An H I region (pronounced "aitch one region") is an interstellar cloud composed of neutral atomic hydrogen (H1). These regions are non-luminous but are detectable because molecules present in small quantities emit radiation in the 21-cm (1,420 MHz) region of the radio-spectrum, even at low temperatures. At ionization fronts, where HI regions collide with expanding ionized gas (such as an H II region), the latter glows brighter than it otherwise would.