Basic spectrometry: Simple considerations
 
By attaching spectrometric instruments to telescopes, the collected radiation is dispersed to provide spectra of any observed source. Analysis of the records provides information on the overall spectral distribution of the received energy, so allowing temperatures to be assigned to the various objects. Within the overall energy–wavelength envelope, spectral features may be discerned corresponding say to the Balmer sequence of hydrogen as described earlier or to lines associated with other atomic or molecular species.
In addition to the detection of the various chemical elements from identification by their spectral fingerprints, some of the lines may correspond to excited or ionized states of the radiating or absorbing atoms and again such features lead to a determination of the source’s temperature.
From laboratory measurements and theory, the wavelength positions of the lines associated with the various chemical elements are known to high accuracy. In the investigations of astronomical sources, it turns out that many of their velocities are not negligible in comparison with that of light. As a consequence, Doppler shift effects (see later) may be apparent whereby the position of a spectral line may be shifted with respect to its laboratory position. Measurement of the displacement can, therefore, lead to a determination of the line-of-sight velocity of the source.
Examination of the detailed shape of spectral lines, or line profiles, also holds much astrophysical information. For example, careful observation with subsequent data reduction of line profiles can again provide information on the rotation of the investigated body, be it a planet, star or galaxy. The rationale for undertaking spectrometric studies is amplified in the following sections by presenting a selection of mechanisms which influence the behaviour of line profiles in terms of their wavelength position and their shape.