H II region

An H II region is a region where huge blue stars are formed from hydrogen. They are named after the ionised atomic hydrogen which they produce: H II.Pronounced H-two by astronomers (an H I region is neutral atomic hydrogen, and H2 is molecular hydrogen)

The stars form inside a large cloud of hydrogen gas. The short-lived blue stars formed in these regions give off huge amounts of ultraviolet light. This ionizes the surrounding gas. 

H II regions can be several hundred light-years across. The first known H II region was the Orion nebula, which was discovered in 1610.Harrison T.G. 1984. The Orion nebula  where in history is it? Quarterly Journal of the Royal Astronomical Society 25: 65–79. These regions have extremely varied shapes. They often appear clumpy and filamentary, sometimes showing bizarre shapes such as the Horsehead nebula. 

H II regions give birth to thousands of stars over several million years. Eventually, this produces a  star cluster. In the end, supernova explosions and strong stellar winds from the most massive stars blow away the gases of the H II region. This leaves behind a cluster of stars such as the Pleiades.

H II regions can be seen at huge distances in the universe.The study of extragalactic H II regions helps to fix the distance and chemical composition of other galaxies. 

Spiral and irregular galaxies have many H II regions, while elliptical galaxies have almost none. In spiral galaxies, like the Milky Way, H II regions are found in the spiral arms, but in the irregular galaxies they are distributed at random. 

Some galaxies have huge H II regions, with tens of thousands of stars. Examples include the 30 Doradus region in the Large Magellanic Cloud and NGC 604 in the Triangulum galaxy.

 References 

Category:nebulae