immense ionized-hydrogen region in the Large Magellanic Cloud, a satellite galaxy of the Milky Way system (in which the Earth is located). The nebula consists of a cloud of interstellar gas—principally hydrogen—lit from within by young, hot stars that ionize the gas around them. As the atoms in the gas recombine, they emit visible light. The total mass of the nebula is about 1,000,000 solar masses, and its diameter is 170 parsecs (550 light-years), making it the largest region of ionized gas in the entire Local Group (q.v.) of galaxies.