Eventually, after hundreds of thousands of years, the infant star reaches a critical density that triggers nuclear fusion of hydrogen within its core. As the protostar ignites, the resulting radiation blasts away its protostellar cocoon of gas and dust. During this phase, the gas around the star is ionized and emits radio waves. This region of radio-emitting gas is known as an HII (ionized hydrogen) region. Once the ionized gas has been blown from the star, its radio signature fades, leaving a visible star.