An H II region is a cloud of glowing
gas and
plasma, 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 by
astronomers (
H I region being
neutral atomic hydrogen, and H2 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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