T

(d) Continuum

Frequency v v v v

Figure 11.1. Idealized spectra of radiation: sketches of (a) line emission, (b) line absorption, (c) a broadened emission line, and (d) continuum emission. The broadening could arise from differing Doppler shifts due to differing velocities of different portions of the cloud, e.g., from rotation or turbulence.

distribution is the variation of intensity with frequency. One aspect of such studies is the study of spectral lines. This is an excess or deficiency of radiation at a specific frequency relative to nearby frequencies (Figs. 1a,b,c). These are called emission lines and absorption lines respectively. These lines can be quite narrow or they can be substantially broadened due, for example, to Doppler shifts arising from thermal and turbulent gas motions.

Another kind of spectral distribution is the continuum spectrum (Fig. 1d). This is a spectrum that varies smoothly with frequency. An emission line may or may not be superposed upon a continuum spectrum. By its very nature, an absorption line must lie upon a continuum background (Fig. 1b).

The intent of this chapter is to familiarize the reader with plotting conventions and with three commonly encountered continuum spectral shapes (optically thin thermal bremsstrahlung, synchrotron radiation and blackbody radiation). Spectral lines are formed as radiation passes through a medium of atoms. We develop the radiative transfer equation which describes this process. Its solution for various limits provides insight into the formation of spectral lines. Finally, we examine the significance of line strengths and shapes.

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