[ General Glossary, Quantum Glossary, Atomic Glossary ]

The Hydrogen (1H) 21cm Line

This line occurs at radio frequencies (see below). It's importance in astronomy is due to the combination of the facts that

  • hydrogen (1H) is the most abundant element/isotope in the universe
  • the "physics" (atomic/quantum) of 1H is the "easiest" to calculate (it is the simplest case)
Thus the 21cm line is strong (there's a lot of 1H out there), so it can be accurately measured, the atomic physics is well-known, so these measurements can provide (in many cases the most) stringent physical constraints on the astronomical system under study.

The ground-state configuration of the neutral hydrogen atom is 1s1 2S1/2 (see also Electron Configurations for some Elemental Ground-states). The interaction between the magnetic moment of the proton and the magnetic moment of the electron results in so-called a hyperfine splitting of the energy level.

The two resultant energy levels associated with this hyperfine splitting (in the case of neutral hydrogen) have
an energy difference 5.874328324 x 10-6 eV

Transitions from the upper to the lower of these two hyperfine levels (which is a magnetic dipole transition) therefore gives rise to the emission of a photon with this energy, ie
at a frequency 1.420405751 x 109 Hz
at a wavelength 21.206114 cm

The main uses of the observed 1H line are
  • determination of the velocity (wrt to us) of material
    • from the Doppler or gravitational shift of the line center.
    Hence it can be used as a redshift indicator.
  • determination of the relative velocity field (aka dispersion)
    • from the breath/profile of the line, again due to the distribution of Doppler/gravitational shifts
  • determination of the amount of 1H "there" (as a function of relative velocity)
    • and hence the amount of other isotopes/elements if one assumes a set of relative abundances (wrt 1H)
  • determination of the density and temperature of the material.
    • The natural width of the line is very small (its a truely "narrow line")
      • the probability of the transition happening is "Low", ie the excited state can be thought of having a "long lifetime"
        (actually A21 = 2.876x10-15 s-1)
        • so the "lifetime" is 11.025x106 years
      • so from Heisenburgs uncertanty principle the "width" of the line (in energy or wavelength) is relatively small
    This narrow width makes it an ideal diagnostic of the temperature and density of 1H.
Indeed the "Hydrogen 21cm" line is recognized as being so important that
  • there is an internationally-recognized ban on using these frquencies for other uses (ie broadcasting).
  • it is an "obvious" frequency at which searches for extraterristrial life (SETI) are being performed.

[Other Astronomically Important Lines]