spectral line shape

The shapes of spectral lines (spectral line shape or line profile) are of interest for analysis. Instruments that measure the flux density of received electromagnetic radiation (EMR) reveal the shape of the line in terms of brightness graphed over wavelength or frequency. A general shape common to lines is something like a bell curve, from each side (higher and lower wavelengths), there is an upward curve, reaching a peak toward the center which may be rounded or pointed. For emission lines, this is the shape of the strength of the EMR across the line's wavelength, and for absorption lines, it is the opposite: instead of a hill-like shape, it is like an inverted hill, a valley with the same general shape up-side down. The tall central portion is termed the line's core and the edges are termed the wings. The relatively flat portion of the SED to either side of the line is termed the continuum.

The particular shape of a given spectral line stems from characteristics of the source material and can be analyzed to determine information about those characteristics. Spectral lines stem from the fact that identical atoms or molecules emit or absorb an identical set of EMR wavelengths (such as the 656.3 nm H-alpha line from atomic hydrogen) so collections of many atoms/molecules produce EMR with these wavelengths added or subtracted. Any width of the line represents a divergence from this wavelength, which is what makes the line wider than infinitely thin. The mechanisms that do this fall under the general term line broadening, and each adjusts the shape in a characteristic way. The term line shape function refers to a mathematical function that models a line, or models one or more broadening mechanisms. Current practice includes taking characteristics of possible source material and simulating the line it would produce (e.g., by appropriately combining line shape functions) to compare with observed lines.