STROMGREN SPHERE

theoretical spherical volume of gas heated and ionized by a hot central star. It is named for the Swedish-born astrophysicist Bengt Strmgren, who postulated its existence in the 1930s. Bright stars with surface temperatures higher than 20,000 kelvins (K) normally are surrounded by such volumes of gas. The density of the ionized gas ranges from a few to about 1,000 atomic particles per cubic centimetre, and its temperature is about 10,000 K. The radius of Strmgren spheres varies from a few to several hundred light-years, depending on the gas densities and temperatures of the central hot stars. At the edge of such a sphere exists an equilibrium between the rate of ionizations of neutral atoms and the rate of forming neutral atoms by recombinations of atomic nuclei with free electrons.