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STS426 Didier Fraix-Burnet
dust (in reality cold gas and very small grains) that can absorb the blue part of
the spectrum and make it steeper than in reality. This phenomenon is very
probably present in the spectra of the first group, at least.
Absorption lines give some indication on the chemical composition of the
stars. It is easy to see that their intensity and their ratios generally differ from
one group to the other. Sometimes a more detailed examination of the
median spectra is necessary.
The emission lines are characteristics of several atomic elements
(hydrogen, carbon, nitrogen, oxygen...) and their ratios give invaluable
information on the physical conditions of the hot gas. This gas can be heated
by young and massive stars, by shocks in the interstellar medium or by the
presence of a black hole in its vicinity, most generally near the center of the
galaxies. The differences between the last eight groups in the two figures are
spectacular and reveal really different kinds of objects.

Figure 2. Idem Fig. 1 for the set of 300 000 spectra. Seventeen groups are shown

4. Discussion and Conclusion
The unsupervised classification results we have obtained on a very large
sample of astrophyscial spectra appear remarkable in that it can automatically
identify recognizable typical galaxy properties. We have shown that the

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