An H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet light, ionizing the surrounding gas. H II regions—sometimes several hundred light-years across—are often associated with giant molecular clouds in which star formation takes place, and from which the stars that produce the H II region were born. The first known H II region was the Orion Nebula, which was discovered in 1610 by Nicolas-Claude Fabri de Peiresc.