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 excitationenergy 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.