Ism

opaque

Log photon energy (eV)

Figure 10.8. Distances of photon travel in the interstellar medium as dictated by the photoelectric effect. (The earth's atmosphere is not included nor is extinction by dust.) This plot can be derived (approximately) from the cross section data of Fig. 7, but here we omit most of the K edges. The hydrogen column density NH = nHr is plotted against photon energy. The shaded region is excluded from observation. The solid lines indicate, for a given energy photon, the "distance" (actually column density NH) that a photon beam has traveled when it has been reduced to e-025 (78%),e-1 (37%), and e-4(1.8%) of its initial numbers. The values of Nh to three celestial objects are shown for an assumed uniform hydrogen number density of nH = 0.5 x l06 m-3. [Adapted from M. Oda, in Proceedings of 9th International Cosmic Ray Conference, 1965; also in Rossi, IAU Symposium #37, p. 1]

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