H II Region

An H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet light, ionizing the surrounding gas. H II regions—sometimes several hundred light-years across—are often associated with giant molecular clouds in which star formation takes place, and from which the stars that produce the H II region were born. The first known H II region was the Orion Nebula, which was discovered in 1610 by Nicolas-Claude Fabri de Peiresc.

H II regions are named for the large amount of ionised atomic hydrogen they contain, referred to as H II, pronounced H-two, or (IPA), by astronomers (an H I region being neutral atomic hydrogen, and H₂ being molecular hydrogen). They have extremely diverse morphologies, because the distribution of the stars and gas inside them is inhomogeneous. They often appear clumpy and filamentary, sometimes showing bizarre shapes like 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 such as the Pleiades.

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. Spiral and irregular galaxies contain a lot of H II regions, while elliptical galaxies are almost devoid of them. In the spiral galaxies, including the Milky Way, H II regions are concentrated in the spiral arms, while in the 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.