An H II region is a region of interstellar hydrogen that is ionized. It is typically 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 that ionize the surrounding gas. H II regions—sometimes several hundred light-years across—are often associated with giant molecular clouds. The first known H II region was the Orion Nebula, which was discovered in 1610 by Nicolas-Claude Fabri de Peiresc, although its nature was not known.