Detection of Neutral Hydrogen


Neutral hydrogen is the major source of radio wave data that has been collected from space.  Neutral hydrogen emits radio waves of 21 cm wavelength.  The source of the radiation is the photon that is released as the hydrogen atom transitions from a higher level energy state to a lower state.  These energy levels are very close in energy, which is the reason a low energy photon associated with the radio wave portion of the spectrum is released.


When the magnetic dipole of the nucleus and the single electron of neutral hydrogen are aligned, that is the spin is in the same direction, the energy state of the electron is slightly higher than when the spins are in opposite directions to one another. (See Figures 1 and 2).  There is not a very high probability for this transition to occur spontaneously.  It was calculated that an atom in the higher energy state would remain there on average for 11 million years before falling to the lower state.  It was postulated in 1945 by H. C. van de Hulst that the spectral line due to this “hyperfine transition” could be observed from interstellar hydrogen.  The first emission lines from space were detected in 1951 by Ewen and Purcell at Harvard.  It seemed unlikely that the radiation would be detected because of the low probability of occurrence of this transition – in a laboratory situation, the emission can’t be observed because of constant collisions with other atoms, which greatly restricts the number of atoms that can exist in the higher energy state. Since space is so cold, the likelihood of collisions is reduced, and since the amount of neutral hydrogen is so large, the emission line is readily observed.







Figure 1:  Hyperfine transition lines for the neutral hydrogen atom are produced via the difference between the energies of the two spin configurations that are possible.

(Foundations of Astronomy, 6th Edition)




Figure 2:  This shows that the energy differences are very small between the two spin states.

(Seeds, Foundations of Astronomy, 6th Edition)


The 21 cm emission line of hydrogen can be shifted due to the Doppler Effect, as shown in Figure 3.  This is a way to observe if clouds of interstellar neutral hydrogen are moving toward or away from the observer.  The neutral hydrogen in our own galaxy was mapped in the 1960’s and the spiral rotating structure of our galaxy was deduced as can be seen in Figure 4. 



Figure 3:  Frequency bands for neutral hydrogen showing shifts due to Doppler Effect.

(Kraus, Radio Astronomy, 2nd Ed.)



Figure 4:  Neutral hydrogen distribution in the Milky Way galaxy.

(Kraus, Radio Astronomy, 2nd Ed.)