compact H II region
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.
density-bounded H II region
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.
Fr.: H I
Atomic or → neutral hydrogen.
From H, abbreviation of hydrogen + I "one" in Roman number system, nomenclature convention representing neutral atoms.
H I region
nâhiye-ye H I
Fr.: région H I
A region of neutral (atomic) hydrogen in interstellar space. At least 95 percent of interstellar hydrogen is H I. It emits radio waves that are 21 cm long.
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
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
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.
H II region
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 I→ recombination 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 luminosity
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.
ultracompact H II region
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.