A region of hot gas surrounding a young star or stars that is mostly ionized. The energetic light from these young stars ionizes the existing gas. This region typically appears red as it glows with the photons emitted when elections recombine with hydrogen protons.
cloud of ionized hydrogen around a hot, luminous star (usually O or B-type). Produced by the copious ultraviolet light from the hot star(s) causing the hydrogen to fluoresce (atoms are ionized and then when the electron recombine, they produce energy in the visible band).
An H II region is a cloud of glowing gas and plasma, sometimes several hundred light-years across, in which star formation is taking place. Young, hot, blue stars which have formed from the gas emit copious amounts of ultraviolet light, ionising the nebula surrounding them.