<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>

Where can ionized hydrogen be found in astronomy