An H II region is a cloud of glowing gas, sometimes several hundred light years across, in which star formation is taking place. Young, hot, blue stars which have formed from the gas emit copious amounts of ultraviolet light, ionising the nebula surrounding them.
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 such as the Pleiades.
H II regions are named for the large amount of ionised atomic hydrogen they contain, referred to as H II (pronounced 'aitch two') by astronomers (H I ('aitch one') being neutral atomic hydrogen, and H2 (also 'aitch two') being molecular hydrogen). 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.
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