Abstract
THE well-known sources of line-broadening in stellar spectra may be summarized in two main groups. (1) Broadening resulting from the finite length of the wave-trains emitted by the atoms. The length is determined by (a) the finite life-time of the excited level, that is, radiation damping, and by (b) abrupt phase-changes in the emitted wave caused by perturbations of the emitting atoms by other particles, that is, collision damping and Stark effect. (2) Broadening resulting from Doppler effect of different types. The Doppler velocities may be of a random character, for example, thermal or turbulent motions of emitting atoms, resulting in a line profile of gaussian shape, or the broadening may result from the integration of contributions to the line from different parts of a rotating star, an expanding gas shell or a pulsating star. In addition, broadening or splitting of lines by magnetic fields (Zeeman effect) and by coupling between the nuclear spin and the angular momentum of the electrons of the emitting atom (hyperfine structure) can occur.
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