 H II region

An H II region is a volume of ionized hydrogen surrounding one or more sources of ultraviolet photons. Photoionization heating, recombination, gas dynamics, and dust determine its structure and emission spectrum.

An H II region is a large cloud of gas and dust in space that is ionized by the radiation from young, hot stars. The term "H II" refers to the presence of hydrogen ions (protons) in the region.