continuum

The term continuum in spectrography is commonly used for portions of a continuous spectrum not affected by a spectral feature such as a spectral line. To characterize a spectral line quantitatively, the received energy within the wavelength-range surrounding the line is measured, and compared with that of the continuum, by sampling nearby wavelength-ranges unaffected by any lines. The goal is to approximate the difference in received energy due specifically to the spectral line of interest. A typical star's spectral energy distribution (SED) shows an underlying smooth shape, largely that of a black-body spectrum, but small portions of it have higher and lower spectral lines and bands. The overall shape is referred to as the continuum. The term can also be used on a smaller scale, e.g., around a spectral line in a portion of the spectrum that does not correspond to the black-body spectrum.

The term continuum with a modifier can refer to a flat or straight-line portion of a wide spectral feature, i.e., of a feature considerably wider than a single spectral line. An example is a Lyman continuum, a type of region of a spectrum showing shows less energy-per-wavelength than "the continuum" that is associated with a black-body spectrum.