Hydrogen Line Emissions

While the more familiar type of signal received in radio astronomy is broadband noise - e.g., from thermal or synchrotron radiation - some narrowband emissions exist caused by quantum energy state transitions in atoms or molecules.  In general experience the most familiar of these are the optical emissions emanating from high energy transitions of hot gas. In order for this process to produce radio frequency waves the energy released in the transition must be small.  Serendipitously, the most common element in interstellar space - hydrogen - has a ground state transition in its un‑ionised form which produces an emission in the microwave frequency range.  The transition is a spin-flip of the electron from the higher-energy parallel spin (same direction as proton spin) to the lower energy anti-parallel spin (opposite direction to the proton spin).  The difference in energy between the two states is emitted as microwave radiowaves.

The frequency, ν, of the emission produced by this transition is related to the difference in the two different energy levels by the Planck-Einstein equation...

where h, is Planck's constant.

The frequency of the narrowband emission is 1420.405752 MHz (21 cm wavelength).

Left to its own devices a single hydrogen atom in the higher energy spin state will drop back to the lower state after some 10 million years and it would be expected that such a low rate of emission production would make it a very weak signal.  Two main factors contribute to make the signal much stronger than might be expected - huge numbers of the hydrogen atoms and jostling from other atoms.

It is worthy of note that the reverse process can occur - where hydrogen atoms in the lower energy state can be excited to the higher energy state by absorbing the energy from photons @ 1420.40575 MHz - thereby creating absorption lines.

An interesting background article describing the first detection of the hydrogen line in 1954 can be read here.

Philosophical Note: the author finds that articles written at the time of initial discoveries of particular phenomena are the most useful, principally because they are necessarily focused on basic information - which is the level most appropriate for citizen scientists 'coming up to speed' on the subject.  Later articles, while interesting in their own right, assume basic knowledge and contain more esoteric material.

Use in Radio Astronomy

By courtesy of the abundance of galactic neutral hydrogen and by observing the doppler shift present on the emissions, a map of the distribution of the gas can be made as shown above.

At 1420.405752 MHz a doppler shift of 1 MHz corresponds to ~ 211 km/s of radial velocity.

Note that observations from Earth (topocentric) require doppler correction to the Local Standard of Rest (LSR) such that stationary (w.r.t the LSR) hydrogen clouds produce a signal @ 1420.405752 MHz.