An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



Number of Results: 7 Search : H II
compact H II region
  ناحیه‌ی ِ H IIی ِ همپک   
nâhiye-ye H II-ye hampak

Fr.: région H II compacte   

A Galactic H II region with an electron density ≥ 103 cm-3 and of a linear dimension ≤ 1 pc.

compact; → H II region.

density-bounded H II region
  ناحیه‌ی ِ H IIی ِ چگالی‌کرانمند   
nâhiye-ye H II-ye cagâli karânmand

Fr.: bornée par la densité   

An → H II region which lacks enough matter to absorb all → Lyman continuum photons of the → exciting star(s). In such an H II region a part of the ionizing photons escape into the → interstellar medium. See also → ionization-bounded H II region.

density; → bounded; → region.

  H II   

Fr.: H II   

Ionized → hydrogen, that is a proton nucleus that has lost its unique electron.

From H, abbreviation of hydrogen + II "two" in Roman number system, nomenclature convention representing singly ionized atoms.

H II galaxy
  کهکشان ِ H II   
kahkešân-e H II

Fr.: galaxie H II   

A low-mass and → metal-poor galaxy (1/30-1/3 Zsun), experiencing strong episodes of → star formation, characterized by the presence of bright → emission lines on a faint → blue continuum. The fact that H II galaxies are metal poor and very blue objects seems to suggest that they are young. Nevertheless, several studies show the existence of an → old stellar population underlying the present → star burst in most of these galaxies. This fact indicates that these objects are not young systems forming their first generation of stars. Same as → blue compact dwarf galaxy.
Spectroscopically, H II galaxies are essentially identical to the → giant H II regions found in nearby → irregular and → late-type galaxies. The correlation among structural parameters (→ H-beta → luminosity, → velocity dispersion, → linewidths) and between these parameters and the → chemical composition favors the interpretation of H II galaxies as giant H II regions in distant → dwarf irregular galaxies similar to the ones found nearby. Some examples of H II galaxies are: I Zw 18, SBS 0335-052, II Zw 33, UM 408.

H II; → galaxy.

H II region
  ناحیه‌ی ِ H II   
nâhiye-ye H II

Fr.: région H II   

A type of → emission nebulae composed of very hot gas (about 104 K), mainly ionized hydrogen, created by the ultraviolet radiation of → massive stars. H II regions originate when O or early-type stars, born in → giant molecular clouds, start heating up the cold gas, causing it to become → ionized and "glow". The effective temperatures of the → exciting stars are in the range 3 x 104 to 5 x 104 K, and throughout the nebula hydrogen is ionized. Helium is → singly ionized, and other elements are mostly singly or → doubly ionized. Typical densities in the H II region are of the order 10 to 102 cm-3, ranging as high as 104 cm-3. Internal motions occur in the gas with velocities of order 10 km s-1. The spectra of H II regions are mainly composed of strong → H Irecombination lines and → forbidden lines such as [O III], [O II], [N II]. See also → ionization-bounded H II region; → density-bounded H II region; → compact H II region; → ultracompact H II region.

H II; → region

H II region luminosity
  تابندگی ِ ناحیه‌ی ِ H II   
tâbandegi-ye nâhiye-ye H II

Fr.: luminosité de région H II   

The total number of → Lyman continuum photons emitted by an → H II region. It is usually derived using → radio continuum observations which are less affected by → interstellar extinction. The measured value is often a lower limit because of photon leakage from the H II region and absorption. See also → density-bounded H II region.

H II; → region; → luminosity.

ultracompact H II region
  ناحیه‌ی ِ H IIی ِ اولتر-همپک   
nâhiye-ye H II-ye ultar hampak

Fr.: région H II ultracompacte   

A very young → H II region fully embedded in its natal molecular cloud. Ultracompact H II regions are distinguished from classical H II regions by their small sizes (diameter ≥ 0.1 pc), high densities (Ne ≥ 105 cm-3), and high emission measures (EM ≥ 107 pc cm-6). Their typical ionized gas content is about 10-2  → solar masses, in contrast to classical H II regions with a mass of about 105 solar masses. Due to very important extinction, ultracompact H II regions are not accessible to visible wavelengths.

ultra-; → compact; → H II region.