Radio waves of 21-cm wavelength originate from which component of the interstellar medium?

Question

Quizplus · Accepted Answer

Radio waves of 21-cm wavelength are emitted by the transition between two energy levels of the electron in a hydrogen atom. These transitions occur when hydrogen atoms change their spin orientation relative to the magnetic field in the interstellar medium. This emission comes from cool, neutral atomic hydrogen, which is found mainly in the interstellar gas clouds. Cold molecular hydrogen, cool carbon monoxide, and hot, ionized atomic hydrogen do not emit radio waves of this wavelength.

Radio Waves of 21-Cm Wavelength Originate from Which Component of the Interstellar