Since an H+ region is at a much higher temperature than the neutral gas which surrounds it, it exerts an outward pressure at the ionization front. The main characteristic of the evolution of an H+ region is therefore one of expansion. The birth of an H+ region is generally a by-product of the birth of […]

Although most of the visible light from H+ regions is in the form emission lines, the same is not true at longer wavelengths, where most of the energy is radiated in a continuum. There are, in fact two continua: bremsstrahlung or free-free emission from the ionized plasma, which causes the radio flux, and thermal radiation […]

The light from an H+ region is of a quite different character from that of a star. Whereas a normal star emits a continuous spectrum with dark absorption lines, an H+ region emits almost all its light in a few comparatively narrow emission lines .The most important of these emission lines are those of hydrogen, […]

H+ regions As its name implies an H+ REGION (or mi REGION as it is often written) is a part of space where the hydrogen in the interstellar medium is in an ionized rather than in a neutral state. The most common situation where this occurs is in the immediate vicinity of O-type stars, since […]

The evolution of a cloud of interstellar gas depends on a balance between internal gravitational forces tending to make it contract and thermal pressure tending to make it expand. James Jeans, in 1926, first showed that a cloud of given temperature and density can collapse only if its mass is above a certain minimum value. […]

Less is known about the intercloud medium than about the clouds. The intercloud medium appears to contain both atomic and ionized hydrogen, and there are several lines of evidence to suggest that the average concentration of free electrons in the intercloud medium is about 3 X 104 atoms m-3. Observations at 21-cm wavelength suggest that […]

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 […]

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 […]

The unevenness of the interstellar medium The previous chapter described the ways in which the interstellar medium can be observed, and summarized the major properties of the dust, the gas, and the cosmic rays which comprise it. This chapter is concerned with the way in which the interstellar medium is generally concentrated into clouds, particularly […]