In spiral galaxies, including our Milky Way, H II regions are concentrated in the spiral arms, while in irregular galaxies they are distributed chaotically. Spiral and irregular galaxies contain many H II regions, while elliptical galaxies are almost devoid of them.
H II regions can be observed at considerable distances in the universe, and the study of extragalactic H II regions is important in determining the distances and chemical composition of galaxies. In the end, supernova explosions and strong stellar winds from the most massive stars in the resulting star cluster disperse the gases of the H II region, leaving a cluster of stars which have formed.
H II regions may give birth to thousands of stars over a period of several million years. They often appear clumpy and filamentary, sometimes showing intricate shapes such as the Horsehead Nebula. H II regions-sometimes several hundred light-years across-are often associated with giant molecular clouds. The short-lived blue stars created in these regions emit copious amounts of ultraviolet light that ionize the surrounding gas. The regions may be of any shape because the distribution of the stars and gas inside them is irregular. 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. It is typically in a molecular 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 centimetre. Large, low-density interstellar cloud of partially ionized gas NGC 604, a giant H II region in the Triangulum GalaxyĪn H II region or HII region is a region of interstellar atomic hydrogen that is ionized.
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