An atom which possesses one valence electron orbiting about an atomic nucleus within an electron shell well outside all the other electrons in the atom. Such an atom approximates the hydrogen atom in that a single electron is interacting with a positively charged core. Early observations of atomic electrons in such Rydberg quantum states involved studies of the Rydberg series in optical spectra. Electrons jumping between Rydberg states with adjacent principal quantum numbers, n and n − 1, with n near 80 produce microwave radiation. Microwave spectral lines due to such electronic transitions in Rydberg atoms have been observed both in laboratory experiments and in the emissions originating from certain low-density partially ionized portions of the universe called HII regions.