The most crucial factor on which the properties of an interstellar cloud depend is the state of its most prevalent constituent, namely hydrogen; the state of the helium and of the heavier elements is of secondary importance. Although the various different forms of hydrogen can coexist to some extent, it is a reasonable simplification to divide space into three basic types of region: those in which the hydrogen is in predominantly molecular form (H ) those in which it is predominantly atomic (HI or H°) and those in which it is predominantly ionized (HII or H+). Molecular hydrogen and ionized hydrogen appear to exist mainly in discrete clouds, leaving most of interstellar space filled with atomic hydrogen.

The unevenness of the atomic hydrogen can be seen in. This diagram shows the strength of the 21-cm hydrogen emission line around the whole sky, with the exception of the galactic plane and some southern regions. Most of the gas seen in is within 100pc of the Sun; it exists largely in the form of loops, filaments, and clouds rather than as a uniform galactic atmosphere. In the case of this local hydrogen the interstellar magnetic field is one of the most important influences; for example, comparison of with shows that many of the hydrogen filaments are elongated along magnetic field lines.

Close to the galactic plane, 21-cm spectra generally indicate the existence of atomic hydrogen at a variety of distances along line of sight. Part of the variation is due to the presence of spiral arms and part is due to the presence of clouds. As described on page 200, spectra such as those in figure 13.11 indicate that different parts of the gas are at different temperatures. In particular, there are patches of hydrogen which are much colder than the average, and so absorb strongly. These patches are cool atomic clouds, which lie within a hotter ‘intercloud medium’. Clouds vary greatly, but typical ones may have densities of between 5 X 106 and 108 atoms nr3, and sizes of up to a few parsecs across. The temperature of the gas inside a cloud is about 100K, whereas that of the intercloud medium is much greater. In the region of the galactic plane a few per cent of the volume is occupied by clouds, whereas the rest is filled by the intercloud medium.

This separation of the interstellar atomic hydrogen into ‘cloud’ and ‘intercloud’ regions is partly the result of THERMAL INSTABILITIES in the interstellar medium. Interstellar gas is heated by a mixture of ultraviolet starlight, X-rays and cosmic rays. It is cooled by photons which are emitted as a result of collisions between the interstellar particles. At typical interstellar temperatures and densities an increase of density at some point leads to an increase in the rate of collisions and therefore in the rate of cooling. The resultant drop in temperature is under certain circumstances enough actually to decrease the pressure at that point, leading to an instability as neighbouring gas rushes in and increases the density even more. A gas which is initially at uniform density will therefore, if it is disturbed slightly, separate itself spontaneously into cold, dense regions and hot, rare regions. The theory behind this process is known as the TWO-PHASE MODEL of the interstellar medium.

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