H II region

large, low-density cloud of partially ionized gas

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.[2]

NGC 604, a giant H II region in the Triangulum galaxy
A small portion of the Tarantula nebula, a giant H II region in the Large Magellanic Cloud
Bubbles of brand new stars LHA 120-N 180B.[1]

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.[3] 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 behind a cluster of stars such as the Pleiades.[4]

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.


  1. "Bubbles of Brand New Stars". www.eso.org. Retrieved 8 February 2019.
  2. Pronounced H-two by astronomers (an H I region is neutral atomic hydrogen, and H2 is molecular hydrogen)
  3. Harrison T.G. 1984. The Orion nebula – where in history is it? Quarterly Journal of the Royal Astronomical Society 25: 65–79.
  4. Anderson L.D.; et al. (2009). "The molecular properties of galactic HII regions". The Astrophysical Journal Supplement Series. 181 (1): 255–271. arXiv:0810.3685. Bibcode:2009ApJS..181..255A. doi:10.1088/0067-0049/181/1/255.