H II region
A region of interstellar hydrogen that is ionized. The notation H II refers to the fact that the hydrogen atoms (H) are ionized (H I is neutral, un-ionized hydrogen). Each ionized hydrogen atom contributes two particles to the gas, namely a proton and an electron. H II regions are hot, with typical temperatures of 10 000 K, and densities of 10 to 100 000 atom/cm3 (i.e. 10–100 000 times denser than H I regions). They are usually found around massive young O and B stars, the strong ultraviolet light from which ionizes the gas, causing it to glow. The Orion Nebula is a famous H II region. H II regions can be detected throughout the Galaxy by their strong radio and infrared emissions. The radio emission is bremsstrahlung from the ionized gas, and the infrared is thermal emission from dust.