Editing Spectral line (section)

=== Broadening due to non-local effects ===
Certain types of broadening are the result of conditions over a large region of space rather than simply upon conditions that are local to the emitting particle.

==== Opacity broadening ====
Opacity broadening is an example of a non-local broadening mechanism. Electromagnetic radiation emitted at a particular point in space can be reabsorbed as it travels through space. This absorption depends on wavelength. The line is broadened because the photons at the line center have a greater reabsorption probability than the photons at the line wings. Indeed, the reabsorption near the line center may be so great as to cause a '''self reversal''' in  which the intensity at the center of the line is less than in the wings. This process is also sometimes called '''self-absorption'''.

==== Macroscopic Doppler broadening ====
Radiation emitted by a moving source is subject to [[Doppler shift]] due to a finite line-of-sight velocity projection. If different parts of the emitting body have different velocities (along the line of sight), the resulting line will be broadened, with the line width proportional to the width of the velocity distribution. For example, radiation emitted from a distant rotating body, such as a [[star]], will be broadened due to the line-of-sight variations in velocity on opposite sides of the star (this effect usually referred to as rotational broadening). The greater the rate of rotation, the broader the line. Another example is an imploding [[Plasma (physics)|plasma]] shell in a [[Z-pinch]].