A region in interstellar space containing hot hydrogen gas which emits a characteristic red spectral line at 656.3 nm; such a region is the visible counterpart of an H II region. The 656.3 nm red line is the brightest line in the optical spectrum of hydrogen, and is known as the Ηα‎ (hydrogen-alpha) line (see balmer series). It gives nebulae their characteristic red colour in photographs.