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 be 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.
Hayashi forbidden zone
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.
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.
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.
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.
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.
1) Full of risk; perilous; risky.
Adj. from → hazard.
Fr.: brume sèche
Maybe from M.E. *hase, O.E. hasu, variant of haswa "ashen, dusky."
Nezm "mist, fog, vapor."
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.
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.
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.
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.
He II line
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).
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."
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.
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
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.
Energy possessed by a substance in the form of kinetic energy of atomic or molecular translation, rotation, or vibration.
Heat, from O.E. hætu, hæto, from P.Gmc. *khaitin- "heat," from *khaitaz "hot" (cf. O.N. hiti, Ger. hitze "heat," Goth. heito "fever").
Garmâ "heat, warmth," from Mid.Pers. garmâg; O.Pers./Av. garəma- "hot, warm;" cf. Skt. gharmah "heat;" Gk. thermos "warm;" L. formus "warm," fornax "oven;" P.Gmc. *warmaz; O.E. wearm; E. warm; O.H.G., Ger. warm; PIE *ghworm-/*ghwerm- "warm."
gonjâyeš-e garmâyi (#)
Fr.: capacité thermique, ~ calorifique
The ratio of an amount of heat, dQ, transferred to a body in some process to the corresponding change in the temperature of the body: C = dQ/dT. The heat capacity depends upon the mass of the body, its chemical composition, thermodynamic state, and the kind of process employed to transfer the heat. The word "capacity" may be misleading because it suggests the essentially meaningless statement "the amount of heat a body can hold," whereas what is meant is the heat added per unit temperature rise. → specific heat.
Fr.: conduction de chaleur
A type of → heat transfer by means of molecular agitation within a material without any motion of the material as a whole.
hambaz-e garmâ (#)
Fr.: convection de chaleur
A type of → heat transfer involving mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it.