Editing Interstellar medium (section)

===The three-phase model===

{{harvtxt|Field|Goldsmith|Habing|1969}} put forward the static two ''phase'' equilibrium model to explain the observed properties of the ISM. Their modeled ISM included a cold dense phase (''T''&nbsp;<&nbsp;300&nbsp;[[Kelvin|K]]), consisting of clouds of neutral and molecular hydrogen, and a warm intercloud phase (''T''&nbsp;~&nbsp;10<sup>4</sup>&nbsp;K), consisting of rarefied neutral and ionized gas. {{harvtxt|McKee|Ostriker|1977}} added a dynamic third phase that represented the very hot (''T''&nbsp;~&nbsp;10<sup>6</sup>&nbsp;K) gas that had been shock heated by [[supernova]]e and constituted most of the volume of the ISM.
These phases are the temperatures where heating and cooling can reach a stable equilibrium. Their paper formed the basis for further study over the subsequent three decades. However, the relative proportions of the phases and their subdivisions are still not well understood.<ref name=Ferriere2001 />

The basic physics behind these phases can be understood through the behaviour of hydrogen, since this is by far the largest constituent of the ISM. The different phases are roughly in pressure balance over most of the Galactic disk, since regions of excess pressure will expand and cool, and likewise under-pressure regions will be compressed and heated. Therefore, since [[Ideal gas law|''P = n k T'']], hot regions (high ''T'') generally have low particle number density ''n''. Coronal gas has low enough density that collisions between particles are rare and so little radiation is produced, hence there is little loss of energy and the temperature can stay high for periods of hundreds of millions of years. In contrast, once the temperature falls to O(10<sup>5</sup> K) with correspondingly higher density, protons and electrons can recombine to form hydrogen atoms, emitting photons which take energy out of the gas, leading to runaway cooling. Left to itself this would produce the warm neutral medium. However, [[OB stars]] are so hot that some of their photons have energy greater than the [[Lyman limit]], ''E'' > 13.6 [[electron volt|eV]], enough to ionize hydrogen. Such photons will be absorbed by, and ionize, any neutral hydrogen atom they encounter, setting up a dynamic equilibrium between ionization and recombination such that gas close enough to OB stars is almost entirely ionized, with temperature around 8000 K (unless already in the coronal phase), until the distance where all the ionizing photons are used up. This ''ionization front'' marks the boundary between the Warm ionized and Warm neutral medium.

OB stars, and also cooler ones, produce many more photons with energies below the Lyman limit, which pass through the ionized region almost unabsorbed. Some of these have high enough energy (> 11.3 eV) to ionize carbon atoms, creating a C II ("ionized carbon") region outside the (hydrogen) ionization front. In dense regions this may also be limited in size by the availability of photons, but often such photons can penetrate throughout the neutral phase and only get absorbed in the outer layers of molecular clouds. Photons with ''E'' > 4 eV or so can break up molecules such as H<sub>2</sub> and CO, creating a [[photodissociation region]] (PDR) which is more or less equivalent to the Warm neutral medium. These processes contribute to the heating of the WNM. The distinction between Warm and Cold neutral medium is again due to a range of temperature/density in which runaway cooling occurs.

The densest molecular clouds have significantly higher pressure than the interstellar average, since they are bound together by their own gravity.  When stars form in such clouds, especially OB stars, they convert the surrounding gas into the warm ionized phase, a temperature increase of several hundred. Initially the gas is still at molecular cloud densities, and so at vastly higher pressure than the ISM average: this is a classical H II region. The large overpressure causes the ionized gas to expand away from the remaining molecular gas (a [[Champagne flow model|Champagne flow]]), and the flow will continue until either the molecular cloud is fully evaporated or the OB stars reach the end of their lives, after a few millions years. At this point the OB stars explode as [[supernova]]s, creating blast waves in the warm gas that increase temperatures to the coronal phase ([[supernova remnants]], SNR). These too expand and cool over several million years until they return to average ISM pressure.