H II region

An H II region is an emission nebula associated with hot, young, blue stars, and star forming regions. H II, or singly-ionized hydrogen, is nebular gas ionized by ultraviolet light emitted by these hot, young stars of spectral type O and B. The sizes of H II regions are determined both by the amount of gas present, and by the luminosity of the O and B stars -- the more luminous the stars are, the larger the H II region can be.

H II regions are found within the spiral arms of galaxies, because the spiral arms are the location of most star formation within galaxies. They are some of the largest and brightest features within star-forming galaxies, and have been observed even in high-redshift galaxies. Examples within the Milky Way include the Orion Nebula and the Eagle nebula.

In visible light, H II regions are characterized by their red color, caused by a strong emission line of hydrogen at 656.3 nm. However, other atomic species are also observed, and forbidden lines of oxygen, nitrogen, and sulphur are common.