HII regions are bubbles of ionized gas surrounded by denser, colder neutral material. They are created by hot young stars that produce high-energy electromagnetic radiation. Any photon with an energy of at least 13.6 eV is powerful enough to ionize a hydrogen atom; this corresponds to light with wavelengths of shorter than 19.2 nm (which is in the ultraviolet portion of the electromagnetic spectrum). Only massive stars emit significant quantities of such high-energy photons, so HII regions tend to form around one or more massive stars. Since massive stars don’t live very long, HII regions are often seen in regions of ongoing star formation with lots of young stars, still surrounded by neutral dust and gas. The energy expelled from a young massive star will spread out into the volume around it, ionizing any gas it encounters.
Eventually, the high-energy photons will be spread too thinly to continue to ionize the surrounding material faster than the newly ionized protons and electrons can recombine to reform neutral hydrogen atoms. Setting the rate at which a central star emits ionizing photons equal to the recombination rate of the surrounding gas defines a volume within which the gas is almost completely ionized. Outside this volume, the gas quickly becomes almost completely neutral. If the density of the surrounding material is roughly constant and we assume that the region contains only hydrogen gas, then the so-called HII region will be roughly spherical and the radius will be given by:
ionizing photon emission rate = recombination rate
where is the recombination coefficient and is the number density of the gas.
Spherical HII regions are sometimes called “Stromgren spheres” after the scientist who first estimated what their radius should be (see Stromgren 1939). For typical values of and , we expect 10-100 parsecs (about 30-300 light years, or 7-70 times the distance from the Sun to the nearest star). This is somewhat larger than the radius of the HII region in the Orion Nebula that we visit on the tour, which has a radius of only about 12 light years.