<P> An H II region or HII region is a region of interstellar atomic hydrogen that is ionized . It is typically a cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic cm . The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas - Claude Fabri de Peiresc by telescope, the first such object discovered . </P> <P> They may be of any shape, because the distribution of the stars and gas inside them is irregular . The short - lived blue stars created in these regions emit copious amounts of ultraviolet light that ionize the surrounding gas . H II regions--sometimes several hundred light - years across--are often associated with giant molecular clouds . They often appear clumpy and filamentary, sometimes showing bizarre shapes such as the Horsehead Nebula . H II regions may give birth to thousands of stars over a period of several million years . In the end, supernova explosions and strong stellar winds from the most massive stars in the resulting star cluster will disperse the gases of the H II region, leaving behind a cluster of stars which have formed, such as the Pleiades . </P> <P> H II regions can be observed from considerable distances in the universe, and the study of extragalactic H II regions is important in determining the distance and chemical composition of galaxies . Spiral and irregular galaxies contain many H II regions, while elliptical galaxies are almost devoid of them . In spiral galaxies, including our Milky Way, H II regions are concentrated in the spiral arms, while in irregular galaxies they are distributed chaotically . Some galaxies contain huge H II regions, which may contain tens of thousands of stars . Examples include the 30 Doradus region in the Large Magellanic Cloud and NGC 604 in the Triangulum Galaxy . </P> <P> The term H II is pronounced "H two" by astronomers . "H" is the chemical symbol for hydrogen, and "II" is the Roman numeral for 2 . It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly - ionised--H II is H in other sciences--III for doubly - ionised, e.g. O III is O, etc . H II, or H, consists of free protons . An H I region being neutral atomic hydrogen, and a molecular cloud being molecular hydrogen, H. In spoken discussion with non-astronomers there is sometimes confusion between the identical spoken forms of "H II" and "H". </P>

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