Chapter Summary

Much of the matter between stars consists of gas at about 100 K, where hydrogen is in atomic form (HI). Discrete HI clouds permeate the Galaxy and have large velocities in and out of the plane. Warmer and more rarefied gas, with a temperature closer to 104 K, also exists, but its structure is poorly known. So too is the morphology of even hotter gas at 106 K. All of these components appear to coexist in rough pressure equilibrium.

Figure 2.15 Emission from a candidate PAH (arbitrary units). The filled peaks represent emission calculated from the laboratory-measured absorption spectrum of the organic compound shown. The dashed curve displays, for comparison, the observed emission from the reflection nebula NGC 2023. Note again the spike near 3 |m.

Figure 2.15 Emission from a candidate PAH (arbitrary units). The filled peaks represent emission calculated from the laboratory-measured absorption spectrum of the organic compound shown. The dashed curve displays, for comparison, the observed emission from the reflection nebula NGC 2023. Note again the spike near 3 |m.

Yet another form of hydrogen, and the most interesting from the perspective of star formation, is the molecular (H2). While the total mass here is comparable to that in HI, the spatial distribution is very different. The molecular gas surface density in the Galactic disk attains a local peak at a radius of 6 kpc and then climbs steeply toward the center. Molecular cloud interiors are under a higher pressure than the other phases of the interstellar medium because of the influence of self-gravity.

Solid dust grains mixed in with the gas absorb background radiation and redden its color. Both effects are quantified as a function of wavelength by the interstellar extinction curve. Each grain consists of a relatively dense core surrounded by an icy mantle. Radiation from the dust around hot stars shows that the larger grains attain equilibrium between stellar heating and thermal emission. However, the tiniest structures, known as PAHs, reveal themselves through their anomalously high temperatures.

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