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Half of the neutral atomic hydrogen and all of the molecular hydrogen in the ISM is concentrated into relatively high density and low temperature regions called `clouds'. The properties of the atomic hydrogen (HI) clouds have been determined primarily from radio observations of the hyperfine ground state transitions of hydrogen at 1420 MHz (21 cm); interstellar absorption lines of trace elements such as Ca+ also continue to play a key role in the study of these clouds. For the most common clouds, where the 21-cm radiation escapes freely, the brightness of the emission provides a direct measurement of the HI column density NH = integ nH ds where nH is the atomic hydrogen density. When an HI cloud lies in front of a bright source of radio continuum emission, the decrease in the brightness of the background source at 21 cm is proportional to NH = integ nH / TH ds, where TH is the temperature of the HI cloud. Thus, observations of HI clouds at 21 cm in emission and absorption provide direct information about cloud temperatures and column densities. Table 1 summarises basic properties of the cold and warm neutral media. Maps of the sky show that HI clouds have complex shapes resembling thin extended sheets or filaments with embedded small clumps.

Molecular hydrogen is confined to the interiors of the densest and most massive clouds, the dark clouds, where starlight capable of dissociating molecules cannot penetrate. These clouds constitute the active star-forming component of the interstellar medium. Because H2 has no electric dipole moment, radiative transitions of H2 are greatly suppressed. Therefore, most of the structural information about molecular clouds in the ISM is obtained through observations of the rotational transitions of the trace molecule CO at 115 GHz (2.6 mm). In addition, a wide variety of other molecules, including complex hydrocarbon chains, have been detected within the H2 clouds. Molecular clouds are small (40 pc), dense (200 cm-3), with structure on scales of less than 0.1 pc (see Table 1). Some of the small condensations can have densities as high as 105 cm-3. In disk galaxies like our own, the cold neutral and molecular gas are confined to a disk which is much thinner than the stellar disk.

Table 1. The component properties of the interstellar medium

component temperature midplane filling average
(K) density (cm-3) fraction (%) height (pc)

H2 15 200 0.1 75
HI 120 25 2 100
Warm HI 8000 0.3* 35* 500
Warm HII 8000 0.15 20 1000
Hot HII ~ 106 0.002 43* 3000*

* Value uncertain by at least a factor of 2.

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