An Etymological Dictionary of Astronomy and Astrophysics

Sir James Hopwood Jeans (1877-1946), English mathematical physicist, astrophysicist, and popularizer of science. He made important contributions to theoretical astrophysics, especially to the theory of stellar formation. → Jeans escape, → Jeans instability, → Jeans length, → Jeans mass, → Jeans scale, → Rayleigh-Jeans law, → Rayleigh-Jeans spectrum, → thermal Jeans mass, → turbulent Jeans mass, → Jeans escape.

Sir James Hopwood Jeans (1877-1946), English mathematical physicist, astrophysicist, and popularizer of science. He made important contributions to theoretical astrophysics, especially to the theory of stellar formation. → Jeans escape, → Jeans instability, → Jeans length, → Jeans mass, → Jeans scale, → Rayleigh-Jeans law, → Rayleigh-Jeans spectrum, → thermal Jeans mass, → turbulent Jeans mass, → Jeans escape.

A → thermal escape process by which the atmosphere of a planet loses gases to outer space. This form of thermal escape occurs because some molecules, especially low mass ones, are within the higher-velocity end of the → Maxwell-Boltzmann distribution. The possibility for the gases to escape occurs when the thermal energy of air molecules becomes greater than the → gravitational potential energy of the planet: (3/2)kT = (1/2)mv²  >  GmM/R where v is upward velocity of a molecule of mass m, M is the mass of the planet, and R is the radius of the planet at which thermal escape occurs.

The minimum velocity for which this can work is called the → escape velocity is:

v_e = (2MG/R)^1/2.

Hydrogen molecules (H₂) and helium, or their ions tend to have velocities high enough so that they are not bound by Earth’s gravitational field and are lost to space from the top of the atmosphere.

This process is important for the loss of hydrogen, a low-mass species that more easily attains escape speed at a given temperature, because v ~ (2kT/m)^1/2. As such, Jeans’ escape was likely influential in the atmospheric evolution of all the early terrestrial planets. Jeans' escape currently accounts for a non-negligible fraction of hydrogen escaping from Earth, Mars, and Titan, but it is negligible for Venus because of a cold upper atmosphere combined with relatively high gravity

(see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

See also: → Jeans; → escape.

A → thermal escape process by which the atmosphere of a planet loses gases to outer space. This form of thermal escape occurs because some molecules, especially low mass ones, are within the higher-velocity end of the → Maxwell-Boltzmann distribution. The possibility for the gases to escape occurs when the thermal energy of air molecules becomes greater than the → gravitational potential energy of the planet: (3/2)kT = (1/2)mv²  >  GmM/R where v is upward velocity of a molecule of mass m, M is the mass of the planet, and R is the radius of the planet at which thermal escape occurs.

The minimum velocity for which this can work is called the → escape velocity is:

v_e = (2MG/R)^1/2.

Hydrogen molecules (H₂) and helium, or their ions tend to have velocities high enough so that they are not bound by Earth’s gravitational field and are lost to space from the top of the atmosphere.

This process is important for the loss of hydrogen, a low-mass species that more easily attains escape speed at a given temperature, because v ~ (2kT/m)^1/2. As such, Jeans’ escape was likely influential in the atmospheric evolution of all the early terrestrial planets. Jeans' escape currently accounts for a non-negligible fraction of hydrogen escaping from Earth, Mars, and Titan, but it is negligible for Venus because of a cold upper atmosphere combined with relatively high gravity

(see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

See also: → Jeans; → escape.

An instability that occurs in a → self-gravitating  → interstellar cloud which is in → hydrostatic equilibrium. Density fluctuations caused by a perturbation may condense the material
leading to the domination of gravitational force
and the cloud collapse. The advent of instability involves a threshold called the → Jeans length or the → Jeans mass.

See also: → Jeans; → instability.

An instability that occurs in a → self-gravitating  → interstellar cloud which is in → hydrostatic equilibrium. Density fluctuations caused by a perturbation may condense the material
leading to the domination of gravitational force
and the cloud collapse. The advent of instability involves a threshold called the → Jeans length or the → Jeans mass.

See also: → Jeans; → instability.

The critical size of a homogeneous and isothermal interstellar cloud above which the cloud is unstable and must collapse under its own gravity. Below this size the cloud’s internal pressure is sufficient to resist collapse. The Jeans length is defined by:

λ_J = (π c_s²/Gρ)^1/2 = 0.2 pc (T/10 K)^1/2(n_H2/10⁴ cm^-3)^-1/2, where c_s is the → sound speed, G is the → gravitational constant, ρ is the gas density,
T is the gas temperature, and n_H2 is the
molecular hydrogen density.

See also: → Jeans; → length.

The critical size of a homogeneous and isothermal interstellar cloud above which the cloud is unstable and must collapse under its own gravity. Below this size the cloud’s internal pressure is sufficient to resist collapse. The Jeans length is defined by:

λ_J = (π c_s²/Gρ)^1/2 = 0.2 pc (T/10 K)^1/2(n_H2/10⁴ cm^-3)^-1/2, where c_s is the → sound speed, G is the → gravitational constant, ρ is the gas density,
T is the gas temperature, and n_H2 is the
molecular hydrogen density.

See also: → Jeans; → length.

The → minimum mass for an → interstellar cloud below which the → thermal pressure of the gas prevents its → collapse under the force of its own → gravity. It is given by the formula M_J = (π^5/2 / 6) G ^-3/2ρ₀^-1/2c_s³, where G is the → gravitational constant, ρ₀ the initial → density, and c_s the isothermal → sound speed.

It can be approximated to M_J
~ 45 (T_K) ^3/2 (n_cm^-3) ^-1/2 in units of solar masses, where T_K is the temperature in → Kelvin, and n_cm^-3 the gas density per cm³. High density favors collapse, while high temperature favors larger Jeans mass. See also: → thermal Jeans mass, → turbulent Jeans mass.

See also: → Jeans; → mass.

The → minimum mass for an → interstellar cloud below which the → thermal pressure of the gas prevents its → collapse under the force of its own → gravity. It is given by the formula M_J = (π^5/2 / 6) G ^-3/2ρ₀^-1/2c_s³, where G is the → gravitational constant, ρ₀ the initial → density, and c_s the isothermal → sound speed.

It can be approximated to M_J
~ 45 (T_K) ^3/2 (n_cm^-3) ^-1/2 in units of solar masses, where T_K is the temperature in → Kelvin, and n_cm^-3 the gas density per cm³. High density favors collapse, while high temperature favors larger Jeans mass. See also: → thermal Jeans mass, → turbulent Jeans mass.

See also: → Jeans; → mass.

Same as → Jeans length.

See also: → Jeans; → scale.

Same as → Jeans length.

See also: → Jeans; → scale.

1. A soft somewhat elastic food product made usually with gelatin or pectin; especially, a fruit product made by boiling sugar and the juice of fruit.
   

   2. A substance resembling jelly in consistency (Merriam-Webster.com).

Etymology (EN): M.E. gely, from O.Fr. gelee “a jelly,” from L. gelare “to freeze, congeal, stiffen,” from PIE *gel- “cold; to freeze.”

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

1. A soft somewhat elastic food product made usually with gelatin or pectin; especially, a fruit product made by boiling sugar and the juice of fruit.
   

   2. A substance resembling jelly in consistency (Merriam-Webster.com).

Etymology (EN): M.E. gely, from O.Fr. gelee “a jelly,” from L. gelare “to freeze, congeal, stiffen,” from PIE *gel- “cold; to freeze.”

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

Any of various marine coelenterates of a soft, gelatinous structure, especially one with an umbrella like body and long, trailing tentacles; medusa (dictionary.com).

Etymology (EN): → jelly; → fish.

Etymology (PE): Medusâ, from Gk. Medousa, literally “guardian,” from medein “to protect, rule over.”

Any of various marine coelenterates of a soft, gelatinous structure, especially one with an umbrella like body and long, trailing tentacles; medusa (dictionary.com).

Etymology (EN): → jelly; → fish.

Etymology (PE): Medusâ, from Gk. Medousa, literally “guardian,” from medein “to protect, rule over.”

A type of galaxy exhibiting “tentacles” (tails) of material that appear to be stripped from the main body of the galaxy, making it resemble a jellyfish. Such type of galaxies occur in → galaxy clusters and are produced by a process called → ram pressure stripping. The mutual → gravitational attraction between galaxies causes them to fall at high speed into the clusters, where they encounter a hot → intracluster medium (ICM) with dense gas. The falling galaxy feels a powerful wind, forcing tails of gas out of the galaxy’s disk and triggering → starbursts within it.

Jellyfish galaxies have mainly been observed in nearby clusters (e.g., Virgo, Coma, A1367, A3627, Shapley). A few examples have been identified in clusters at → redshifts z ~ 0.2-0.4, and there is accumulating evidence for a correlation between the efficiency of the stripping phenomenon and the presence of shocks and strong gradients in the X-ray → intergalactic medium

(Poggianti et al., 2016, AJ 151, 78).

See also: → jellyfish; → galaxy.

A type of galaxy exhibiting “tentacles” (tails) of material that appear to be stripped from the main body of the galaxy, making it resemble a jellyfish. Such type of galaxies occur in → galaxy clusters and are produced by a process called → ram pressure stripping. The mutual → gravitational attraction between galaxies causes them to fall at high speed into the clusters, where they encounter a hot → intracluster medium (ICM) with dense gas. The falling galaxy feels a powerful wind, forcing tails of gas out of the galaxy’s disk and triggering → starbursts within it.

Jellyfish galaxies have mainly been observed in nearby clusters (e.g., Virgo, Coma, A1367, A3627, Shapley). A few examples have been identified in clusters at → redshifts z ~ 0.2-0.4, and there is accumulating evidence for a correlation between the efficiency of the stripping phenomenon and the presence of shocks and strong gradients in the X-ray → intergalactic medium

(Poggianti et al., 2016, AJ 151, 78).

See also: → jellyfish; → galaxy.

1. A stream of a liquid, gas, or small solid particles forcefully shooting forth from a nozzle, orifice, etc.
   
2. Fountain-like formations of gas and/or dust gushing out from compact regions of some astronomical objects. → astrophysical jets.
   
3. Meteo.: A common abbreviation for → jet stream.
   
4. Shortened of → jet plane.

Etymology (EN): Jet, from M.Fr. jeter “to throw,” from V.L. *jectare, alter. of L. jactare, from jac- “throw” + -t- frequentative suffix + -are infinitive suffix; PIE base *ye- “to do” (cf. Gk. hienai “to send, throw;” Hittite ijami “I make”).

Etymology (PE): Ešân, from ešândan, → eject; šân contraction of ešân.

1. A stream of a liquid, gas, or small solid particles forcefully shooting forth from a nozzle, orifice, etc.
   
2. Fountain-like formations of gas and/or dust gushing out from compact regions of some astronomical objects. → astrophysical jets.
   
3. Meteo.: A common abbreviation for → jet stream.
   
4. Shortened of → jet plane.

Etymology (EN): Jet, from M.Fr. jeter “to throw,” from V.L. *jectare, alter. of L. jactare, from jac- “throw” + -t- frequentative suffix + -are infinitive suffix; PIE base *ye- “to do” (cf. Gk. hienai “to send, throw;” Hittite ijami “I make”).

Etymology (PE): Ešân, from ešândan, → eject; šân contraction of ešân.

An → engine that works by taking in air at the front and expelling exhaust gases at the rear so that the reaction to this exhaust propels the vehicle forward.

See also: → jet; → engine.

An → engine that works by taking in air at the front and expelling exhaust gases at the rear so that the reaction to this exhaust propels the vehicle forward.

See also: → jet; → engine.

The mechanism whereby → astrophysical jets are thrown out of → accretion disks . Observed correlations between emission from the accretion disk and from the jet provide evidence that the jets are launched from the disks directly.

As the energy emitted from the jets is a → synchrotron radiation, the presence of a → magnetic field is deduced for the ejection.

The most promising model for such “accretion-ejection” structures is based on a scenario where a large-scale magnetic field threads an accretion disk. This model, using a → magnetohydrodynamic (MHD) approach, shows that the magnetic field can azimuthally brake the matter inside the disk (carrying off → angular momentum allowing accretion) and accelerate matter above the disk surface. The → collimation of the flow is achieved via → magnetic tension due to the presence of a → toroidal component of the magnetic field. The magnetic field provides an effective alternative to the radially outward transport of disk angular momentum by → viscosity. The interaction of the magnetic structure with the disk plasma can create a MHD → Poynting flux leaving the disk along the magnetic surface. This energy flux can then be converted into → kinetic energy of the matter within the jet. Because the → mass density in the jet is smaller than in the disk, it is thereby possible to reach high → terminal velocities for a given amount of angular momentum removed from the disk

(Casse & Keppens, 2002, ApJ 581, 988, and references therein).

See also: → jet; → launch; → -ing.

The mechanism whereby → astrophysical jets are thrown out of → accretion disks . Observed correlations between emission from the accretion disk and from the jet provide evidence that the jets are launched from the disks directly.

As the energy emitted from the jets is a → synchrotron radiation, the presence of a → magnetic field is deduced for the ejection.

The most promising model for such “accretion-ejection” structures is based on a scenario where a large-scale magnetic field threads an accretion disk. This model, using a → magnetohydrodynamic (MHD) approach, shows that the magnetic field can azimuthally brake the matter inside the disk (carrying off → angular momentum allowing accretion) and accelerate matter above the disk surface. The → collimation of the flow is achieved via → magnetic tension due to the presence of a → toroidal component of the magnetic field. The magnetic field provides an effective alternative to the radially outward transport of disk angular momentum by → viscosity. The interaction of the magnetic structure with the disk plasma can create a MHD → Poynting flux leaving the disk along the magnetic surface. This energy flux can then be converted into → kinetic energy of the matter within the jet. Because the → mass density in the jet is smaller than in the disk, it is thereby possible to reach high → terminal velocities for a given amount of angular momentum removed from the disk

(Casse & Keppens, 2002, ApJ 581, 988, and references therein).

See also: → jet; → launch; → -ing.

An airplane moved by → jet propulsion.

Etymology (EN): → jet; plane, short for airplane, from Fr. aeroplane, from aero-, → air, + plane feminine of plan “flat, level,” from L. planus, perhaps by association with forme plane; apparently coined and first used by Fr. sculptor and inventor Joseph Pline in 1855.

Etymology (PE): → jet; havâpeymâ “airplane,” from havâ, → air, + peymâ “travelling; traveller,” from peymudan, peymâyidan “to travel, traverse, pass over,”
from Mid.Pers. patmudan, paymudan “to measure (against),” from *pati-māya-. The first element *pati- “against, back” (cf. Mod.Pers. pâd- “against, contrary to;” Mid.Pers. pât-; O.Pers. paity “agaist, back, opposite to, toward, face to face, in front of;” Av. paiti; Skt. práti “toward, against, again, back, in return, opposite;” Pali pati-; Gk. proti, pros “face to face with, toward, in addition to, near;” PIE *proti). The second element from *mā- “to measure;” O.Pers./Av. mā(y)- “to measure;” cf. Skt. mati “measures,” matra- “measure;” Gk. metron “measure;” L. metrum; PIE base *me- “to measure.” Apart from peymâ, several other terms in Mod.Pers. are related to this second element, which occurs also as mun, mân, man, mâ, mu, and mây:
pirâmun “perimeter,” âzmun, âzmây- “test, trial,”
peymân “measuring, agreement,” peymâné “a measure; a cup, bowl,” man “a measure weighing forty seers),”
nemudan, nemâ- “to show, display,”
âmâdan, âmây- “to prepare.”

An airplane moved by → jet propulsion.

Etymology (EN): → jet; plane, short for airplane, from Fr. aeroplane, from aero-, → air, + plane feminine of plan “flat, level,” from L. planus, perhaps by association with forme plane; apparently coined and first used by Fr. sculptor and inventor Joseph Pline in 1855.

Etymology (PE): → jet; havâpeymâ “airplane,” from havâ, → air, + peymâ “travelling; traveller,” from peymudan, peymâyidan “to travel, traverse, pass over,”
from Mid.Pers. patmudan, paymudan “to measure (against),” from *pati-māya-. The first element *pati- “against, back” (cf. Mod.Pers. pâd- “against, contrary to;” Mid.Pers. pât-; O.Pers. paity “agaist, back, opposite to, toward, face to face, in front of;” Av. paiti; Skt. práti “toward, against, again, back, in return, opposite;” Pali pati-; Gk. proti, pros “face to face with, toward, in addition to, near;” PIE *proti). The second element from *mā- “to measure;” O.Pers./Av. mā(y)- “to measure;” cf. Skt. mati “measures,” matra- “measure;” Gk. metron “measure;” L. metrum; PIE base *me- “to measure.” Apart from peymâ, several other terms in Mod.Pers. are related to this second element, which occurs also as mun, mân, man, mâ, mu, and mây:
pirâmun “perimeter,” âzmun, âzmây- “test, trial,”
peymân “measuring, agreement,” peymâné “a measure; a cup, bowl,” man “a measure weighing forty seers),”
nemudan, nemâ- “to show, display,”
âmâdan, âmây- “to prepare.”

Powerful, forward thrust that results from the rearward expulsion of a jet of fluid, especially propulsion by jet engines.

See also: → jet; → propulsion.

Powerful, forward thrust that results from the rearward expulsion of a jet of fluid, especially propulsion by jet engines.

See also: → jet; → propulsion.

Meteo.: An area of relatively strong winds that are concentrated in a narrow band in the upper troposphere of the middle latitudes and subtropical regions of the Northern and Southern Hemispheres.

See also: → jet; → stream.

Meteo.: An area of relatively strong winds that are concentrated in a narrow band in the upper troposphere of the middle latitudes and subtropical regions of the Northern and Southern Hemispheres.

See also: → jet; → stream.

Same as → Hebrew calendar

Etymology (EN): Jewish, adj. of jew, from M.E. jewe, giu, gyu, ju, from O.Fr. juiu, juieu, gyu,
from L.L. judeus, from L. juaeus, from Gk. ioudaios, from Aramaic yehudhai, from Heb. yəhudhi “Jew,”
from Yəhudah “Judah,” literally “celebrated,” name of Jacob’s fourth son and of the tribe descended from him; → calendar.

Etymology (PE): Gâhšomâr→ calendar; yahud, from Ar., from Heb., as above.

Same as → Hebrew calendar

Etymology (EN): Jewish, adj. of jew, from M.E. jewe, giu, gyu, ju, from O.Fr. juiu, juieu, gyu,
from L.L. judeus, from L. juaeus, from Gk. ioudaios, from Aramaic yehudhai, from Heb. yəhudhi “Jew,”
from Yəhudah “Judah,” literally “celebrated,” name of Jacob’s fourth son and of the tribe descended from him; → calendar.

Etymology (PE): Gâhšomâr→ calendar; yahud, from Ar., from Heb., as above.