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 ?
It's particularly fascinating to zoom into a galaxy such as M81 at high resolution and then search for M51, which cinematically zooms you out of M81 and into M51 to reveal tiny HII regions in this popular target.
This study will constrain the key physical parameters for the SF process on only 50pc scales - the scale of large HII regions and their predecessors, giant molecular clouds.
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.
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.
They were the first to suggest that this object is a planetary nebula, although it was previously listed as object 200 in the 1959 Catalogue of HII Regions, a second catalog by Stewart Sharpless, which extended and superseded his first.
OB stars are associated with HII regions and, though not mentioned much in this book, PAH (polycyclic aromatic hydrocarbons) emissions.
A wide field and low power will give the best view, and with a large telescope you should be able to pick out the HII regions and the stars that make up the arms of this spiral, and give it its common name, the Pinwheel Galaxy.
Detail within the spiral arms consists of HII regions, star clouds, star clusters, and dark lanes of dust and gas.
These are huge, star-rich HII regions and a few faint Milky Way foreground stars.
In the old theoretical model, a high-mass star forms and the HII region lights up and begins to expand.