Widespread stirring of the neutral gas could be provided by supernovae. Although supernova explosions are rare, occurring only once every 50 years or so in a galaxy like our own, the expanding material from each event sweeps out a large volume and imparts considerable momentum to the surrounding gas. The extremely hot gas within the advancing shell of a supernova remnant cools so slowly that new remnants inevitably intersect older ones, resulting in a network of connected bubbles. Indeed, there is good evidence that the solar system is located within such a rarified region (see § 7.2). On a much greater scale, it is thought that the hot gas fills a large volume of the disk, surrounding the neutral component and effectively constituting a third phase of the interstellar medium. The temperature of this hot intercloud medium is predicted theoretically to be 106 K, and its density 0.003 cm-3.
Verification of the properties, including the spatial distribution, of the third phase has been slow, as the relevant data are difficult to obtain. Thus far, spectral lines from highly ionized heavy elements have provided the only solid clues. Since these lines lie in the ultraviolet, they must be observed from space. In the early 1970s, the Copernicus satellite observed pervasive emission from OVI. Other spaceborne instruments have subsequently detected additional species. It is interesting that optical lines from less highly ionized elements are also seen. These lines, together with widespread, diffuse emission in Ha, point to the existence of yet another gas component. This warm ionized medium, first systematically explored in the 1980s, shares the same density and temperature as the warm neutral medium, but its hydrogen is mostly ionized. Accounting for the energy input required to maintain this ionization poses another challenge to theory.
It should be clear from this brief discussion that many aspects of the interstellar gas remain unexplained. In particular, recent observations have emphasized the general principle that the medium is essentially a dynamic entity. This feature naturally renders the theorist's task much more difficult. Nevertheless, the insight that the gas consists of discrete phases in approximate pressure equilibrium will continue to play a key role. As a convenient reference for the reader, we summarize in Table 2.2 the basic properties of the known phases. Note that we have included the important molecular component, although this gas is not in pressure balance with the other phases.
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