An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



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Number of Results: 457
Hawking radiation
  تابش ِ هاؤکینگ   
tâbeš-e Hawking (#)

Fr.: rayonnement de Hawking   

The radiation produced by a → black hole when → quantum mechanical effects are taken into account. According to quantum physics, large fluctuations in the → vacuum energy occurs for brief moments of time. Thereby virtual particle-antiparticle pairs are created from vacuum and annihilated. If → pair production happens just outside the event horizon of a black hole, as soon as these particles are formed they would both experience drastically different → gravitational attractions due to the sharp gradient of force close to the black hole. One particle will accelerate toward the black hole and its partner will escape into space. The black hole used some of its → gravitational energy to produce these two particles, so it loses some of its mass if a particle escapes. This gradual loss of mass over time means the black hole eventually evaporates out of existence. See also → Bekenstein formula, → Hawking temperature.

Named after the British physicist Stephen Hawking (1942-2018), who provided the theoretical argument for the existence of the radiation in 1974; → radiation.

Hawking temperature
  دمای ِ هاؤکینگ   
damâ-ye Hawking

Fr.: température de Hawking   

The temperature inferred for a → black hole based on the → Hawking radiation. For a → Schwarzschild black hole, one has TH = ħc3/(8πGMk) where ħ is the → reduced Planck's constant, c is the → speed of light, G is the → gravitational constant, M is the mass, and k is → Boltzmann's constant. The formula can approximately be written as: TH≅ 6.2 x 10-8 (Msun/M) K. Thus radiation from a solar mass black hole would be exceedingly cold, about 5 x 107 times colder than the → cosmic microwave background. Larger black holes would be colder still. Moreover, smaller black holes would have higher temperatures. A → mini black hole of mass about 1015 g would have TH≅ 1011 K.

Hawking radiation; → temperature.

Hayashi forbidden zone
  زنار ِ بژکم ِ هایاشی   
zonâr-e baſkam-e Hayashi

Fr.: zone interdite de Hayashi   

The region to the right the → Hayashi track, representing objects that cannot be in → hydrostatic equilibrium. Energy transport in these objects would take place with a → superadiabatic temperature gradient.

Hayashi track; → forbidden; → zone.

Hayashi phase
  فاز ِ هایاشی   
fâz-e Hayashi

Fr.: phase de Hayashi   

A period in the → pre-main sequence evolution of a low mass star during which the star has negligible nuclear energy production and low internal temperature. Hence energy transport inside the star takes place dominantly through → convection. The star contracts homologously and evolves in the → H-R diagram along the → hayashi track with decreasing → luminosity and nearly constant → effective temperature. The time taken by a star of mass M* to contract to radius R* along a Hayashi track is of the order of the → Kelvin-Helmholtz time: tKH = 107(M*/Msun)2/(R*/Rsun)3 yr.

Hayashi track; → phase.

Hayashi temperature
  دمای ِ هایاشی   
damâ-ye Hayashi

Fr.: température de Hayashi   

The minimum → effective temperature required for a → pre-main sequence star of given mass and radius to be in → hydrostatic equilibrium. This temperature delimits the boundary of the → Hayashi forbidden zone.

Hayashi track; → temperature.

Hayashi track
  تر ِ هایاشی   
tor-e Hayashi

Fr.: trajet de Hayashi   

The path on the → Hertzsprung-Russell diagram that is followed by a fully → convective  → pre-main sequence star to reach the → zero-age main sequence. Hayashi tracks for → low-mass stars are near vertical. At higher masses, stars become increasingly radiative as they contract and the Hayashi tracks are almost horizontal.

Named after the Japanese astrophysicist Chushiro Hayashi (1920-2010), who published his paper in 1961 (PASJ 13, 450); → track.


Fr.: hasard, risque, danger   

1) A danger that one can foresee but cannot avoid.
2) Something causing unavoidable danger, peril, risk, or difficulty.
3) The absence or lack of predictability; chance; uncertainty (

M.E. hasard, from O.Fr. hasard, hasart "game of chance played with dice," possibly from Sp. azar "an unfortunate card or throw at dice," postulated to derive from Ar. az-zahr "the die," but this etymology is controversial.

Âpé, from Av. au-pat-, "to fall down, off," from pat- "to fall, fly;" Proto-Ir. *pat- "to fall; fly; rise;" related to Pers. oftâdan "to fall; to befall; to happen," → fall. Pers. âfat "blight, pest, curse," may belong to this family.


Fr.: dangereux   

1) Full of risk; perilous; risky.
2. Dependent on chance (

Adj. from → hazard.

nezm (#)

Fr.: brume sèche   

A phenomenon where fine particles of → dust and/or → smoke suspended in the → atmosphere near Earth reduce the → visibility by → scattering light.

Maybe from M.E. *hase, O.E. hasu, variant of haswa "ashen, dusky."

Nezm "mist, fog, vapor."

HD 43317
HD 43317

Fr.: HD 43317   

A → hot star of → apparent visual magnitude 6.61 lying in the constellation → Orion. HD 43317 has a B3.5V → spectral type and has no detected binary companion. Its chemical surface abundances agree with the solar abundances, but with some co-rotating He abundance spots at the stellar surface. The CoRoT satellite revealed that HD 43317 is a → hybrid pulsator of → Slowly Pulsating B star (SPB)/ → Beta Cephei type. Its → rotation period is 0.897673(4) days. Zeeman signatures in the Stokes V profiles of HD 43317 are clearly detected and rotationally modulated, which proves that this star exhibits an oblique magnetic field. The strength of the dipolar magnetic field is of the order of 1 kG to 1.5 kG (Buysschaert et al., 2017, A&A 605, A104).

HD number.

HD 5980
HD 5980

Fr.: HD 5980   

A remarkable → binary star system composed of → massive stars that is associated with NGC 346, the largest → H II region + OB star cluster in the → Small Magellanic Cloud. HD 5980 is a rather complex system because it consists of at least three stars: two stars form an → eclipsing binary with a period of 19.266 days, while the third component, an → O star, is detected by means of a set of absorption lines. Whether or not the third star is physically bound to the eclipsing binary remains currently unclear. HD 5980 underwent an → LBV-type event in August 1994. Before the LBV eruption, both components of the eclipsing binary already showed emission lines in their spectra and were thus classified as → Wolf-Rayet stars. However, as shown by the analysis of the spectra taken during and after the LBV event, at least the star that underwent the eruption was not a classical, helium-burning, Wolf-Rayet object, but rather a WNha star. This means a rather massive star with substantial amounts of hydrogen present in its outer layers. These WNha stars have → stellar wind properties that are intermediate between those of extreme → Of stars and classical → WN Wolf-Rayet stars.

Henry Draper system; → number.

HD 93129
HD 93129

Fr.: HD 93129   

A multiple → O-type star in the → Trumpler 14 cluster, which consists of at least three components. The main component, HD 93129A, is of spectral type O2 If*, a very rare hot star and the closest known O2 star (Walborn et al. 2002). It is one of the most luminous stars known. HD 93129B, lying 2.7 arcsec apart, is an O3.5 V((f+)) type. Recent → HST observations have shown that component A is itself probably a double or → binary star with a separation of 0.055 arcsec.

HD number.

HD 97950
HD 97950

Fr.: HD 97950   

The core of the Galactic → giant H II region, → NGC 3603. It is a multiple object composed of several → massive stars with a collective → spectral type of around WN6+O5. One of the stars, → NGC 3603A-1, is a double-eclipsing binary with an orbital period of 3.77 days. The component masses are 116 Msun for the primary and 89 Msun for the secondary, respectively. The primary WN6ha component of A1 is the most massive star ever directly weighed. A second star, C, has newly been identified, which has been classified as an SB1 binary, and in which only the primary (WN6ha) component is visible. The third star, B, shows constant radial velocities over the observed time interval, and therefore is most likely not a binary. While the primary component of C might have a mass similar to or even greater than that of A1's primary, it is possible that star B, be the most massive member in NGC 3603 and, therefore, the most massive main-sequence star known in the Galaxy (Schnurr et al., 2008, MNRAS, 389, L38).

HD number.

HD number
  عدد ِ HD   
adad-e HD (#)

Fr.: numéro HD   

An identifying number assigned to the stars in the Henry Draper catalog. For example, the star Vega is HD 172167.

Henry Draper system; → number.

He II line
  خط ِ He II   
xatt-e He II

Fr.: raie He II   

Any of the → spectral lines arising from → singly ionized helium in the atmosphere of → O-type and → Wolf-Rayet stars. He II lines are chiefly in absorption, but some of them, such as 4686 Å occur in emission in hotter stars. The presence of He II → absorption lines separates O types from → B-type stars. A number of these He II lines belong to the → Pickering series involving transitions with → principal quantum numbern = 4 and higher. Although the n = 3-4 (4686 Å) transition also belongs to ionized helium and often occurs in these hot stars, it does not belong to the Pickering series because it has a lower landing level quantum number (n = 3). The same goes for n = 2-5 (4026 Å).

helium; → line.

He-strong star
  ستاره‌ی ِ هلیوم-سترگ   
setâre-ye heliom-sotorg

Fr.: étoile forte en hélium   

An early → B-type star showing helium lines with abnormally large equivalent widths. The surface → chemical abundances of He-strong stars are influenced by the presence of a strong → magnetic field, resulting in a He overabundance that typically varies in strength over the stellar surface. Examples include HR 735, HD 184927, and CPD-62°2124.

helium; → strong; → line.

He-weak star
  ستاره‌ی ِ هلیوم-نزار   
setâre-ye heliom-nazâr

Fr.: étoile faible en hélium   

A → chemically peculiar star with very weak helium lines. Examples include 3 Sco, HD 176582, HD 217833, HR 2949, and HD 21699. The He-weak stars do not form a homogeneous group. Some of them display intense Si, or Ti and Sr lines, and are considered a hot extension of the magnetic → Ap/Bp stars. Others show overabundances of P and Ga, typically noted for → HgMn stars. The star HD 139160 belongs to the non-magnetic subgroup of He-weak stars.

helium; → weak; → line.

sar (#)

Fr.: tête   

1) That part of anything that forms or is situated at the top, summit, or upper end.
2) The → nucleus and surrounding → coma of a → comet.

Head, from O.E. heafod "top of the body," also "chief person" (cf. O.S. hobid; Goth. haubiþ Ger. Haupt "head"), from PIE *kauput- "head;" cf. Skt. kaput-, kapala- "skull;" L. caput "head;" Pers. dialect Lori: kapu "head," kapulek "skull, middle of the head;" Kurd. kapol "skull;" Pashto kaparay "skull."

Sar "head," soru, sorun "horn" (karnâ "a trumpet-like wind instrument," variant sornâ "a wind instrument"); Mid.Pers. sar "head," sru "horn;" Av. sarah- "head," srū- "horn, nail;" cf. Skt. śiras- "head, chief;" Gk. kara "head," karena "head, top," keras "horn;" L. cornu "horn," cerebrum "brain;" P.Gmc. *khurnaz (Ger. Horn, Du. horen; cognate with E. horn, as above, from PIE *ker- "head, horn;" O.E. horn "horn of an animal," also "wind instrument;" E. horn); PIE base *ker- "head, horn, top, summit."

head-tail galaxy
  کهکشان ِ \"سر-دم\"   
kahkešân-e sar-dom

Fr.: galaxie tête-queue, ~ têtard   

A member of the class of radio galaxies (→ radio galaxy) that have a strong radio emission coming from a bright "head" and a more diffuse emission from a "tail." They are often found in clusters.

head; → tail; → galaxy.

del (#)

Fr.: cœur   

1) A hollow muscular organ that pumps the blood through the circulatory system by rhythmic contraction and dilation. In vertebrates there may be up to four chambers (as in humans), with two atria and two ventricles.
2) The heart regarded as the centre of a person's thoughts and emotions, especially love or compassion (

M.E. herte, from O.E. heorte "heart breast, soul, spirit, will, desire; courage; mind, intellect;" cf. O.Saxon herta, O.Frisian herte, O.Norse hjarta, Du. hart, O.H.G. herza, Ger. Herz; cognate with Pers. del, as below; PIE root *kerd- "heart."

Del "heart" (Pashtu z'rrah, zyah; Baluci zirde "heart, mind, soul;" Kurd. zar; Sogd. žyâwar); Mid.Pers. dil; Av. zərəd-; cf. Skt. hrd-; Gk. kardia; L. cor "heart" (Fr. cœur; Sp. corazon, It. cuore); Russ. serdtse; Arm. sirt; E. heart, as above.

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