H II region

A H II region is a region of ionised molecular hydrogen gas excited by intense ultraviolet radiation released by O class and very hot B class stars. H II regions glow a dull red colour and are also detectable through characteristic radio waves.

Importance in astronomy

Extremely hot O and B class stars release powerful ultraviolet radiation that can strip the electrons from hydrogen atoms. The resulting ionised hydrogen gas glows at several frequencies, including 6563 angstroms, a reddish colour, which astronomers call hydrogen-alpha (Hα). In many cases, hot stars create glowing gas clouds over a wide area. These red patches, called HII regions, can easily be seen in many galaxies and are an important marker of spiral arms.

Astronomers mapping the Milky Way galaxy have made cataloguing and estimating the distances of HII regions a key priority.