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The spread in wavelength range of a feature in a spectrum produced by internal motion of the emitting atoms and molecules in the source of light. The Doppler effect causes a change in the wavelength of a spectral line when the source and observer are in relative motion, either towards or away from each other. The size of the change increases with the relative speed. Light from a star, for example, consists of individual photons emitted by atoms that are moving rapidly in the hot gas of the star's outer layers. Some atoms will always be moving towards the observer, others away. The result is to broaden the range of wavelength over which the line is spread. The hotter the star, the faster the gas atoms move and the greater the broadening effect.
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