha; Phacd.

Etymology (EN): Phad, from Ar. al-Fakhidh (ad-Dubb) (الفخذ‌الدب) “the thigh (of the Bear)”.

Etymology (PE): Faxez, from Ar., as above.

A blue, → main sequence star of → apparent visual magnitude 2.44 and
→ spectral type A0 Ve located in → Ursa Major. Other designations: Phecda; Phekda; Phegda; Phekha; Phacd.

Etymology (EN): Phad, from Ar. al-Fakhidh (ad-Dubb) (الفخذ‌الدب) “the thigh (of the Bear)”.

Etymology (PE): Faxez, from Ar., as above.

A hypothetical → planet which once was postulated to have existed between the orbits of → Mars and → Jupiter and its destruction supposedly led to the formation of the → asteroid belt. The idea of such a hypothetical planet was first put forward by the German astronomer Heinrich Wilhelm Olbers (1758-1840).

See also: In Greek mythology Phaeton was the sun god Helios. Phaeton tried to drive his father’s solar chariot but crashed after almost setting fire to the whole earth.

A hypothetical → planet which once was postulated to have existed between the orbits of → Mars and → Jupiter and its destruction supposedly led to the formation of the → asteroid belt. The idea of such a hypothetical planet was first put forward by the German astronomer Heinrich Wilhelm Olbers (1758-1840).

See also: In Greek mythology Phaeton was the sun god Helios. Phaeton tried to drive his father’s solar chariot but crashed after almost setting fire to the whole earth.

The tube or cavity, with its surrounding membrane and muscles, that connects the mouth and nasal passages with the esophagus (Dictionary.com). → throat.

Etymology (EN): From Gk pharynx (genitive pharyngos) “windpipe, throat.”

Etymology (PE): Halq, loan from Ar.

The tube or cavity, with its surrounding membrane and muscles, that connects the mouth and nasal passages with the esophagus (Dictionary.com). → throat.

Etymology (EN): From Gk pharynx (genitive pharyngos) “windpipe, throat.”

Etymology (PE): Halq, loan from Ar.

1. A particular stage or point in a course, development,
   or graph varying cyclically; the fractional part of the period through which the time has advanced, measured from some arbitrary origin. Phase is measured like an angle, when a complete cycle is equivalent to a phase of 360° (or 2π radians), or, sometimes, as a number between 0 and 1. Two or more waves of the same frequency are → in phase when their maxima and minima take place at the same moments. Otherwise, they are said to be → out of phase or that they have a → phase difference.
   

2. A state in which matter can exist, depending on temperature and pressure, e.g. the → solid, → liquid, → gaseous, and → plasma states.
   

3. A recurring form of the → Moon or a → planet seen in the sky. → lunar phase, → phases of Venus.
   

4. In a → binary star system, → orbital phase.

Etymology (EN): Mod.L. phases, plural of phasis, from Gk. phasis “appearance,” from stem of phainein “to show, to make appear.”

Etymology (PE): 1) Fâz, loanword from Fr., as above.

2. Simâ “face, aspect, resemblance.”

1. A particular stage or point in a course, development,
   or graph varying cyclically; the fractional part of the period through which the time has advanced, measured from some arbitrary origin. Phase is measured like an angle, when a complete cycle is equivalent to a phase of 360° (or 2π radians), or, sometimes, as a number between 0 and 1. Two or more waves of the same frequency are → in phase when their maxima and minima take place at the same moments. Otherwise, they are said to be → out of phase or that they have a → phase difference.
   

2. A state in which matter can exist, depending on temperature and pressure, e.g. the → solid, → liquid, → gaseous, and → plasma states.
   

3. A recurring form of the → Moon or a → planet seen in the sky. → lunar phase, → phases of Venus.
   

4. In a → binary star system, → orbital phase.

Etymology (EN): Mod.L. phases, plural of phasis, from Gk. phasis “appearance,” from stem of phainein “to show, to make appear.”

Etymology (PE): 1) Fâz, loanword from Fr., as above.

2. Simâ “face, aspect, resemblance.”

1. Physics: Of a → periodic wave, the number of suitable units of angular measure between a point on the wave and a reference point.
   

2. Astro.: For an object in the solar system, the angle “Sun-object-Earth” that is, the angle between the Sun and the observer as seen from the given object. It is 0° when the object is fully illuminated, 90° when the object is half-illuminated (like the Moon at first quarter and last quarter), and 180° when the object is between Earth and the Sun.
   

3. More generally, the angle between star light incident onto a related revolving object and the light reflected from the object to the observer (Earth).

See also: → phase; → angle.

1. Physics: Of a → periodic wave, the number of suitable units of angular measure between a point on the wave and a reference point.
   

2. Astro.: For an object in the solar system, the angle “Sun-object-Earth” that is, the angle between the Sun and the observer as seen from the given object. It is 0° when the object is fully illuminated, 90° when the object is half-illuminated (like the Moon at first quarter and last quarter), and 180° when the object is between Earth and the Sun.
   

3. More generally, the angle between star light incident onto a related revolving object and the light reflected from the object to the observer (Earth).

See also: → phase; → angle.

1. Astro.: A curve describing the → brightness of a reflecting → natural satellite as a function of its → phase angle.
   

2. Math.: A plot of the solution to a set of equations of motion in a phase space as a function of time.

See also: → phase; → curve.

1. Astro.: A curve describing the → brightness of a reflecting → natural satellite as a function of its → phase angle.
   

2. Math.: A plot of the solution to a set of equations of motion in a phase space as a function of time.

See also: → phase; → curve.

The ratio of the phase shift of a sinusoidal signal in transmission through a system to the frequency of the signal.

See also: → phase; → delay.

The ratio of the phase shift of a sinusoidal signal in transmission through a system to the frequency of the signal.

See also: → phase; → delay.

A graph showing the equilibrium relationships between phases (such as vapor-liquid, liquid-solid) of a chemical compound, mixture of compounds, or solution.

See also: → phase; → diagram.

A graph showing the equilibrium relationships between phases (such as vapor-liquid, liquid-solid) of a chemical compound, mixture of compounds, or solution.

See also: → phase; → diagram.

The difference of phase (usually expressed as a time or an angle) between two periodic quantities which vary sinusoidally and have the same frequency.

See also: → phase; → difference.

The difference of phase (usually expressed as a time or an angle) between two periodic quantities which vary sinusoidally and have the same frequency.

See also: → phase; → difference.

The condition of temperature and pressure under which different phases (e.g. gas, liquid, and solid) of a substance coexist.

See also: → phase; → equilibrium.

The condition of temperature and pressure under which different phases (e.g. gas, liquid, and solid) of a substance coexist.

See also: → phase; → equilibrium.

The variation in brightness of a target as the phase angle (the angle between Sun and observer as seen from the target) varies between 0° and 180°. The directional distribution of reflected (or scattered) radiation. The phase angle is the supplement of the scattering angle (the angle between the incident ray and the emerging ray); in other words, the sum of the phase angle and the scattering angle is always 180° (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → phase; → function.

The variation in brightness of a target as the phase angle (the angle between Sun and observer as seen from the target) varies between 0° and 180°. The directional distribution of reflected (or scattered) radiation. The phase angle is the supplement of the scattering angle (the angle between the incident ray and the emerging ray); in other words, the sum of the phase angle and the scattering angle is always 180° (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → phase; → function.

1. General: Same as → phase difference.
   

2. Cepheids: The observed phase difference between luminosity and velocity in classical (radially pulsating) → Cepheids. On the basis of adiabatic pulsation theory, one would expect the maximum luminosity to occur when the radius of the star is minimal. This means that the maximum outward velocity would be one quarter period out of phase with the maximum velocity. However, in the observations the maximum luminosity and maximum outward velocity are nearly in phase. This effect is due to the → kappa mechanism which is responsible for driving the → pulsations. The pulsations in Cepheids
   are excited by the helium → partial ionization zone, He⁺↔ He⁺⁺, which is located below the He ↔ He⁺ and H ↔ H⁺ zones. These latter two regions are too shallow to contribute significantly to the driving of the fundamental modes of Cepheids; so their only effect is to introduce a phase shift.

Etymology (EN): → phase; lag, possibly from a Scandinavian source; cf. Norw. lagga “go slowly.”

Etymology (PE): Degarsâni, → difference; fâz→ phase.

1. General: Same as → phase difference.
   

2. Cepheids: The observed phase difference between luminosity and velocity in classical (radially pulsating) → Cepheids. On the basis of adiabatic pulsation theory, one would expect the maximum luminosity to occur when the radius of the star is minimal. This means that the maximum outward velocity would be one quarter period out of phase with the maximum velocity. However, in the observations the maximum luminosity and maximum outward velocity are nearly in phase. This effect is due to the → kappa mechanism which is responsible for driving the → pulsations. The pulsations in Cepheids
   are excited by the helium → partial ionization zone, He⁺↔ He⁺⁺, which is located below the He ↔ He⁺ and H ↔ H⁺ zones. These latter two regions are too shallow to contribute significantly to the driving of the fundamental modes of Cepheids; so their only effect is to introduce a phase shift.

Etymology (EN): → phase; lag, possibly from a Scandinavian source; cf. Norw. lagga “go slowly.”

Etymology (PE): Degarsâni, → difference; fâz→ phase.

In electronics, a technique of adjusting the phase of an oscillator signal so that it will follow the phase of a reference signal.

Etymology (EN): → phase; lock, from O.E. loc “bolt, fastening, enclosure;” cf. O.N. lok “fastening, lock,” Goth. usluks “opening,” O.H.G. loh “dungeon,” Ger. Loch “opening, hole,” Du. luck “shutter, trapdoor.”

Etymology (PE): Fâz, → phase; bast “fastening, lock,” from bastan, from Mid.Pers. bastan/vastan “to bind, shut,” Av./O.Pers. band- “to bind, fetter,” banda- “band, tie,” Skt. bandh- “to bind, tie, fasten,” PIE *bhendh- “to bind,” cf. Ger. binden, E. bind, → band.

In electronics, a technique of adjusting the phase of an oscillator signal so that it will follow the phase of a reference signal.

Etymology (EN): → phase; lock, from O.E. loc “bolt, fastening, enclosure;” cf. O.N. lok “fastening, lock,” Goth. usluks “opening,” O.H.G. loh “dungeon,” Ger. Loch “opening, hole,” Du. luck “shutter, trapdoor.”

Etymology (PE): Fâz, → phase; bast “fastening, lock,” from bastan, from Mid.Pers. bastan/vastan “to bind, shut,” Av./O.Pers. band- “to bind, fetter,” banda- “band, tie,” Skt. bandh- “to bind, tie, fasten,” PIE *bhendh- “to bind,” cf. Ger. binden, E. bind, → band.

Modulation in which the phase angle of a sine-wave carrier is caused to depart from the carrier angle by an amount proportional to the instantaneous magnitude of the modulating wave.

See also: → phase; → modulation.

Modulation in which the phase angle of a sine-wave carrier is caused to depart from the carrier angle by an amount proportional to the instantaneous magnitude of the modulating wave.

See also: → phase; → modulation.

An angular shift in phase by 180°.

See also: → phase; → reversal.

An angular shift in phase by 180°.

See also: → phase; → reversal.

Any change in the phase of a periodic quantity or in the phase difference between two or more periodic quantities.

See also: → phase; → shift.

Any change in the phase of a periodic quantity or in the phase difference between two or more periodic quantities.

See also: → phase; → shift.

Of a dynamical system, a six-dimensional space consisting of the
set of values that the position and velocity can take together (x, y, z, v_x, v_y, v_z). → velocity space.

See also: → phase; → space.

Of a dynamical system, a six-dimensional space consisting of the
set of values that the position and velocity can take together (x, y, z, v_x, v_y, v_z). → velocity space.

See also: → phase; → space.

A measure of the relative phase in the image as function of frequency. It is the phase component of the → optical transfer function. A relative phase change of 180°, for example, results in an image with the black and white areas reversed.

See also: → phase; → transfer;
→ function.

A measure of the relative phase in the image as function of frequency. It is the phase component of the → optical transfer function. A relative phase change of 180°, for example, results in an image with the black and white areas reversed.

See also: → phase; → transfer;
→ function.

The changing of a substance from one phase to another, by → freezing, → melting, → boiling, → condensation, or → sublimation. Also known as phase transformation.
A well known phase transition is the transition from → water to → ice. Phase transitions are often associated with → symmetry breaking. In water there is a complete symmetry under rotations with no preferred direction. Ice has a crystal structure, in which certain orientations in space are preferred. Therefore, in transition from water to ice the continuous rotational symmetry is lost.

See also: → phase; → transition.

The changing of a substance from one phase to another, by → freezing, → melting, → boiling, → condensation, or → sublimation. Also known as phase transformation.
A well known phase transition is the transition from → water to → ice. Phase transitions are often associated with → symmetry breaking. In water there is a complete symmetry under rotations with no preferred direction. Ice has a crystal structure, in which certain orientations in space are preferred. Therefore, in transition from water to ice the continuous rotational symmetry is lost.

See also: → phase; → transition.

The speed at which any fixed phase (individual wave) in a → wave packet travels. It is expressed as v_ph = ω/k, where ω is the → angular frequency and k the → wave number. See also the → group velocity.

See also: → phase; → velocity.

The speed at which any fixed phase (individual wave) in a → wave packet travels. It is expressed as v_ph = ω/k, where ω is the → angular frequency and k the → wave number. See also the → group velocity.

See also: → phase; → velocity.

→ Lunar phase.

See also: → phase; → Moon.

→ Lunar phase.

See also: → phase; → Moon.

The gradual variation of the apparent shape of → Venus between a small, full → disk and a larger → crescent. The first telescopic observation of the phases of Venus by Galileo (1610) proved the → Ptolemaic system could not be correct. The reason is that with the → geocentric system the phases of Venus would be impossible. More specifically, in that model Venus lies always between Earth and Sun. Hence its fully bright surface would always be toward the Sun; so Venus could not be seen in full phase from Earth. Only slim crescents would be possible. On the other hand, this phenomenon could not prove the → heliocentric system, because it could equally be explained with the → Tychonic model.

See also: → phase; → Venus.

The gradual variation of the apparent shape of → Venus between a small, full → disk and a larger → crescent. The first telescopic observation of the phases of Venus by Galileo (1610) proved the → Ptolemaic system could not be correct. The reason is that with the → geocentric system the phases of Venus would be impossible. More specifically, in that model Venus lies always between Earth and Sun. Hence its fully bright surface would always be toward the Sun; so Venus could not be seen in full phase from Earth. Only slim crescents would be possible. On the other hand, this phenomenon could not prove the → heliocentric system, because it could equally be explained with the → Tychonic model.

See also: → phase; → Venus.

The study of the biological recurring phenomena in plants and animals (such as blossoming, hibernation, reproduction, and migration) and of their relation to changes in season and climate.

Etymology (EN): From L. phaeno-, from Gk. phaino-, from phainein “bring to light, cause to appear, show,” from PIE root *bha- “to shine”

• → -logy.

Etymology (PE): Zistcarxe-šenâsi, literally study of “life cycle,” from zistcarxe “life cycle,” from zist, → bio-, + carxe, → cycle, + -šenâsi, → -logy.

The study of the biological recurring phenomena in plants and animals (such as blossoming, hibernation, reproduction, and migration) and of their relation to changes in season and climate.

Etymology (EN): From L. phaeno-, from Gk. phaino-, from phainein “bring to light, cause to appear, show,” from PIE root *bha- “to shine”

• → -logy.

Etymology (PE): Zistcarxe-šenâsi, literally study of “life cycle,” from zistcarxe “life cycle,” from zist, → bio-, + carxe, → cycle, + -šenâsi, → -logy.

1. An occurrence, circumstance, or fact, in matter or spirit, which can be perceived by human senses. → physical phenomenon.
   

2. Philosophy: For Kant, a thing as it is apprehended by the human senses as distinguished from a noumenon, or thing-in-itself.

Etymology (EN): From L.L. phænomenon, from Gk. phainomenon “that which appears or is seen,” from phainesthai “to appear,” passive of phainein “to bring to light; to show,” from PIE base *bhhā- “to shine;” cf. Skt. bhāati “shines, glitters;” Av. bā- “to shine, appear, seem,” bāmya- “light, luminous, bright,” bānu- “light, ray;” Mid.Pers. bâm “beam of light, splendor,” bâmik “brilliant,”
bâmdâd “morning, dawn.”

Etymology (PE): Padidé, noun from padid “manifest, evident, conspicuous, in sight,” variant padidâr, from Mid.Pers. pad didâr “visible,” from pad “to, at, for, in,” evolved to bé “to; for; in; on; with; by” in Mod.Pers. (O.Pers. paity; Av. paiti “to, toward, in, at;” cf. Skt. práti, Gk. poti)

• did past stem of didan “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”).

1. An occurrence, circumstance, or fact, in matter or spirit, which can be perceived by human senses. → physical phenomenon.
   

2. Philosophy: For Kant, a thing as it is apprehended by the human senses as distinguished from a noumenon, or thing-in-itself.

Etymology (EN): From L.L. phænomenon, from Gk. phainomenon “that which appears or is seen,” from phainesthai “to appear,” passive of phainein “to bring to light; to show,” from PIE base *bhhā- “to shine;” cf. Skt. bhāati “shines, glitters;” Av. bā- “to shine, appear, seem,” bāmya- “light, luminous, bright,” bānu- “light, ray;” Mid.Pers. bâm “beam of light, splendor,” bâmik “brilliant,”
bâmdâd “morning, dawn.”

Etymology (PE): Padidé, noun from padid “manifest, evident, conspicuous, in sight,” variant padidâr, from Mid.Pers. pad didâr “visible,” from pad “to, at, for, in,” evolved to bé “to; for; in; on; with; by” in Mod.Pers. (O.Pers. paity; Av. paiti “to, toward, in, at;” cf. Skt. práti, Gk. poti)

• did past stem of didan “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”).

A blue → giant star in the constellation → Ursa Minor, also known as HR 5735, HD 137422, HIP 75097, BD+72°79, and SAO 8220. It has an → apparent visual magnitude of +3.0, → color indices of B -V = +0.09, U - B = +0.08, and a → spectral type of A2 III. Pherkad has a → luminosity of 1,100 Lsun, a radius of 15 Rsun, and a → surface temperature of 8,200 K. It lies 487 → light-years away from Earth.

See also: From Ar. Al-Farqad (الفرقد) “calf.”

A blue → giant star in the constellation → Ursa Minor, also known as HR 5735, HD 137422, HIP 75097, BD+72°79, and SAO 8220. It has an → apparent visual magnitude of +3.0, → color indices of B -V = +0.09, U - B = +0.08, and a → spectral type of A2 III. Pherkad has a → luminosity of 1,100 Lsun, a radius of 15 Rsun, and a → surface temperature of 8,200 K. It lies 487 → light-years away from Earth.

See also: From Ar. Al-Farqad (الفرقد) “calf.”

A correlation between the peak brightness of → Type Ia supernovae and the decline rate of their → light curve (15 days after the maximum). The decline rate is also correlated to the width of the peak brightness of the supernova. The brightest events are the broadest in time and brighter SNe Ia decline more slowly than dimmer ones.
Applying the Phillips relation reduces the dispersion in the light curves of Type Ia SNe thus making them precise distance indicators which can be observed over large distances.

See also: Named after Mark M. Phillips (1951-), American astronomer (Phillips et al. 1993, ApJ 413, L105); → relation.

A correlation between the peak brightness of → Type Ia supernovae and the decline rate of their → light curve (15 days after the maximum). The decline rate is also correlated to the width of the peak brightness of the supernova. The brightest events are the broadest in time and brighter SNe Ia decline more slowly than dimmer ones.
Applying the Phillips relation reduces the dispersion in the light curves of Type Ia SNe thus making them precise distance indicators which can be observed over large distances.

See also: Named after Mark M. Phillips (1951-), American astronomer (Phillips et al. 1993, ApJ 413, L105); → relation.

A person who engages in → philosophy.

Etymology (EN): M.E., from O.E. philosophe, from L. philosophus “philosopher,” from Gk. philosophos “philosopher, sage,” literally “lover of wisdom,” → philosophy; the agent noun ending -er appears in early 14th century from an Anglo-French or O.Fr. variant of philosophe.

Etymology (PE): Filsuf, from Ar., from Gk., as above. Falsafedân, literally “philosophy knower,” with -dân present stem of dânestan “to know,” → science.

A person who engages in → philosophy.

Etymology (EN): M.E., from O.E. philosophe, from L. philosophus “philosopher,” from Gk. philosophos “philosopher, sage,” literally “lover of wisdom,” → philosophy; the agent noun ending -er appears in early 14th century from an Anglo-French or O.Fr. variant of philosophe.

Etymology (PE): Filsuf, from Ar., from Gk., as above. Falsafedân, literally “philosophy knower,” with -dân present stem of dânestan “to know,” → science.

To explain or argue in terms of philosophical speculations or theories.

See also: → philosophy; → -ize.

To explain or argue in terms of philosophical speculations or theories.

See also: → philosophy; → -ize.

A conceptual study that attempts to understand reality and answer fundamental questions about knowledge, existence, life, morality, and human nature. Philosophy deals with issues that generally are not subject to investigation through experimental verification.
It focuses on questions which cannot be answered by means of observation alone. See also → philosophy of science.

Etymology (EN): From O.Fr. filosofie “philosophy, knowledge,” from L. philosophia, from Gk. philosophia “love of wisdom,” from philo- “loving” combining form of philos “dear; friend,” from philein  “to love,” of unknown origin, +  sophia “knowledge, wisdom,” from sophis “wise, learned;” of unknown origin.

Etymology (PE): Falsafé, from Ar. falsafah, loan from Gk. philosophia, as above.

A conceptual study that attempts to understand reality and answer fundamental questions about knowledge, existence, life, morality, and human nature. Philosophy deals with issues that generally are not subject to investigation through experimental verification.
It focuses on questions which cannot be answered by means of observation alone. See also → philosophy of science.

Etymology (EN): From O.Fr. filosofie “philosophy, knowledge,” from L. philosophia, from Gk. philosophia “love of wisdom,” from philo- “loving” combining form of philos “dear; friend,” from philein  “to love,” of unknown origin, +  sophia “knowledge, wisdom,” from sophis “wise, learned;” of unknown origin.

Etymology (PE): Falsafé, from Ar. falsafah, loan from Gk. philosophia, as above.

The critical study of the basic principles and concepts of a particular branch of  knowledge. The philosophy of science is particularly concerned with the nature
of scientific facts, the structure of scientific statements, and relations between them.

See also: → philosophy; → science.

The critical study of the basic principles and concepts of a particular branch of  knowledge. The philosophy of science is particularly concerned with the nature
of scientific facts, the structure of scientific statements, and relations between them.

See also: → philosophy; → science.

A hypothetical substance that, prior to the discovery of → oxygen, was thought to be released during → combustion. → phlogiston theory.

Etymology (EN): From New Latin, from Gk. phlogiston, neuter of phlogistos “inflammable, burnt up,” from phlogizein “to set on fire, burn,” from phlox “flame, blaze;” from PIE root *bhel- “to shine, burn.”

Etymology (PE): Fložiston, loan from Fr, as above.

A hypothetical substance that, prior to the discovery of → oxygen, was thought to be released during → combustion. → phlogiston theory.

Etymology (EN): From New Latin, from Gk. phlogiston, neuter of phlogistos “inflammable, burnt up,” from phlogizein “to set on fire, burn,” from phlox “flame, blaze;” from PIE root *bhel- “to shine, burn.”

Etymology (PE): Fložiston, loan from Fr, as above.

An obsolete theory of combustion in which all flammable objects were supposed to contain a substance called → phlogiston, which was released when the object burned. The existence of this hypothetical substance was proposed in 1669 by Johann Becher, who called it terra pinguis “fat earth.” For example, as wood burns it releases phlogiston into the air, leaving ash behind. Ash was therefore wood minus phlogiston. In the early 18th century Georg Stahl renamed the substance phlogiston. The theory was disproved by Antoine Lavoisier in 1783, who proved the principle of conservation of mass, refuted the phlogiston theory
and proposed the oxygen theory of burning.

See also: → phlogiston; → theory.

An obsolete theory of combustion in which all flammable objects were supposed to contain a substance called → phlogiston, which was released when the object burned. The existence of this hypothetical substance was proposed in 1669 by Johann Becher, who called it terra pinguis “fat earth.” For example, as wood burns it releases phlogiston into the air, leaving ash behind. Ash was therefore wood minus phlogiston. In the early 18th century Georg Stahl renamed the substance phlogiston. The theory was disproved by Antoine Lavoisier in 1783, who proved the principle of conservation of mass, refuted the phlogiston theory
and proposed the oxygen theory of burning.

See also: → phlogiston; → theory.

The inner → satellite of → Mars orbiting less than 6,000 km above the surface of Mars, closer to its → primary than any other → moon in the → solar system. Phobos is irregularly shaped, 27 x 22 x 18 km in size and orbits Mars in 0.319 days. Phobos’ orbit is decaying at a rate of about 2 centimeters per year; it is therefore expected to break up and crash onto Mars within the next 50 million years. See also: → Roche limit, → orbit decay.

See also: In Gk. mythology, Phobos is one of the sons of Ares (Mars) and Aphrodite (Venus). The name means “fear, panic, flight.”

The inner → satellite of → Mars orbiting less than 6,000 km above the surface of Mars, closer to its → primary than any other → moon in the → solar system. Phobos is irregularly shaped, 27 x 22 x 18 km in size and orbits Mars in 0.319 days. Phobos’ orbit is decaying at a rate of about 2 centimeters per year; it is therefore expected to break up and crash onto Mars within the next 50 million years. See also: → Roche limit, → orbit decay.

See also: In Gk. mythology, Phobos is one of the sons of Ares (Mars) and Aphrodite (Venus). The name means “fear, panic, flight.”

The outermost of Saturn’s known satellites, also known as
Saturn IX. It is 220 km in diameter and orbits Saturn in 550.5 days at a distance of 12,952,000 km.

See also: In Gk. mythology, Phoebe is the daughter of Uranus and Gaia; grandmother of Apollo and Artemis.

The outermost of Saturn’s known satellites, also known as
Saturn IX. It is 220 km in diameter and orbits Saturn in 550.5 days at a distance of 12,952,000 km.

See also: In Gk. mythology, Phoebe is the daughter of Uranus and Gaia; grandmother of Apollo and Artemis.

A giant ring around Saturn spanning an area of space from a distance of ~ 128 Saturn equatorial radii, R_S (60,330 km) to 207 R_S, that is, from about 7.7 × 10⁶ to 12.4 × 10⁶ km from the planet. Its vertical thickness is about 40 R_S. The Phoebe ring was detected in 2009 using NASA’s infrared → Spitzer Space Telescope. The Phoebe ring is made up mainly of dust particles about 10 to 20 microns in size, or about one-tenth to one-fifth the average width of a human hair. Rocks that are the size of soccer balls or larger with diameters of more than about 20 cm make up no more than about 10 percent of the ring (Verbiscer et al., 2009, Nature, 461, 1098).

See also: → Phoebe; → ring.

A giant ring around Saturn spanning an area of space from a distance of ~ 128 Saturn equatorial radii, R_S (60,330 km) to 207 R_S, that is, from about 7.7 × 10⁶ to 12.4 × 10⁶ km from the planet. Its vertical thickness is about 40 R_S. The Phoebe ring was detected in 2009 using NASA’s infrared → Spitzer Space Telescope. The Phoebe ring is made up mainly of dust particles about 10 to 20 microns in size, or about one-tenth to one-fifth the average width of a human hair. Rocks that are the size of soccer balls or larger with diameters of more than about 20 cm make up no more than about 10 percent of the ring (Verbiscer et al., 2009, Nature, 461, 1098).

See also: → Phoebe; → ring.

A constellation in the southern hemisphere, at 0h 30m → right ascension, 50° south → declination. Its brightest star Alpha Phoenicis is of magnitude 2.4. Abbreviation: Phe; genitive: Phoenicis.

Etymology (EN): L. Phoenix, also phenix, from Gk. phoinix a mythical bird of great beauty which according to one account lived 500 years, burned itself to ashes on a pyre, and rose alive from the ashes to live another period.

Etymology (PE): Qoqnos, from Ar., from Gk., as above, or, for some reasons (mistake?),
from Gk. kuknos, → Cygnus.

A constellation in the southern hemisphere, at 0h 30m → right ascension, 50° south → declination. Its brightest star Alpha Phoenicis is of magnitude 2.4. Abbreviation: Phe; genitive: Phoenicis.

Etymology (EN): L. Phoenix, also phenix, from Gk. phoinix a mythical bird of great beauty which according to one account lived 500 years, burned itself to ashes on a pyre, and rose alive from the ashes to live another period.

Etymology (PE): Qoqnos, from Ar., from Gk., as above, or, for some reasons (mistake?),
from Gk. kuknos, → Cygnus.

A speech sound considered as a physical event without regard to its place in the sound system of a language.

Etymology (EN): From Gk. phone “voice, sound,” phonein “to speak;” cf. L. fama “talk, reputation, fame.”

Etymology (PE): Ãva “voice, sound,” related to âvâz “voice, sound, song” (both prefixed forms), bâng “voice, sound, clamour” (Mid.Pers. vâng), vâžé “word,” variants vâj-, vâk-, vâ-, vâz-, vât-;
Av. vacah- “word,” vaocanghê “to decalre” (by means of speech), from vac- “to speak, say;” cf. Skt. vakti “speaks, says,” vacas- “word;” Gk. epos “word;” L. vox “voice;” PIE base *wek- “to speak.”

A speech sound considered as a physical event without regard to its place in the sound system of a language.

Etymology (EN): From Gk. phone “voice, sound,” phonein “to speak;” cf. L. fama “talk, reputation, fame.”

Etymology (PE): Ãva “voice, sound,” related to âvâz “voice, sound, song” (both prefixed forms), bâng “voice, sound, clamour” (Mid.Pers. vâng), vâžé “word,” variants vâj-, vâk-, vâ-, vâz-, vât-;
Av. vacah- “word,” vaocanghê “to decalre” (by means of speech), from vac- “to speak, say;” cf. Skt. vakti “speaks, says,” vacas- “word;” Gk. epos “word;” L. vox “voice;” PIE base *wek- “to speak.”

The smallest phonetic unit in a language that can distinguish one word from another.

Etymology (EN): From Fr. phonème, from Gk. phonema “speech sound, utterance,” from phonein “to sound,” → phone.

Etymology (PE): Vâj “voice,” variant of vâž, vâz-, âvâz etc., → phone.

The smallest phonetic unit in a language that can distinguish one word from another.

Etymology (EN): From Fr. phonème, from Gk. phonema “speech sound, utterance,” from phonein “to sound,” → phone.

Etymology (PE): Vâj “voice,” variant of vâž, vâz-, âvâz etc., → phone.

A branch of linguistics dealing with the analysis, description, and classification of speech sounds. More specifically, phonetics deals with the physical production of → phonemes regardless of language, while → phonology studies how those sounds are put together to create meaningful words in a particular language.

Etymology (EN): From phonetic, from N.L. phoneticus, from Gk. phonetikos “vocal,” from phonet(os) “utterable,” verbal adj. of phonein “to speak clearly, utter,” from → phone + -ikos, → -ics.

Etymology (PE): Âvâyik, from âvâ, → phone, + -ik,
→ -ics.

A branch of linguistics dealing with the analysis, description, and classification of speech sounds. More specifically, phonetics deals with the physical production of → phonemes regardless of language, while → phonology studies how those sounds are put together to create meaningful words in a particular language.

Etymology (EN): From phonetic, from N.L. phoneticus, from Gk. phonetikos “vocal,” from phonet(os) “utterable,” verbal adj. of phonein “to speak clearly, utter,” from → phone + -ikos, → -ics.

Etymology (PE): Âvâyik, from âvâ, → phone, + -ik,
→ -ics.

The branch of medical science dealing with the study and treatment of voice disorders.

See also: From Gk. phon-, → phone; → -iatrics.

The branch of medical science dealing with the study and treatment of voice disorders.

See also: From Gk. phon-, → phone; → -iatrics.

A combining form meaning “sound, voice,” used in the formation of compound words. Also phon-, especially before a vowel.

See also: From Gk. phon-, phono-, form → phone
“voice, sound, speech”

A combining form meaning “sound, voice,” used in the formation of compound words. Also phon-, especially before a vowel.

See also: From Gk. phon-, phono-, form → phone
“voice, sound, speech”

A branch of linguistics that studies the rules in any given language that govern how → phonemes are combined to create meaningful words. Phonology and → phonetics study two different aspects of sound, but the concepts are dependent on each other in the creation of language.

See also: → phono-; → -logy.

A branch of linguistics that studies the rules in any given language that govern how → phonemes are combined to create meaningful words. Phonology and → phonetics study two different aspects of sound, but the concepts are dependent on each other in the creation of language.

See also: → phono-; → -logy.

A quantum of vibrational or acoustic energy in a crystal lattice, being the analog of a photon of electromagnetic energy.

See also: → phono- + → -on.

A quantum of vibrational or acoustic energy in a crystal lattice, being the analog of a photon of electromagnetic energy.

See also: → phono- + → -on.

A colorless, flammable, and explosive gas at ambient temperature with unpleasant smell of rotten fish or garlic.

Named also hydride of phosphorus (PH₃), it is highly poisonous in nature. On cooling to 185.5 K, phosphine condenses to a liquid and on cooling to 139.5 K, it solidifies. By heating in the absence of air at 713 K or by passing an electric spark through it, phosphine breaks into its elements.

Small amounts occur naturally from the break down of organic matter. It is heavier than air and slightly soluble in water. Phosphine is used in semiconductor and plastics industries, in the production of a flame retardant, and as a pesticide in stored grain. Phosphine has two strong absorption bands in → infrared at 10 and 9 μm.

See also: From phosph-, variant of phospho-, denoting → phosphorus, used before a vowel + suffix -ine, ultimately from L. -inus, used to form names of chemical substances, especially basic (alkaline) substances, alkaloidal substances, or halogen elements.

A colorless, flammable, and explosive gas at ambient temperature with unpleasant smell of rotten fish or garlic.

Named also hydride of phosphorus (PH₃), it is highly poisonous in nature. On cooling to 185.5 K, phosphine condenses to a liquid and on cooling to 139.5 K, it solidifies. By heating in the absence of air at 713 K or by passing an electric spark through it, phosphine breaks into its elements.

Small amounts occur naturally from the break down of organic matter. It is heavier than air and slightly soluble in water. Phosphine is used in semiconductor and plastics industries, in the production of a flame retardant, and as a pesticide in stored grain. Phosphine has two strong absorption bands in → infrared at 10 and 9 μm.

See also: From phosph-, variant of phospho-, denoting → phosphorus, used before a vowel + suffix -ine, ultimately from L. -inus, used to form names of chemical substances, especially basic (alkaline) substances, alkaloidal substances, or halogen elements.

A specific type of → photoluminescence that continues for an appreciable time after the stimulating process has ceased. Phosphorescence is due to the existence of metastable → excited states of the atoms and molecules from which a change to the normal state is hindered for some reason or other. The change from the → metastable metastable state to the normal one becomes possible only as a result of some additional excitation, for example the application of heat.

See also: → phosphorus; → -escence.

A specific type of → photoluminescence that continues for an appreciable time after the stimulating process has ceased. Phosphorescence is due to the existence of metastable → excited states of the atoms and molecules from which a change to the normal state is hindered for some reason or other. The change from the → metastable metastable state to the normal one becomes possible only as a result of some additional excitation, for example the application of heat.

See also: → phosphorus; → -escence.

1. Nonmetallic chemical element; symbol P. → Atomic number 15; → atomic weight 30.97376; → melting point 44.1°C; → boiling point about 280°C. It was discovered by the German merchant Hennig Brand in 1669.
   
2. Greek name for the planet → Venus when it appears as a → morning star.

Etymology (EN): L. Phosphorus “morning star,” from Gk. Phosphoros “morning star,” literally “light bearing,” from phos “light” + phoros “bearer,” from pherein “to carry,” cognate with Pers. bordan “to carry, lead” (→ periphery).
The chemical element is such called because of its white color.

Etymology (PE): 1) Fosfor, loan from Fr.
2) → morning star.

1. Nonmetallic chemical element; symbol P. → Atomic number 15; → atomic weight 30.97376; → melting point 44.1°C; → boiling point about 280°C. It was discovered by the German merchant Hennig Brand in 1669.
   
2. Greek name for the planet → Venus when it appears as a → morning star.

Etymology (EN): L. Phosphorus “morning star,” from Gk. Phosphoros “morning star,” literally “light bearing,” from phos “light” + phoros “bearer,” from pherein “to carry,” cognate with Pers. bordan “to carry, lead” (→ periphery).
The chemical element is such called because of its white color.

Etymology (PE): 1) Fosfor, loan from Fr.
2) → morning star.

The supersymmetric partner of the → photon.

See also: From phot, from → photon + -ino supersymmetric particle suffix.

The supersymmetric partner of the → photon.

See also: From phot, from → photon + -ino supersymmetric particle suffix.

Etymology (EN): From Gk. combining form of phos (genitive photos).

Etymology (PE): Šid- “light, sunlight,” from Mid.Pers. šÃªt “shining, radiant, bright;” Av. xšaēta- “shining, brilliant, splendid, excellent.”
Nur-, → light.

Etymology (EN): From Gk. combining form of phos (genitive photos).

Etymology (PE): Šid- “light, sunlight,” from Mid.Pers. šÃªt “shining, radiant, bright;” Av. xšaēta- “shining, brilliant, splendid, excellent.”
Nur-, → light.

A situation in which all of the energy of a photon is transferred to an atom, molecule, or nucleus.

See also: → photo- + → absorption.

A situation in which all of the energy of a photon is transferred to an atom, molecule, or nucleus.

See also: → photo- + → absorption.

Electrode capable of releasing electrons when illuminated.

See also: → photo- + → cathode.

Electrode capable of releasing electrons when illuminated.

See also: → photo- + → cathode.

The study of the chemical and physical changes occurring when a molecule or atom absorbs photons of light.

See also: → photo- + → chemistry.

The study of the chemical and physical changes occurring when a molecule or atom absorbs photons of light.

See also: → photo- + → chemistry.

Th desorption of surface substances by ultraviolet radiation.

See also: → photo-; → desorption.

Th desorption of surface substances by ultraviolet radiation.

See also: → photo-; → desorption.

The process by which atomic nuclei are broken apart into their constituent protons and neutrons by the impact of high energy gamma photons. Photodisintegration takes place during the core collapse phase of a → Type II supernova explosion.

See also: → photo- + → disintegration.

The process by which atomic nuclei are broken apart into their constituent protons and neutrons by the impact of high energy gamma photons. Photodisintegration takes place during the core collapse phase of a → Type II supernova explosion.

See also: → photo- + → disintegration.

To dissociate a → molecule by → radiation. See also → photodissociation.

See also: → photo-; → dissociate.

To dissociate a → molecule by → radiation. See also → photodissociation.

See also: → photo-; → dissociate.

The → dissociation of a → chemical compound by → radiation  → energy.

See also: Verbal noun of → photodissociate; → -tion.

The → dissociation of a → chemical compound by → radiation  → energy.

See also: Verbal noun of → photodissociate; → -tion.

A neutral region at the boundary of a → molecular cloud created by the penetration of → far ultraviolet (FUV) radiation from associated stars. The FUV radiation (6 eV ≤ hν ≤ 13.6 eV) dissociates the molecules and heats the gas and dust. A warm, atomic → H I region is thus created and the chemistry and thermal balance of the region are determined by the penetrating FUV photons. The progressive absorption of FUV photons leads to the occurrence of transitions between atomic and molecular phases, such as H I/H₂ and C II/C I/CO transitions. By extension, any neutral region where the physics is controlled by FUV photons can be called a PDR, as it is the case for → diffuse interstellar clouds or the edge of → circumstellar disks.
The PDR concept was first studied by A. G. G. M. Tielens and D. Hollenbach (1985, ApJ 291, 722).

See also: → photodissociation + → region.

A neutral region at the boundary of a → molecular cloud created by the penetration of → far ultraviolet (FUV) radiation from associated stars. The FUV radiation (6 eV ≤ hν ≤ 13.6 eV) dissociates the molecules and heats the gas and dust. A warm, atomic → H I region is thus created and the chemistry and thermal balance of the region are determined by the penetrating FUV photons. The progressive absorption of FUV photons leads to the occurrence of transitions between atomic and molecular phases, such as H I/H₂ and C II/C I/CO transitions. By extension, any neutral region where the physics is controlled by FUV photons can be called a PDR, as it is the case for → diffuse interstellar clouds or the edge of → circumstellar disks.
The PDR concept was first studied by A. G. G. M. Tielens and D. Hollenbach (1985, ApJ 291, 722).

See also: → photodissociation + → region.

Pertaining to electronic or other electrical effects that are due to the action of electromagnetic radiation, especially visible light.

See also: → photo- + → electric.

Pertaining to electronic or other electrical effects that are due to the action of electromagnetic radiation, especially visible light.

See also: → photo- + → electric.

The current produced in an → photoelectric effect process when → photoelectrons are received at the positive electrode.

See also: → photoelectric; → current.

The current produced in an → photoelectric effect process when → photoelectrons are received at the positive electrode.

See also: → photoelectric; → current.

The process of release of electrically charged particles (usually → electrons) as
a result of irradiation of matter by light or other → electromagnetic radiation. The classical electromagnetic theory was unable to account for the following characteristics of the phenomenon. Light below a certain threshold frequency, no matter how intense, will not cause any electrons to be emitted.
Light above that frequency, even if it is not very intense, will always cause electrons to be ejected. The electrons are ejected after some nanoseconds, independently of the light intensity.
The maximum kinetic energy of the emitted electrons is a function of the frequency and does not dependent on the intensity of the incident light. The classical theory could not explain how a train of light waves spread out over a large number of atoms could, in a very short time interval, concentrate enough energy to knock a single electron out of the metal.
In 1905, based on Planck’s idea of → quanta, Einstein proposed that light consisted of quanta (later called → photons);
that a given source could emit and absorb radiant energy only in units which are all exactly equal to the radiation frequency multiplied by a constant (→ Planck’s constant);
and that a photon with a frequency over a certain threshold would have sufficient energy to eject a single electron. His photoelectric equation is descibed as
(1/2)mu² = hν - A, where m is the electron mass, u is the electron velocity, h is Planck’s constant, ν is the frequency, and A the → work function, which represents the amount of work needed by electrons to get free of the surface. See also → photoelectron, → photoelectric current, → external photoelectric effect, → internal photoelectric effect.

See also: → photoelectric; → effect.

The process of release of electrically charged particles (usually → electrons) as
a result of irradiation of matter by light or other → electromagnetic radiation. The classical electromagnetic theory was unable to account for the following characteristics of the phenomenon. Light below a certain threshold frequency, no matter how intense, will not cause any electrons to be emitted.
Light above that frequency, even if it is not very intense, will always cause electrons to be ejected. The electrons are ejected after some nanoseconds, independently of the light intensity.
The maximum kinetic energy of the emitted electrons is a function of the frequency and does not dependent on the intensity of the incident light. The classical theory could not explain how a train of light waves spread out over a large number of atoms could, in a very short time interval, concentrate enough energy to knock a single electron out of the metal.
In 1905, based on Planck’s idea of → quanta, Einstein proposed that light consisted of quanta (later called → photons);
that a given source could emit and absorb radiant energy only in units which are all exactly equal to the radiation frequency multiplied by a constant (→ Planck’s constant);
and that a photon with a frequency over a certain threshold would have sufficient energy to eject a single electron. His photoelectric equation is descibed as
(1/2)mu² = hν - A, where m is the electron mass, u is the electron velocity, h is Planck’s constant, ν is the frequency, and A the → work function, which represents the amount of work needed by electrons to get free of the surface. See also → photoelectron, → photoelectric current, → external photoelectric effect, → internal photoelectric effect.

See also: → photoelectric; → effect.

A heating process occurring in → diffuse molecular clouds which is believed to be the main heating mechanism in cool → H I regions. Far-ultraviolet (FUV) photons, in the energy range 6 eV «i>hν < 13.6 eV, expel electrons from → interstellar dust grains
and the excess → kinetic energy of the electrons is
converted into gas → thermal energy through → collisions. The high energy limit corresponds to the cut-off in the → far-ultraviolet (FUV) radiation field caused by
the hydrogen absorption (hν = 13.6 eV), while the low energy limit corresponds to the energy needed to free electrons from the grains (hν ~ 6 eV). In the cold neutral medium (T_kin≥ 200 K) photoelectric heating accounts for most of the heating, the → X-ray and → cosmic ray heating rates (→ cosmic-ray ionization) being more than an order of magnitude smaller. In a relatively dense neutral medium (n_H≥ 100 cm^-3), where a significant fraction of carbon is in the neutral form, carbon ionization becomes an important heating source, but it is still not comparable to the photoelectric effect.

The heating rate cannot be directly measured, but it can be estimated through observations of the [C II] line emission, since this is believed to be the main → coolant in regions where the photoelectric heating is dominant (See, e.g., Juvela et al., 2003, arXiv:astro-ph/0302365).

See also: → photoelectric; → heating.

A heating process occurring in → diffuse molecular clouds which is believed to be the main heating mechanism in cool → H I regions. Far-ultraviolet (FUV) photons, in the energy range 6 eV «i>hν < 13.6 eV, expel electrons from → interstellar dust grains
and the excess → kinetic energy of the electrons is
converted into gas → thermal energy through → collisions. The high energy limit corresponds to the cut-off in the → far-ultraviolet (FUV) radiation field caused by
the hydrogen absorption (hν = 13.6 eV), while the low energy limit corresponds to the energy needed to free electrons from the grains (hν ~ 6 eV). In the cold neutral medium (T_kin≥ 200 K) photoelectric heating accounts for most of the heating, the → X-ray and → cosmic ray heating rates (→ cosmic-ray ionization) being more than an order of magnitude smaller. In a relatively dense neutral medium (n_H≥ 100 cm^-3), where a significant fraction of carbon is in the neutral form, carbon ionization becomes an important heating source, but it is still not comparable to the photoelectric effect.

The heating rate cannot be directly measured, but it can be estimated through observations of the [C II] line emission, since this is believed to be the main → coolant in regions where the photoelectric heating is dominant (See, e.g., Juvela et al., 2003, arXiv:astro-ph/0302365).

See also: → photoelectric; → heating.

The magnitude of an object as measured with a photoelectric photometer.

See also: → photoelectric; → magnitude.

The magnitude of an object as measured with a photoelectric photometer.

See also: → photoelectric; → magnitude.

A photometry in which the magnitudes are obtained using a photoelectric photometer.

See also: → photoelectric; → photometry.

A photometry in which the magnitudes are obtained using a photoelectric photometer.

See also: → photoelectric; → photometry.

An electron emitted from an atom or molecule by an incident photon in the → photoelectric effect.

See also: → photo-; + → electron.

An electron emitted from an atom or molecule by an incident photon in the → photoelectric effect.

See also: → photo-; + → electron.

The emission of electrons as a result of incident radiation in the → photoelectric effect. Also called → external photoelectric effect.

See also: → photo- + → emissive; → effect.

The emission of electrons as a result of incident radiation in the → photoelectric effect. Also called → external photoelectric effect.

See also: → photo- + → emissive; → effect.

A process going on in a molecular cloud surface whereby the surface material ionized by ultraviolet photons of neighboring stars is dissipated.

See also: → photo- + → evaporation.

A process going on in a molecular cloud surface whereby the surface material ionized by ultraviolet photons of neighboring stars is dissipated.

See also: → photo- + → evaporation.

The mechanism of raising an electron to higher energies by photon absorption, when the energy of the photon is too low to cause photoionization.

See also: → photo- + → excitation.

The mechanism of raising an electron to higher energies by photon absorption, when the energy of the photon is too low to cause photoionization.

See also: → photo- + → excitation.

A picture produced by photography. → picture.

Etymology (EN): From → photo- + → -graph.

Etymology (PE): Aks, from Ar. ‘aks “to inverse, reverse.” Šidnegâr, nurnegâr, from šid, nur, → photo-, + negâr, → graph.

A picture produced by photography. → picture.

Etymology (EN): From → photo- + → -graph.

Etymology (PE): Aks, from Ar. ‘aks “to inverse, reverse.” Šidnegâr, nurnegâr, from šid, nur, → photo-, + negâr, → graph.

The apparent magnitude of a star as determined by measuring its brightness on a photographic plate. The photographic magnitude scale is now considered obsolete.

See also: Adj. of → photography; → magnitude.

The apparent magnitude of a star as determined by measuring its brightness on a photographic plate. The photographic magnitude scale is now considered obsolete.

See also: Adj. of → photography; → magnitude.

Recording a large area of the night sky by photographic techniques, as practiced in the past before the advent of electronic detectors.

See also: Adj. of → photography; → survey.

Recording a large area of the night sky by photographic techniques, as practiced in the past before the advent of electronic detectors.

See also: Adj. of → photography; → survey.

The process of recording and producing images by exposing light-sensitive detectors to light or other forms of radiation.

Etymology (EN): → photo-, → -graphy.

Etymology (PE): Aksbardâri, literally “taking photograph,” from aks,
→ photograph,

• bardâri verbal noun of bardâštan “to take,” composite verb from bar- “on; up; upon; in; into; at; forth; with; near; before; according to” (Mid.Pers. abar; O.Pers. upariy “above; over, upon, according to;” Av. upairi “above, over,” upairi.zəma- “located above the earth;” cf. Gk. hyper- “over, above;” L. super-; O.H.G. ubir “over;” PIE base *uper “over”) + dâštan “to have, to possess” (Mid.Pers. dâštan; O.Pers./Av. root dar- “to hold, keep back, maintain, keep in mind;” cf.
  Skt. dhr-, dharma- “law;”
  Gk. thronos “elevated seat, throne;” L. firmus “firm, stable;” Lith. daryti “to make;” PIE *dher- “to hold, support”).
  Šidnegâri, nurnegâri, action noun from šidnegâr, nurnegâr,
  → photograph.

The process of recording and producing images by exposing light-sensitive detectors to light or other forms of radiation.

Etymology (EN): → photo-, → -graphy.

Etymology (PE): Aksbardâri, literally “taking photograph,” from aks,
→ photograph,

• bardâri verbal noun of bardâštan “to take,” composite verb from bar- “on; up; upon; in; into; at; forth; with; near; before; according to” (Mid.Pers. abar; O.Pers. upariy “above; over, upon, according to;” Av. upairi “above, over,” upairi.zəma- “located above the earth;” cf. Gk. hyper- “over, above;” L. super-; O.H.G. ubir “over;” PIE base *uper “over”) + dâštan “to have, to possess” (Mid.Pers. dâštan; O.Pers./Av. root dar- “to hold, keep back, maintain, keep in mind;” cf.
  Skt. dhr-, dharma- “law;”
  Gk. thronos “elevated seat, throne;” L. firmus “firm, stable;” Lith. daryti “to make;” PIE *dher- “to hold, support”).
  Šidnegâri, nurnegâri, action noun from šidnegâr, nurnegâr,
  → photograph.

The physical process in which an incident high-energy photon ejects one or more electrons from an atom, ion, or molecule.

See also: → photo- + → ionization.

The physical process in which an incident high-energy photon ejects one or more electrons from an atom, ion, or molecule.

See also: → photo- + → ionization.

To cause, or to undergo → photoionization.

See also: → photo-; → ionize.

To cause, or to undergo → photoionization.

See also: → photo-; → ionize.

Subject to, or produced by → photoionization.

See also: → photo-; → ionized.

Subject to, or produced by → photoionization.

See also: → photo-; → ionized.

A process in which → absorption of photons at → ultraviolet (UV) / → optical wavelengths is followed by → electronic transitions associated with the emission of longer wavelength optical and → near-IR photons. Photoluminescence has two types: → phosphorescence and → luminescence.

The excitation of the photoluminescence process under astrophysical conditions results from the absorption of a single UV/optical photon, leading to an electronic transition from a → ground state (1) to a higher state (2). State (2) typically is a bound, high-lying vibrational-rotational level of the first or second electronically excited state of a molecule or molecular ion, or a high state in the → conduction band of a semiconductor particle. The excited system relaxes through a series of → vibrational-rotational transitions until the electron finds itself in an intermediate state (3), from where an optical electronic transition back to the ground state (1) is possible. In a → polycyclic aromatic hydrocarbon (PAH) molecule, for example, state (3) can either be the lowest state in the → singlet or → triplet vibrational-rotational manifold of the first excited electronic level (Witt, A. N., Vijh, U. P., 2003, astro-ph/0309674).

See also: → photo-; → luminescence.

A process in which → absorption of photons at → ultraviolet (UV) / → optical wavelengths is followed by → electronic transitions associated with the emission of longer wavelength optical and → near-IR photons. Photoluminescence has two types: → phosphorescence and → luminescence.

The excitation of the photoluminescence process under astrophysical conditions results from the absorption of a single UV/optical photon, leading to an electronic transition from a → ground state (1) to a higher state (2). State (2) typically is a bound, high-lying vibrational-rotational level of the first or second electronically excited state of a molecule or molecular ion, or a high state in the → conduction band of a semiconductor particle. The excited system relaxes through a series of → vibrational-rotational transitions until the electron finds itself in an intermediate state (3), from where an optical electronic transition back to the ground state (1) is possible. In a → polycyclic aromatic hydrocarbon (PAH) molecule, for example, state (3) can either be the lowest state in the → singlet or → triplet vibrational-rotational manifold of the first excited electronic level (Witt, A. N., Vijh, U. P., 2003, astro-ph/0309674).

See also: → photo-; → luminescence.

An instrument for measuring the amount of light.

See also: → photo- + → -metry.

An instrument for measuring the amount of light.

See also: → photo- + → -metry.

Pertaining to or related to → photometry.

See also: → photometer + → -ic.

Pertaining to or related to → photometry.

See also: → photometer + → -ic.

The range of → wavelengths allowed by a → filter used in a → photometric system.

See also: → photometric + → band.

The range of → wavelengths allowed by a → filter used in a → photometric system.

See also: → photometric + → band.

A binary star whose binarity is detectable from its variability and light-curve that has certain specific characteristics.

See also: → photometric + → binary.

A binary star whose binarity is detectable from its variability and light-curve that has certain specific characteristics.

See also: → photometric + → binary.

A calibration which converts the measured relative magnitudes into an absolute photometry.

See also: → photometric + → calibration.

A calibration which converts the measured relative magnitudes into an absolute photometry.

See also: → photometric + → calibration.

A method of deriving the distance of a star using its
→ apparent magnitude and the
→ absolute magnitude inferred from its → spectral type.

See also: This is a misnomer, because the method has nothing to do with parallax; → photometric; → parallax.

A method of deriving the distance of a star using its
→ apparent magnitude and the
→ absolute magnitude inferred from its → spectral type.

See also: This is a misnomer, because the method has nothing to do with parallax; → photometric; → parallax.

A system of → magnitudes, each of them characterized by a set of
well-defined → passbands (or → filters) with known → response curves. The system is defined by the values given for the → standard stars.
See also:
→ AB magnitude system, → five-color system, → Stromgren system, → JHK system, → UBV system, → uvby system.

See also: → photometric; → system.

A system of → magnitudes, each of them characterized by a set of
well-defined → passbands (or → filters) with known → response curves. The system is defined by the values given for the → standard stars.
See also:
→ AB magnitude system, → five-color system, → Stromgren system, → JHK system, → UBV system, → uvby system.

See also: → photometric; → system.

In astronomy, the measurement of the light of astronomical objects, generally in the visible or infrared bands, in which a wavelength band is normally specified.

See also: → photo- + → -metry.

In astronomy, the measurement of the light of astronomical objects, generally in the visible or infrared bands, in which a wavelength band is normally specified.

See also: → photo- + → -metry.

Electronic tube which converts photons into electrons, multiplies the electrons via a series of electrodes, and produces a measurable current from a very small input signal.

See also: → photo- + → multiplier.

Electronic tube which converts photons into electrons, multiplies the electrons via a series of electrodes, and produces a measurable current from a very small input signal.

See also: → photo- + → multiplier.

The → quantum of the → electromagnetic field, which mediates the interaction between charged particles. It is the mass-less → boson with zero → electric charge, which propagates with the → speed of light in vacuum. The energy of a photon is connected to its → frequency ν, through the formula E = hν, where h is → Planck’s constant.

See also: From phot-, variant of → photo- before a vowel + → -on a suffix used in the names of subatomic particles (gluon; meson; neutron), quanta (photon, graviton), and other minimal entities or components. The term photon was coined by Gilbert N. Lewis in 1926 in a letter to the editor of Nature magazine (Vol. 118, Part 2, December 18, page 874).

The → quantum of the → electromagnetic field, which mediates the interaction between charged particles. It is the mass-less → boson with zero → electric charge, which propagates with the → speed of light in vacuum. The energy of a photon is connected to its → frequency ν, through the formula E = hν, where h is → Planck’s constant.

See also: From phot-, variant of → photo- before a vowel + → -on a suffix used in the names of subatomic particles (gluon; meson; neutron), quanta (photon, graviton), and other minimal entities or components. The term photon was coined by Gilbert N. Lewis in 1926 in a letter to the editor of Nature magazine (Vol. 118, Part 2, December 18, page 874).

The time required for a photon created in the Sun’s core to attain the → photosphere and leave the Sun. If the photons were free to escape, they would take a time of only R/c (a couple of seconds) to reach the surface, where R is the Solar radius and c the speed of light. The solar material is, however, very opaque, so that photons travel only a short distance before interacting with other particles. Therefore, photons undergo a very large number of → random walks
before arriving at the surface by chance. The typical time is approximately
5 x 10⁴ years for a constant density Sun.

See also: → photon; → escape;
→ time.

The time required for a photon created in the Sun’s core to attain the → photosphere and leave the Sun. If the photons were free to escape, they would take a time of only R/c (a couple of seconds) to reach the surface, where R is the Solar radius and c the speed of light. The solar material is, however, very opaque, so that photons travel only a short distance before interacting with other particles. Therefore, photons undergo a very large number of → random walks
before arriving at the surface by chance. The typical time is approximately
5 x 10⁴ years for a constant density Sun.

See also: → photon; → escape;
→ time.

→ Electromagnetic radiation in equilibrium in a → black body cavity. Photons can be treated as the simplest → ideal gas because all the particles move at the same velocity, the → speed of light. There are, nevertheless, two main differences. 1) Photons are → bosons and → Bose-Einstein statistics must be used. However, photons do not interact with each others so that no approximation is made by neglecting inter-particle forces. 2) Some photons scatter off the walls, with some being absorbed and new ones being emitted continually;
so that no constraint can be placed on their number.

See also: → photon; → gas.

→ Electromagnetic radiation in equilibrium in a → black body cavity. Photons can be treated as the simplest → ideal gas because all the particles move at the same velocity, the → speed of light. There are, nevertheless, two main differences. 1) Photons are → bosons and → Bose-Einstein statistics must be used. However, photons do not interact with each others so that no approximation is made by neglecting inter-particle forces. 2) Some photons scatter off the walls, with some being absorbed and new ones being emitted continually;
so that no constraint can be placed on their number.

See also: → photon; → gas.

An effect occurring in the outer zones of → H II regions where
the number of high-energy ultraviolet photons with energies well above the → ionization potential of hydrogen increases with respect to
the number of → Lyman continuum photons. The effect is due to stronger absorption of weaker photons.

See also: → hard; → photon.

An effect occurring in the outer zones of → H II regions where
the number of high-energy ultraviolet photons with energies well above the → ionization potential of hydrogen increases with respect to
the number of → Lyman continuum photons. The effect is due to stronger absorption of weaker photons.

See also: → hard; → photon.

An intrinsic noise caused by the quantum nature of light. Same as → quantum noise.

See also: → photon; → noise.

An intrinsic noise caused by the quantum nature of light. Same as → quantum noise.

See also: → photon; → noise.

A surface where if a photon is emitted from one of its points the photon follows a closed orbit and returns periodically to its departure point. Such a surface exists only near sufficiently → compact objects where the → curvature of → space-time is very important. In other words, a body can take a stable orbit around a → black hole provided that it moves with the → speed of light.
However, only photons can have such a velocity; hence the term “photon sphere.” For a non-rotating → Schwarzschild black hole, the photon sphere has a radius of R = 3GM/c² = 3 R_S/2, where G is the → gravitational constant, M is the mass, c is the → speed of light, and R_S is the → Schwarzschild radius.
For a rotating, → Kerr black hole, the situation is much more
complex due to the → Lense-Thirring effect. In that case circular paths exist for radii whose values depend on the rotation direction. More specifically, in the equatorial plane there are two possible circular light paths: a smaller one in the direction of the rotation, and a larger one in the opposite direction.

See also: → photon; → sphere.

A surface where if a photon is emitted from one of its points the photon follows a closed orbit and returns periodically to its departure point. Such a surface exists only near sufficiently → compact objects where the → curvature of → space-time is very important. In other words, a body can take a stable orbit around a → black hole provided that it moves with the → speed of light.
However, only photons can have such a velocity; hence the term “photon sphere.” For a non-rotating → Schwarzschild black hole, the photon sphere has a radius of R = 3GM/c² = 3 R_S/2, where G is the → gravitational constant, M is the mass, c is the → speed of light, and R_S is the → Schwarzschild radius.
For a rotating, → Kerr black hole, the situation is much more
complex due to the → Lense-Thirring effect. In that case circular paths exist for radii whose values depend on the rotation direction. More specifically, in the equatorial plane there are two possible circular light paths: a smaller one in the direction of the rotation, and a larger one in the opposite direction.

See also: → photon; → sphere.

The maximum → mass loss rate of a star when the → wind luminosity equals the total available → stellar luminosity. The mechanical luminosity of the wind at infinity is given by: L_wind = Mdot (v_∞²/2 + GM/R) = Mdot (v_∞²/2 + v_esc²/2). For
L_wind = L, the mass loss rate is
Mdot_max = 2L/(v_∞² + v_esc²). Following Owoki & Gayly (1997), Mdot_tir is the maximum mass loss rate when the wind just escapes the gravitational potential, with v_∞ tending toward zero. Mdot_tir is much larger than typical mass loss rates from → line-driven winds, where the driving lines become saturated with increasing density limiting the wind mass loss rates to about 10^-4 Msun yr^-1 in even the most luminous stars.

See also: → photon; tiring, from tire “to weary; become weary,” → tired; → limit.

The maximum → mass loss rate of a star when the → wind luminosity equals the total available → stellar luminosity. The mechanical luminosity of the wind at infinity is given by: L_wind = Mdot (v_∞²/2 + GM/R) = Mdot (v_∞²/2 + v_esc²/2). For
L_wind = L, the mass loss rate is
Mdot_max = 2L/(v_∞² + v_esc²). Following Owoki & Gayly (1997), Mdot_tir is the maximum mass loss rate when the wind just escapes the gravitational potential, with v_∞ tending toward zero. Mdot_tir is much larger than typical mass loss rates from → line-driven winds, where the driving lines become saturated with increasing density limiting the wind mass loss rates to about 10^-4 Msun yr^-1 in even the most luminous stars.

See also: → photon; tiring, from tire “to weary; become weary,” → tired; → limit.

The plasma filling space before the → recombination epoch that mainly consisted of → cosmic microwave background radiation photons, electrons, protons, and → light elements.

See also: → photon; → baryon; → plasma.

The plasma filling space before the → recombination epoch that mainly consisted of → cosmic microwave background radiation photons, electrons, protons, and → light elements.

See also: → photon; → baryon; → plasma.

The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems.

See also: → photon + → -ics.

The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems.

See also: → photon + → -ics.

1. The visible surface of the Sun (temperature 5700 K), just below the → chromosphere and just above the → convective zone.
   The solar photosphere is a thin layer of roughly 300 km wide. Its temperature decreases uniformly with height, from about 6,600 K (pressure 0.868 → millibars) at its bottom, to about 4,400 K (pressure 125 mb), where it merges with the chromosphere. The photosphere has a “rice-grain” appearance, called → granulation, caused by rising (hot) and falling (cool) material in the → convective cells just below the photosphere. Other main features of the photosphere are → sunspots,
   → faculae, and → supergranulation.
   

2. The region of a star which gives rise to the continuum radiation emitted by the star.

See also: → photo- + → sphere.

1. The visible surface of the Sun (temperature 5700 K), just below the → chromosphere and just above the → convective zone.
   The solar photosphere is a thin layer of roughly 300 km wide. Its temperature decreases uniformly with height, from about 6,600 K (pressure 0.868 → millibars) at its bottom, to about 4,400 K (pressure 125 mb), where it merges with the chromosphere. The photosphere has a “rice-grain” appearance, called → granulation, caused by rising (hot) and falling (cool) material in the → convective cells just below the photosphere. Other main features of the photosphere are → sunspots,
   → faculae, and → supergranulation.
   

2. The region of a star which gives rise to the continuum radiation emitted by the star.

See also: → photo- + → sphere.

Of or pertaining to a → photosphere.

See also: → photosphere; → -ic.

Of or pertaining to a → photosphere.

See also: → photosphere; → -ic.

The process in green plants, algae, diatoms, and certain forms of bacteria by which carbohydrates are synthesized from carbon dioxide and water using light as an energy source. Most forms of photosynthesis release oxygen as a byproduct.

See also: → photo- + → synthesis.

The process in green plants, algae, diatoms, and certain forms of bacteria by which carbohydrates are synthesized from carbon dioxide and water using light as an energy source. Most forms of photosynthesis release oxygen as a byproduct.

See also: → photo- + → synthesis.

Magnitude defined for the combination of a photographic plate and a yellow filter, approximating the spectral sensitivity of the eye.

See also: → photo- + → visual; → magnitude.

Magnitude defined for the combination of a photographic plate and a yellow filter, approximating the spectral sensitivity of the eye.

See also: → photo- + → visual; → magnitude.

A detector usually constituted by a p-n junction. Upon irradiation, the electron-hole pairs which are created, are immediately separated by the strong electric field across the junction, and a current is generated, which is proportional to the number of incident photons per second.

See also: → photo- + → voltaic; → detector.

A detector usually constituted by a p-n junction. Upon irradiation, the electron-hole pairs which are created, are immediately separated by the strong electric field across the junction, and a current is generated, which is proportional to the number of incident photons per second.

See also: → photo- + → voltaic; → detector.

A sequence of two or more words arranged in a grammatical construction and acting as a unit in a → sentence.

Etymology (EN): From L.L. phrasis “diction,” from Gk. phrasis “speech, way of speaking, enunciation,” from phrazein “to express, tell,” from phrazesthai “to consider.”

Etymology (PE): Vatpâr, literally “part of speech,” from vat-, “to speak, say;” cf. (Kurd.) wittin “to speak, say,” → letter,

• pâr “piece, part, portion,” → partial.

A sequence of two or more words arranged in a grammatical construction and acting as a unit in a → sentence.

Etymology (EN): From L.L. phrasis “diction,” from Gk. phrasis “speech, way of speaking, enunciation,” from phrazein “to express, tell,” from phrazesthai “to consider.”

Etymology (PE): Vatpâr, literally “part of speech,” from vat-, “to speak, say;” cf. (Kurd.) wittin “to speak, say,” → letter,

• pâr “piece, part, portion,” → partial.

Pertaining to the physical sciences, especially physics.

See also: → physics + → -al.

Pertaining to the physical sciences, especially physics.

See also: → physics + → -al.

Same as → physisorption.

See also: → physical; → adsorption.

Same as → physisorption.

See also: → physical; → adsorption.

The branch of chemistry dealing with the relations between the physical properties of substances and their chemical composition and transformations.

See also: → physical; → chemistry.

The branch of chemistry dealing with the relations between the physical properties of substances and their chemical composition and transformations.

See also: → physical; → chemistry.

The state of a → physical system
regarding its temperature, density, pressure, etc. at a given time.

See also: → physical; → condition.

The state of a → physical system
regarding its temperature, density, pressure, etc. at a given time.

See also: → physical; → condition.

A fundamental → physical quantity that is generally believed to be both universal in nature and constant in time.

See also: → physical; → constant.

A fundamental → physical quantity that is generally believed to be both universal in nature and constant in time.

See also: → physical; → constant.

Any of basic physical quantities, such as mass, length, time, electric charge, and temperature in terms of which all other kinds of quantity can be expressed.

See also: → physical; → dimension.

Any of basic physical quantities, such as mass, length, time, electric charge, and temperature in terms of which all other kinds of quantity can be expressed.

See also: → physical; → dimension.

A theoretical principle which is deduced from particular observational facts regarding the behavior of matter. Physical laws are expressed by a general statement that a particular → physical phenomenon always occurs if certain → conditions are present.

See also: → physical; → law.

A theoretical principle which is deduced from particular observational facts regarding the behavior of matter. Physical laws are expressed by a general statement that a particular → physical phenomenon always occurs if certain → conditions are present.

See also: → physical; → law.

A real periodic variation in the rotation rate of a celestial object, as distinct from a → geometrical libration. In particular, slight oscillations in the → Moon’s rotation caused by the → gravitational attraction of the Earth on the → equatorial bulge of the Moon’s near side. The Moon’s physical libration is about 0.03° in longitude and about 0.04° in latitude.

See also: → physical; → libration.

A real periodic variation in the rotation rate of a celestial object, as distinct from a → geometrical libration. In particular, slight oscillations in the → Moon’s rotation caused by the → gravitational attraction of the Earth on the → equatorial bulge of the Moon’s near side. The Moon’s physical libration is about 0.03° in longitude and about 0.04° in latitude.

See also: → physical; → libration.

The branch of optics concerned with the wave properties of light, → diffraction, → polarization,
and other phenomena for which the ray approximation of → geometric optics is not valid. Also called → wave optics.

See also: → physical; → optics.

The branch of optics concerned with the wave properties of light, → diffraction, → polarization,
and other phenomena for which the ray approximation of → geometric optics is not valid. Also called → wave optics.

See also: → physical; → optics.

Any of a set of physical properties whose values determine the characteristics or behavior of a system; for example, → mass, → size, → temperature, → luminosity, etc.

See also: → physical; → parameter.

Any of a set of physical properties whose values determine the characteristics or behavior of a system; for example, → mass, → size, → temperature, → luminosity, etc.

See also: → physical; → parameter.

A natural → phenomenon that can be explained by → physical laws.

See also: → physical; → phenomenon.

A natural → phenomenon that can be explained by → physical laws.

See also: → physical; → phenomenon.

A physical → property that can be measured and/or calculated.

See also: → physical; → quantity.

A physical → property that can be measured and/or calculated.

See also: → physical; → quantity.

A set of physical components chosen to study their relations.

See also: → physical; → system.

A set of physical components chosen to study their relations.

See also: → physical; → system.

A specialist in → physics.

Etymology (EN): From physic, → physics, + → -ist.

Etymology (PE): Fizikdân, from fizik, → physics, + -dân “knower,” present stem of dânestan “to know,” → science.

A specialist in → physics.

Etymology (EN): From physic, → physics, + → -ist.

Etymology (PE): Fizikdân, from fizik, → physics, + -dân “knower,” present stem of dânestan “to know,” → science.

The science that deals with matter and energy and their interactions.

Etymology (EN): M.E. fisyk(e), phisik(e), from O.Fr. fisique, from L. physica (fem. sing.) “study of nature,” from Gk. physike episteme “knowledge of nature,” from fem. of physikos “pertaining to nature,” from physis “nature,” from phyein “to bring forth, produce, make to grow,”
Gk. phy- “to become;” L. fui “I was,” futurus “that is to be, future;” Ger. present first and second person sing. bin, bist; E. to be; O.Ir. bi’u “I am;” Lith. bu’ti “to be;” Rus. byt’ “to be.”

Etymology (PE): Loan from Fr. physique, as above.

The science that deals with matter and energy and their interactions.

Etymology (EN): M.E. fisyk(e), phisik(e), from O.Fr. fisique, from L. physica (fem. sing.) “study of nature,” from Gk. physike episteme “knowledge of nature,” from fem. of physikos “pertaining to nature,” from physis “nature,” from phyein “to bring forth, produce, make to grow,”
Gk. phy- “to become;” L. fui “I was,” futurus “that is to be, future;” Ger. present first and second person sing. bin, bist; E. to be; O.Ir. bi’u “I am;” Lith. bu’ti “to be;” Rus. byt’ “to be.”

Etymology (PE): Loan from Fr. physique, as above.

A kind of → adsorption in which the forces involved are → intermolecular  → van der Waals forces. Same as → physical adsorption. See also → chemisorption.

See also: Physi-, from → physical; → sorption.

A kind of → adsorption in which the forces involved are → intermolecular  → van der Waals forces. Same as → physical adsorption. See also → chemisorption.

See also: Physi-, from → physical; → sorption.