A radio recombination line (RRL) in astrophysics is a spectral emission line produced by the transition of electrons between two energy levels in an atom, and with a rest frequency in the radio band of the electromagnetic spectrum. This effect is produced in ionized gas, driven by the recombination of ions with electrons, when the energy of the electron falls enough to be captured by the ion. Following this capture, the electron cascades down through the energy levels to a stable state, emitting a photon at each transition. In low density regions of the interstellar medium (ISM), these atoms can remain stable up to very high quantum number (n~1000), and have a diameter up to 1 micron across, similar to the size of a human hair.