In absorption spectroscopy, the spectral features of interest appear in absorption with respect to a background continuous spectrum. In the interstellar medium, the background continuum may be supplied by a radiation source, such as a star, located behind the region of interest. The absorbing material may be either in the gas or the solid phase (e.g., interstellar dust or ices). Solid state features are much broader than atomic or molecular absorptions and are consequently more difficult to assign to a specific carrier. Much of the solar (Fraunhofer) spectrum is seen in absorption, as the outer cooler layers of the solar atmosphere absorb radiation from the deeper photosphere. Spectral lines in planetary atmospheres are typically seen in absorption, against the continuous thermal spectrum from the planetary or satellite surface.
The first person to notice a number of dark features in the solar spectrum was the English chemist William Wollaston in 1802. This...