H II region

An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. (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.) 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 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. 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.

The term H II is pronounced "H two" by astronomers (an H I region being neutral atomic hydrogen, and a molecular cloud being molecular hydrogen, H₂. In spoken discussion with non-astronomers there is sometimes confusion between the identical spoken forms of "H II" and "H₂"). Such regions may be of any shape, because the distribution of the stars and gas inside them is irregular. They often appear clumpy and , 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.

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