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	H I region - Wikipedia, the free encyclopedia
		An H I region is an interstellar cloud composed of neutral atomic hydrogen (H
		These regions are non-luminous but are detectable because molecules present in small quantities emit radiation in the 21-cm (1,420 MHz) region of the radio-spectrum, even at low temperatures.
		At ionization fronts, where HI regions collide with expanding ionized gas (such as an H II region), the latter glows brighter than it otherwise would.
	en.wikipedia.org /wiki/H_I_region (128 words)

	H II region - Wikipedia, the free encyclopedia
		H II regions can be seen out to considerable distances in the universe, and the study of extragalactic H II regions is important in determining the distance and chemical composition of other galaxies.
		Before then, the dense regions which contain the new stars are often seen in silhouette against the rest of the ionised nebula — these dark patches are known as Bok globules, after astronomer Bart Bok, who proposed in the 1940s that they might be stellar birthplaces.
		This is the case for NGC 604, a giant H II region in the Triangulum Galaxy.
	en.wikipedia.org /wiki/H_II_region (2386 words)

	Ph 344 Lab 5
		Also prominent are emission lines of N II (once-ionized nitrogen) and O II (once-ionized oxygen), which result from the excitation of electrons in these ions by collisions with free electrons, followed by radiative de-excitation (the emission of a photon).
		H II regions are discussed briefly in section 12.3 of Carroll and Ostlie (pp.
		The Orion Nebula is a ``bubble'' of H II on the near side of a larger cloud of denser atomic and molecular hydrogen.
	www.physics.rutgers.edu /~pryor/ph344/lab5/lab5.html (1748 words)

	H II region concept from the Astronomy knowledge base (Site not responding. Last check: 2007-11-05)
		Most H II regions are red and arise from hot blue O and B stars, whose ultraviolet light can ionize all the hydrogen for dozens or even hundreds of light-years in every direction.
		H II regions occur near stars with high luminosities and high surface temperatures.
		H II condensation (2 facts) - A high-density H II region.
	www.site.uottawa.ca:4321 /astronomy/HIIregion.html (298 words)

	H II Regions (Site not responding. Last check: 2007-11-05)
		The glowing H II regions are called emission nebulae.
		Areas in space which are luminous with the emission spectrum of ionized hydrogen are called H II regions.
		H II regions are associated with young star clusters, and are useful as a part of astronomical distance measurement.
	hyperphysics.phy-astr.gsu.edu /hbase/starlog/h2reg.html (194 words)

	GK Workshop - M.S. Oey & C.J. Clarke
		II is observed in the variation between arm and interarm populations of nebulae in grand-design spiral galaxies.
		II regions represent the current burst of star formation, leaving the interarm regions with significant populations of aging nebulae remaining in the wake of the arms.
		II LF is the same between Figs. 2a and 2b in the regime dominated by unsaturated objects, but that the high-L tail now shows an extremely steep dropoff in Fig. 2b where the saturated population is missing.
	www.astro.uni-bonn.de /~webstw/ws98/oey_t.html (1966 words)

	Preprints, Mar/Apr 99 (Site not responding. Last check: 2007-11-05)
		CO, and CS emission is indicative of a density gradient, with the peak density located in front of the head of the cometary H II region.
		In addition to the exciting star of the H II region, we identify a second star toward the head of the H II region with an extinction similar to that of the exciting star; this appears to be a second OB star in the H II region.
		Given the evidence that the G29.96 H II region exists in a gradient of molecular gas density which peaks in front of the head of the H II region, we favor the champagne flow model for this region.
	cfa-www.harvard.edu /cfa/ep/preprint/marapr99/4780.html (439 words)

	ADIL Project Description (Site not responding. Last check: 2007-11-05)
		The results of the observations are interpreted in terms of a model for the absorption line gas in which some of the OH and H I gas has been accelerated by the passage of a shock front and some of the gas lies outside the shock front at the undisturbed velocity of the molecular cloud.
		This component shows a large peak in opacity at the eastern edge of the H II region near a previously observed molecular cloud core; the H2 mass of this component is estimated to be about 13 solar masses.
		The increased strength of the shocked H I component is probably due to an increased relative abundance of H I from the photodissociation of H2.
	imagelib.ncsa.uiuc.edu /description/95.DR.04 (624 words)

	Embedded clusters in giant extragalactic HII Regions. I. BVRHalpha photometry
		Photometry in BVR continuum bands and in the emission line of Halpha+ [N II] are presented for a sample of H II complexes in disk, spiral arm and nuclear regions of galaxies NGC 1365, 1566, 2366, 2903, 2997, 3351, 4303, 4449, and 5253.
		Colors and Halpha+[N II] equivalent widths of nuclear H II regions are found to be distinctly different from disk H II regions, suggesting different star formation histories.
		E, W, N, and S preceeding the numbers indicate respectively whether the H II region is to the east, west, north or south of the nucleus of the galaxy.
	www.cs.wisc.edu /niagara/data/nasa/AJ_108_1276.xml (1009 words)

	G5.HTML (Site not responding. Last check: 2007-11-05)
		The radio continuum maps show that the H II region is composed of a bright component immersed in a diffuse emission of 6' (or 21 pc) radius.
		The VLA H I 21 cm line maps show that there is an H I cloud in contact with the steep western bounday.
		Therefore, G5.48-0.24 appears to be an H II region-molecular cloud complex where the formation of massive stars occurred recently.
	www.aoc.nrao.edu /~myun/papers/g5.html (339 words)

	Interstellar Medium and the Milky Way
		H II regions are regions of hot (several thousand K), thin hydrogen emission nebulae that glow from the fluorescence of hydrogen atoms.
		The composition and conditions inside the H II regions are easier to determine and understand than for stars, so H II regions provide a valuable tool for understanding the history of star formation in a galaxy.
		H II regions also provide a convenient way to map the structure of a galaxy because they are so large and luminous.
	www.astronomynotes.com /ismnotes/s3.htm (2498 words)

	2MASS Atlas Image Gallery
		To the south of the NGC 2024 region lies NGC 2023 (a well-studied photo-dissociation region) and the Horsehead Nebula (cf.
		The supershell is in an interarm region near the Carina spiral arm in the Milky Way, and the presence of molecular gas and HII regions along the supershell indicate that star formation may have been initiated by the supershell's expansion.
		H II regions are areas of ionized gas, where electrons have been stripped away from once-neutral hydrogen (H) and other atoms, by the intense ultraviolet radiation from hot, young stars.
	www.ipac.caltech.edu /2mass/gallery/images_hii.html (6096 words)

	Ultra-compact SMC HII regions (Site not responding. Last check: 2007-11-05)
		A preliminary analysis of these H II regions show that they are extremely compact (the largest one is less than 3 pc across), extremely reddened (much higher than the typical SMC H II region), and well populated star forming regions.
		The importance of studying the ultra-compact H II regions in the SMC is multi-fold.
		Second, the small sizes of these regions, compared to the average SMC H II regions (50 to 270 pc) are indicative of a very recent star formation process (Elmegreen 2000).
	www.bo.astro.it /report02/node31.html (221 words)

	ASTR 3220 Homework 5
		ANS: An H II region is a region of hot ionized gas, comprised mainly of (singly) ionized hydrogen in which the hydrogen atoms have lost their electrons.
		H II regions are often found surrounding hot O or B stars, which are capable of providing sufficient ultraviolet radiation to ionize the hydrogen atoms.
		Once the atom becomes ionized (as in H II regions), the proton and electron are no longer bound together and their spins no longer interact.
	casa.colorado.edu /~skinners/ast3220_syll/hw/hw5_solns.html (1032 words)

	Notes on individual objects (Site not responding. Last check: 2007-11-05)
		This is a ``composite object'', with a relatively strong H II region and a weak Seyfert 1.9 nebulosity.
		The northern galaxy is a H II region (Wu et al.
		The spectrum is obviously ``composite'' with a H II region and a Seyfert component.
	www.obs-hp.fr /www/preprints/pp131/node5.html (6927 words)

	[No title]
		ADC Dataset: J/A+A/291/261 -- The occurrence of H2O masers in HII regions (1994)
		This value is greater than that expected from the ratio between the lifetime of the H II region 10^6 yr and that of the maser phase, even assuming a maser lifetime as long as 10^5 yr.
		The analysis of a homogeneous sample of classical H II regions allows to confirm in a quantitative fashion the well known result that the maser phenomenon occurs during the earliest phase of the evolution of a massive star and that it as the H II region evolves into the diffuse stage.
	archive.astro.umd.edu /archive/journals/A+A/291/261 (435 words)

	H II Region Fluxes and Diameters at 4.85 GHz (Site not responding. Last check: 2007-11-05)
		H II Region Fluxes and Diameters at 4.85 GHz
		We present an all sky catalog of H II regions larger than \sim 3' at 4.85 GHz.
		Nearly 35% of these H II regions have fluxes measured from these surveys for the first time.
	www.aas.org /publications/baas/v28n4/aas189/abs/S106002.html (107 words)

	Ultracompact H II region (from nebula) -- Encyclopædia Britannica
		Recently “ultracompact” H II regions have been discovered inside the molecular clouds in which they were formed.
		Land, Edwin H. The inventor of instant photography, in the form of the Polaroid Land camera, was Edwin H. Land.
		This region encompasses the coast of the Pacific Ocean and ranges from tropical islands to the freezing temperatures.
	www.britannica.com /eb/article-61227?tocId=61227 (811 words)

	HII region
		A volume of space where the hydrogen in the interstellar medium is in an ionized rather than a neutral state.
		Generally, these are regions where hot, blue OB stars are pouring large amounts of ultraviolet radiation into the surrounding cloud from which they were recently formed.
		HII regions have typical kinetic temperatures of 10,000 to 20,000 K, and a density of about 10 atoms/cm³.
	www.daviddarling.info /encyclopedia/H/HII.html (227 words)

	Galaxy Formation Research Home Page (Site not responding. Last check: 2007-11-05)
		(1998) studied the regions within photoionized nebulae in which the ionizing radiation from the central source is blocked by an intervening neutral, opaque clump - the shadow zones.
		The shadow gas can be partially ionized by diffuse ionizing radiation from the unshadowed H II region which surrounds it and is dynamically affected by the pressure of that surrounding gas.
		An analytical model for the final, steady configuration of the shadows was derived involving a neutral core surrounded by a layer which is ionized by the diffuse radiation of the surrounding H II region.
	galileo.as.utexas.edu /research_shadows.html (154 words)

	H II region populations in Seyfert galaxies. I. Database
		J/ApJS/101/287 J/ApJS/101/287 H II Regions in Seyfert galaxies.
		Data on positions, effective diameters, and absolute fluxes of H II regions on 21 Seyfert spiral galaxies are presented together with the Halpha + [N II] images and identification charts.
		The X,Y positions of the H II regions.
	www.cs.wisc.edu /niagara/data/nasa/ApJS_101_287.xml (164 words)

	Universe Today - High Mass Stars Form From Discs Too
		Adjacent to the south-western edge of the H II region, there is a huge cloud of molecular gas which is believed to be a site of ongoing star formation.
		Against the nebular background of the H II region a large opaque silhouette is seen associated with an hourglass shaped reflection nebula.
		The astronomers have observed the region in the rotational transitions of the 12CO, 13CO and C18O molecules, and in the adjacent continuum at 3 mm.
	www.universetoday.com /am/publish/high_mass_stars_form_discs.html?1352004 (1817 words)

	Oey & Clarke 1998 (Site not responding. Last check: 2007-11-05)
		Observed variations in the H II region luminosity function (HII LF) seen in spiral arm vs. interarm regions, and different galactic Hubble type, can be explained simply by evolutionary effects and maximum number of ionizing stars per cluster.
		We investigate the evolution of the HII LF, as determined by stellar main-sequence lifetimes and ionizing luminosities, for a single burst case and continuous creation of the nebular population.
		Shallower HII LF slopes measured for the arms of spiral galaxies can be explained as a composite slope, expected for a zero-age burst population, whereas the interarm regions tend to be dominated by evolved rich clusters described by a single, steeper slope.
	www.astro.lsa.umich.edu /~msoey/publications/OC98.php (192 words)

	tenorio-tagle82 (Site not responding. Last check: 2007-11-05)
		ABSTRACT:One- and two-dimensional calculations of the gasdynamical evolution of H II regions excited by an evolving OB association are presented.
		However, for large values of the ambient density, phase II can be inhibited and the H II region will continue to expand according to the classical formulation, even though the ionizing photon flux from the source continues to decay.
		The latter evolution will make the H II region appear as a ring-like nebula for a considerable period of time.
	www.cita.utoronto.ca /~mamd/BIB/tenorio-tagle82.html (192 words)

	Jahrbuch-CD der MPG 2003 - The massive star-forming region
		The region contains an H II region, similar to 30 arcsec across, and two Class II methanol maser sites, which are separate from the H II region.
		We show that the spectral type of the star powering the H II region is B0.
		For at least one of these maser sites, this shows a real deficiency in the radio continuum flux, which we suggest is an indication that the star is in an early stage of development, before its H II region becomes visible, or it is a multiple intermediate mass star system.
	www.mpg.de /forschungsergebnisse/wissVeroeffentlichungen/archivListenJahrbuch/2002/21/publZIM120.html (252 words)

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