The 21 cm Hydrogen Line

During the Second World War a highly significant theoretical work by Grote Reber jump started the field of Radio Astronomy. Jan Oort, the Director of Leiden Observatory, Holland (then under Nazi occupation), realized that a spectral line originating from the radio part of the electromagnetic spectrum by atoms or molecules in the interstellar medium could, in principle, reveal information about the matter distribution at the very early stages of cosmic evolution. About 99% of the interstellar medium (ISM) is gaseous, of which about 90% is atomic or molecular hydrogen, 10% helium, and traces of other elements.

Oort’s student, Hendrick van de Hulst, subsequently predicted the existence of a spectral line due to a transition between the two spin states of the ground state of hydrogen (Figure 1). This is the famous 21cm line (wavelength = 21.1 centimeters and frequency = 1420.4 MHz). It turns out that scientists could draw certain important conclusions about the dynamics of galaxies just by studying the 21 cm hydrogen line.

Figure 1: Illustration of the spin-flip transition in the hydrogen atom which gives rise to the 21-cm line. Credit: The Origins of Radio Astronomy.

The 21 cm hydrogen line is a hyperfine transition that arises due to the spin-spin interaction between the electron and the proton in hydrogen: the parallel spins state has a slightly higher energy than the anti-parallel spins state. Since atoms always try to reside in the lowest possible energy state, an electron in the parallel spin direction will eventually flip to the anti-parallel spin direction. This transition is highly forbidden with an extremely low transition probability of 2.6×10-15s-1 (a hydrogen atom on an average needs to wait a few million years before it undergoes this transition). Despite its low probability, the 21 cm hyperfine transition is one of the main tools of observational astronomy owing to the very large amount of hydrogen in the Universe.

A remarkable feature about the 21-cm line radiation is that it not blocked by galactic dust! In fact, the 21-cm line radiation provides one of the best ways to map the structure of a Galaxy. Most of what is known about the distribution of cold gas in our Galaxy, including the mapping of the nearby spiral arms, has come from detailed studies of the variation of 21-cm emission across the sky. In 1959, the famous Morrison-Cocconi conjecture, concerning the possibility of detecting artificial signals at this wavelength, heralded the birth of SETI in its modern form.

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