H II region

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H II region

A region of predominantly ionized hydrogen in interstellar space, existing mainly in discrete clouds. The ionization is usually caused by photoionization by ultraviolet photons in regions of recent star formation, but cosmic rays, X-rays, or shock waves in the medium may sometimes be responsible. In comparison with the 21-cm radio emission of neutral hydrogen in H I regions, the ionized hydrogen of H II regions emits radio waves by bremsstrahlung (thermal emission) and recombination line emission; the ionized hydrogen also emits recombination lines in the infrared, ultraviolet, and optical, the latter making an H II region appear as an emission nebula. Younger H II regions are often roughly spherical with a sharply delineated boundary (see Strömgren sphere). Their size is usually less than 200 parsecs, the largest being relatively constant in diameter from galaxy to galaxy. By studying the apparent diameters of the H II regions in a distant galaxy, the distance to the galaxy can be estimated. See also extragalactic H II region; interstellar medium.

H II region

[¦āch ′tü ‚rē·jən]
(astronomy)
A region of interstellar space occupied by gas that is largely atomic hydrogen and mostly ionized.
References in periodicals archive ?
In the old theoretical model, a high-mass star forms and the HII region lights up and begins to expand.
NGC 2035 is an HII region, or emission nebula, consisting of clouds of gas that glow due to the energetic radiation given off by young stars.
40, the source was considered an SNR; otherwise, it was assumed to be an HII region.
1m and Dec +57[degrees] 30' is IC 1396, a huge and complex HII region of star cluster and nebulosity.
To the southwest of NGC 7000 lies another emission nebula which in reality is part of the same large HII region but separated by a dark dust lane.
He used this for his initial research work at HartRAO which was in the field of HII regions and recombination lines at 2.
The 94 papers that emerged cover the interstellar content of the galactic center region; star formation, young stars, and HII regions in the central molecular zone; stellar dynamics and stellar distributions in the presence of the galactic black hole; the supermassive black hole and accretion onto Sagittarius A*; high-energy diagnostics of galactic nuclear activity; and supermassive black holes and stellar feedback in nuclear environments.
The nebula was also catalogued by Stewart Sharpless, an astronomer at the United States Naval Observatory in Flagstaff, Arizona who examined the Palomar Observatory Sky Survey plates (POSS) for HII regions.
OB stars are associated with HII regions and, though not mentioned much in this book, PAH (polycyclic aromatic hydrocarbons) emissions.
The ration of the optical [SII] emmisson line to the Halpha emission line is the primary means to identify supernova remnants (SNR) and HII regions and confirm the radio identification of each source.