line broadening

The term line broadening (often just broadening) refers to the mechanisms that create the spectral line shapes (e.g., in EMR from a star), i.e., the mechanisms that affect the range and distribution of wavelengths of the emission or absorption of an atom; the simplest model of emission/absorption produces spectral lines at a specific wavelength, which would be infinitely narrow if there were no mechanism(s) to vary the wavelength. These mechanisms include Doppler broadening due to the Doppler effects of the movement of absorbing or emitting atoms, natural broadening from the Heisenberg uncertainty principle, which assures no atom has a precise measurable velocity, and collisional broadening (pressure broadening) from distortion due to absorption by an atom whose excitation energy levels are being affected by a collision (i.e., interaction with a nearby charged particle). Each mechanism produces a specific line shape which can be described by a density function, the resulting line shape of the simultaneous mechanisms being a convolution of these line shape functions. The line's width can be quantified by an equivalent width, the width of a theoretical maximum-depth rectangular-shaped line with the same area as the given spectral line. An alternate measure is full width at half maximum (FWHM), a measure of the width "half way down" to the line's deepest point (or the converse for emission lines). Line depth is the depth of the line as a percentage of the continuum of the spectrum on either side of the line.

A curve of growth is a plot of a model relating an absorption line's equivalent width to the column density, of absorbing atoms, i.e., the number atoms along the path (per unit cross section) over which the light passes to create a the line.

Doppler broadening occurs from the normal movement of atoms due to temperature, e.g., according to the Maxwell-Boltzmann distribution, but also occurs in other patterns due to more-specific movement of the atoms:

• Rotational broadening is Doppler broadening specifically due to the rotation of the source (e.g., star). The portion rotating away from the observer is redshifted by various amounts and the portion rotating toward the observer is blue shifted, both according to the radial velocity of that part of the body. Analysis reveals information about the rotation of planets, stars, galaxies and clouds.
• Microturbulence causes Doppler broadening from turbulence on a very small scale, e.g., if a star's photosphere is stirred by an immediately-underlying convection zone or if the photosphere includes its own small regions of convection.