Once star formation has begun, the hottest and most massive of the newly formed stars (known as OB stars) will emit ultraviolet radiation, which ionize the surrounding gas of the giant molecular cloud, forming an H II region.
Once star formation has begun, the hottest and most massive stars (known as OB stars) will emit copious amounts of ultraviolet radiation. This radiation rapidly ionizes the surrounding gas of the giant molecular cloud, forming an H II region.