The hydrogen 21 -cm line
Interstellar gas is more difficult to see than interstellar dust, despite the fact that there is much more of it. The reason for this is that while solid particles absorb and emit radiation over a wide range of wavelengths, gases emit and absorb only at certain dis¬crete wavelengths. In studying the gas, one is therefore constrained to observing a limited number of spectral lines. Most of what we know about interstellar gas has come from the study of a single spectral line: the 21-cm RADIO WAVELENGTH transition of atomic hydrogen. This transition arises as follows. Hydrogen, in its lowest energy state, can exist in two forms. In one case the spins of the proton and electron are parallel, in the other they are opposed. There is a very slight energy difference between these states; the difference corresponds to a photon of 1420.4MHz frequency or about 21-cm wavelength. In interstellar space very nearly three-quarters of the hydrogen atoms are in the higher energy state (spins parallel) and one-quarter in the lower. Transitions between the states are rare and usually occur only as a result of a collision with another particle. This may happen to a particular atom only about once every 400 years. Nevertheless the number of interstellar hydrogen atoms is so large that the 21-cm line can be detected in nearly every direction of the sky with a moderate-sized radio telescope.

The 21-cm line can lie seen in both emission and absorption. The left a stationary cloud emitting radiation only at frequencies very close to 1420.4MHz. In general the more hydrogen there is in a cloud the stronger will be the emis¬sion line. If the cloud is moving either towards or away from the observer, the frequency of the line will be altered by the Doppler effect (middle diagram). These frequency shifts are extremely use¬ful as they allow us to distinguish between clouds at different velocities along the same line of sight, and hence to study the motions of distant parts of the Galaxy. Absorption at 21cm can occur when radio emission from a bright, distant object such as a quasar, a radio galaxy or a supernova remnant passes through a cloud of hydrogen. The amount of absorption depends on the temperature of the cloud; cold gas absorbs more than hot gas. By comparing the amount of emission and absorption produced by the same cloud of gas one can measure its temperature. The lower profile shows the 21-cm absorption spectrum of the distant radio galaxy Cygnus A due to hydrogen in our Galaxy. The spectrum contains several strong, nar¬row lines. The upper profile is the emission from the same clouds of hydrogen, as measured along a direction a fraction of a degree of arc away from Cygnus A. The emission profile shows that there is much hydrogen moving at velocities between + 20 and —20kms-1 and smaller amounts moving at all velocities between +30 and —130 kms-1. Much of the hydrogen is hot, more than 1000K, and so does not absorb strongly, but there are some patches which are much colder, about 100K. and give rise to the deep absorption lines. The most prominent cold clouds are moving at velocities of +11, +4, 0, — 10, 18 and -85km km-1 away from us.

The 21-cm line is seen only in hydrogen when it exists in the form of free, neutral atoms. Hydrogen which is ionized and hydro¬gen in the form of H., molecules must be studied by other methods.

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