The integrated spectrum of a galaxy is a powerful diagnostic of its stellar content and evolutionary properties. In most normal galaxies the nuclear regions contribute only a few percent or less of the integrated flux, in either the continuum or the emission lines, so in most cases the spectral appearance is dominated by the disk and the bulge. Therefore, the galaxy spectra must take into account the contribution of all these extended and external galactic regions to avoid that spatial undersampling introduces significant errors in the star formation rate determination.
In the following we distinguish between galaxies whose spectra are characterized by absorption features and those in which emission features are dominant. This difference is due to their stellar population properties.