An HII region, also known as an H II region, is a large cloud of gas and dust in space that is ionized by the radiation from nearby hot stars. The "HII" designation refers to the presence of ionized hydrogen, which is created when ultraviolet light from these stars strips electrons from hydrogen atoms. These regions are often sites of active star formation and can be found in galaxies, including our own Milky Way. HII regions are typically characterized by their bright emission lines, particularly in the H-alpha wavelength, which makes them visible in optical telescopes. They can vary in size and density, with some being several hundred light-years across. The study of HII regions helps astronomers understand the processes of star formation and the chemical evolution of galaxies.