  We analyse the UV spectral energy distribution of a sample of normal galaxies
listed in the IUE-INES Guide No. 2-Normal Galaxies (Formiggini & Brosch, 2000)
using a Principal Component Analysis. The sample consists of the IUE-SW spectra
of the central regions of 118 galaxies, where the IUE aperture included more
than 1 per cent of the galaxy size. The principal components are associated
with the main components observed in the UV spectra of galaxies. The first
component, accounting for the largest source of diversity, can be associated
with the UV continuum emission. The second component represents the UV
contribution of an underlying evolved stellar population. The third component
is sensitive to the amount of activity in the central regions of galaxies and
measures the strength of star formation events. In all the samples analysed
here the principal component representative of star-forming activity accounts
for a significant percentage of the variance. The fractional contribution to
the SED by the evolved stars and by the young population are similar.
Projecting the SEDs onto their eigenspectra, we find that none of the
coefficients of the principal components can outline an internal correlation or
can correlate with the optical morphological types. In a sub-sample of 43
galaxies, consisting of almost only compact and BCD galaxies, the third
principal component defines a sequence related to the degree of starburst
activity of the galaxy.
