H II region
LARGE, LOW-DENSITY CLOUD OF PARTIALLY IONIZED GAS
HII regions; HII region; H II Region; H II; HII Region; H II nucleus
An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. Typically the region is in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic cm.