The thickest and densest interstellar clouds consist mainly molecular hydrogen. The reason for this is that the formation A preservation of molecules is facilitated by a high concentration f gas (and consequently a greater frequency of atomic collisions! and also a high concentration of dust grains which assist the formation of hydrogen and shield it from the destructive effects of ultraviolet starlight. Ultraviolet observations with the Copernicus satellite have shown that clouds which have an extinction (Av ) of less than 0.3 mag are predominantly atomic, whereas those with an extinction greater than this contain an appreciable amount of molecular hydrogen.

Clouds that produce visual extinction of a few magnitudes or more may become detectable because of the obscuration they cause. They are often referred to as DARK NEBULAE or DARK CLOUDS. The Horsehead Nebula and the Coalsack are the classic examples of these objects. Sometimes the dark nebulae’ manifest themselves simply as apparently blank areas of sky, devoid of stars . Alternatively, they may appear as very dark patches in front of bright nebulae . It is very common for dark clouds to be found closely associated with H+ regions and with reflection nebulae. Dark clouds occur in a wide variety of sizes. Small clouds, such as those are called GLOBULES, and may have diameters no larger than the Solar System; large clouds may be several parsecs in diameter.

The dust content of dark clouds may in some cases be by comparing the number of stars visible through it number estimated to be behind it. The gas content is more difficult to obtain. The 21-cm line is no use since the hydrogen in dark clouds is predominantly molecular, not atomic. The ultraviolet spectral lines of molecular hydrogen cannot be studied since the cloud is opaque at these wavelengths. It is therefore necessary to observe other molecules such as formaldehyde, hydroxyl or carbon monoxide contours of the strength of the formaldehyde absorption at 6-cm wavelength from different parts of the p-Ophiuchi dark cloud. The regions of strongest absorption correspond well with thick parts of the obscuring cloud, confirming that the dust and the molecules are co-extensive in this cloud. The hydrogen molecule density in dark clouds is typically 109-1010 atoms m-3. Because of the increased rate of cooling at high densities and the fact that ultraviolet and X-radiation cannot penetrate to heat the middle of the cloud, the temperature of dark clouds is lower (around 10K) than in atomic clouds.

The MOLECULAR CLOUDS which are the main focus of attention of the radio spectroscopists are generally hotter (30-l00k) a denser (1010-1012 atoms m-3) than visible dark clouds, though they are much less common in the Galaxy. These warm molecular clouds are also strong sources of infrared radiation by virtue of the heated dust grains they contain. They are frequently found in the vicinity of giant H+ regions, and are thought to be places where new stars are forming. The nearest such clouds to the sun are associated with the Orion Nebula.

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